■■s^* x* mzwwr ^Bt'taCTl (Gv. v/&-i'f.:&s/fiy/'~ OFFICE. i. BESIDE'Xl'r,. >V.W£.VffereonSl,Vt.9fliA-10fh. if, ^uauyelfasuuSt.lrt.lOtkikltfti, A FZ®Q&aO0ZZlCZOZZZOZZZ(JZZZOZ::Z0-■ V./~?v rz . :-?* :*!> *& ?*^- f? -^ -;l>^ (c , organs of great importance in the ani- zz^ mal economy, through the extraordi- <$$ nary motor powers with which they \ul\!if!fV;lflf^ are endowed. The form of the ciliary filaments is usually a little flattened, vibratiie or ciliated Epithelium;-a, and tapering gradually from the base nucleated cells, resting on their smaller r . O- o . / extremities; 6, cilia. to the point. 1 heir size is extremely variable; the largest that have been observed being about l-500th of an inch in length, and the smallest about l-13,000th. When in motion, each filament appears to bend from its root to its point, returning again to its original state, like the stalks of corn when depressed by the wind; and when a number are affected in succession with this motion, the appearance of progressive waves following one another is produced, as when a corn-field is agitated by frequent gusts. When the ciliary motion is taking place in full activity, however, nothing whatever can be distinguished, but the whirl of particles in the surrounding fluid ; and it is only when the rate of movement slackens, that the shape and size of the cilia, and the manner in which their stroke is made, can be clearly seen. The motion of the cilia is not only quite independent (in all the higher animals at least) of the will of the animal, but is also inde- pendent even of the life of the rest of the body ; being seen after the death of the animal, and proceeding with perfect regularity in parts separated from the body. Thus isolated epithelium-cells have been seen to swim about actively in water, by the agency of their cilia, for some hours after they have been detached from the mucous surface of the nose ; and the ciliary movement has been seen fifteen days after death in the body of a Tortoise, in which putrefaction was far advanced. In the gills of the River-Mussel, which are among the SIMPLE ISOLATED CELLS.—EPITHELIUM. • 151 best objects for the study of it, the movement endures with similar pertinacity. 235. The purpose of this ciliary movement is obviously to propel fluids over the surface on which it takes place; and it is consequently limited in the higher animals to the internal surfaces of the body, and always takes place in the direction of the outlets, towards which it aids in propelling the various products of secretion. The case is different, however, among animals of the lower classes, especially those inhabiting the water. Thus the external surface of the gills of Fishes, Tadpoles, &c, is furnished with cilia; the continual move- ment of which renews the water in contact with them, and thus promotes the aeration of the blood. In the lower Mollusca, and in many Zoophytes, which pass their lives rooted to one spot, the motion of the cilia serves not merely to produce currents for respiration, but likewise to draw into the mouth the minute particles that serve as food. And in the free-moving Animalcules, of various kinds, the cilia are the sole instruments which they possess, not merely for pro- ducing those currents in the water which may bring them the requisite supply of air and food, but also for propelling their own bodies through the water. This is the case, too, with many larger animals of the class Acalepha (Jelly-fish), which move through the water, sometimes with great activity, by the combined action of the vast numbers of cilia that clothe the margins of their external surfaces. In these latter cases it would seem as if the ciliary movement were more under the control of the will of the animal, than it is where it is concerned only in the organic functions. In what way the will can influence it, how- ever, it does not seem easy to say; since the ciliated epithelium-cells appear to be perfectly disconnected from the surface on which they lie, and cannot, therefore, receive any direct influence from their nerves. Of the cause of the movement of the cilia themselves, no account can be given ; they are usually far too small to contain even the minutest fibrillae of muscle ; and we must regard them as being, like those fibrillae, organs sui generis, having their own peculiar en- dowment,—which is, in the higher animals at least, that of continuing in ceaseless vibration, during the whole term of the life of the cells to which they are attached. The length of time during which the ciliary movement continues after the general death of the body, is much less in the warm-blooded than in the cold-blooded animals; and in this respect it corresponds with the degree of persistence of muscular irritability, and of other vital endowments. 236. The Tesselated-Epithelium, as already mentioned, covers the Serous and Synovial membranes, the lining membrane of the blood- vessels and absorbents, and the Mucous membranes with their glan- dular prolongations, except where the cylinder-epithelium exists. It presents itself, with some modifications presently to be noticed, in the ultimate follicles of all glands, and also in the air-cells of the lungs. In this latter situation it is furnished with cilia; and these are also found on the cells of the tesselated-epithelium, which covers the deli- 152 SIMPLE ISOLATED CELLS.—EPITHELIUM. cate pia mater lining the cerebral cavities.—The Cylinder-Epithelium commences at the cardiac orifice of the stomach, and lines the whole intestinal tube; and, generally speaking, it lines the larger gland- ducts, giving place to the tesselated form in their smaller ramifica- tions. A similar epithelium, furnished with cilia, is found lining the air-passages and their various offsets,—the nasal cavities, frontal sinuses, maxillary antra, lachrymal ducts and sac, the posterior surface of the pendulous velum of the palate and fauces, the Eustachian tubes, the larynx, trachea, and bronchi,—becoming continuous, however, in the finer divisions of the latter, with the ciliated pavement-epi- thelium. The upper part of the vagini, the uterus, and the Fallopian tubes, are also furnished with a ciliated Cylinder-epithelium. The function of the cilia in all these cases appears to be the same; that of propelling the viscid secretions, which would otherwise accumulate on these membranes, towards the exterior orifices, whence they may be carried off. 237. The simplest office which the Epithelium-cells of Mucous membranes perform, appears to be that of elaborating a peculiar secretion, termed Mucus; which is destined to protect them from the contact of air, or from that of the various irritating substances to which they are exposed, in consequence of their peculiar position and functions. This Mucus is a transparent semi-fluid substance, dis- tinguished by its peculiar tenacity or viscidity. It is quite insoluble in water; but is readily dissolved by dilute alkaline solutions, from which it is precipitated again by the addition of an acid. A substance resembling Mucus may be produced from any fibrinous exudation, or even from pus, by treating it with a small quantity of liquor potassae. The secretion of Mucus, like the formation of Epidermis, appears to take place with an activity proportioned to the degree of irritation of the. subjacent membrane. On many parts of the mucous surface, a sufficient supply is afforded by the epithelium-cells which cover it; but in other situations, especially along the alimentary canal, the demand is much greater, and it is supplied not merely by the cells of the surface, but by those lining the crypts or follicles which are formed by involutions of it. There is reason to believe that the whole epithelial covering of the stomach and intestinal tube (along the upper part of the latter at least) is cast off at every meal (Fig. 27); the cells growing from their germs, elaborating their mucous secretion, and then bursting or liquefying to set this free, in the course of a few hours. The debris of these secreting cells may be recognized in the substances voided from the intestine ; as well as in the mucus taken from the surface of any mucous membrane. 238. The Epithelium-cells, which are thus being continually re- newed on the Mucous surfaces, commonly seem to have their origin in the granular germs diffused through the basement-membrane ; but it is different in regard to the cells of the follicles, which seem rather to occupy their cavity than merely to line their walls, and which appear to be in course of continual production from a germinal spot, SIMPLE ISOLATED CELLS.—SECRETING CELLS. 153 or collection of reproductive granules, at the blind extremity of the follicle. This is the case in the ultimate follicles of the more complex glands; which may be regarded as so many repetitions of the simple crypts or follicles in the substance of the mucous membranes;—the only difference being, that the former pour their secretion into a branch of a duct, which unites with the other ramifications to form a trunk ; and this trunk conveys them to their destination in some cavity lined by a mucous membrane;—whilst the simple follicles or crypts at once pour forth their secretions upon the surface of the membrane. The accompanying figure represents two follicles of the Fig. 23. Fig. 24. Two follicles from the liver of Carcinus Ultimate follicles of Mammary rncsnas, (Common Crab), with their con- gland, with their secreting cells, a, tained secreting cells. a;—6, 6, the nuclei. liver of the Common Crab, which are seen to be filled with secreting cells ; it is evident, from the comparative sizes of these cells in dif- ferent parts, that they originate at the blind extremity of the follicle, where there is a germinal spot; and that, as they recede from that spot, they gradually increase in size, and become filled with their characteristic secretion, being at the same time pushed onwards towards the outlet by the continual new growth of cells at the ger- minal spot. In Fig. 24 are shown the corresponding ultimate folli- cles of the Mammary gland ; filled, like the preceding, with secreting cells. 239. The whole of the acts, then, by which the separation of the different Secretions from the Circulating fluid is accomplished, really consist in the growth and nutrition of a certain set of cells, usually covering the free surfaces of the body, both internal and external, or lining cavities which have a ready communication with these by means of ducts or canals.* These cells differ widely from one an- other, in regard to the kind of matter which they appropriate and assemble in their cavities; although the nature of their walls is pro- bably the same throughout. Thus we find biliary matter and oil, easily recognizable by their colour and refracting power, in the cells of the liver; milk in the cells of the mammary gland; sebaceous or fatty matter in the cells of the sebaceous follicles of the skin; and so on. All these substances are derived from the blood; being either contained in it previously, or being elaborated from its constituents * The Synovial secretion is, perhaps, the only one which is poured into a closed sac. 154 SIMPLE ISOLATED CELLS.—REPRODUCTIVE CELLS. Fig. 25. Secreting cells of Human Liver; a, nu- cleus; b, nucleolus; c, oil-particles. by a simple process of transformation,—as, for example, that which converts the albumen of the blood into the casein of milk. Hence they may be considered as the peculiar aliments of the several groups of cells ; whose acts of nutrition are the means of drawing them off or secreting them, from the general circulating fluid. When they have attained their full growth, and ac- complished their term of life, their walls either burst or dissolve away, and thus the contents of the cells are delivered into the cavity, or upon the surface, at which they are required. Now as all the canals of the glands open either directly outwards upon the surface, or into cavities which communicate with the exterior, it is evident that the various products of the action of these epithelial cells must be destined to be cast forth from the body. This we shall find to be the case ; some of them, as the bile and urine, being excretions, of which it is necessary to get rid by the most direct channel; whilst others, like the tears, the saliva, the gastric fluid, the milk, &c, are separated from the blood, not so much for its purification, but because they are required to answer certain purposes in the economy. 240. Nowr whilst thus actively concerned in the Nutritive functions of the economy, and exercising in the highest degree their powers of selection and transformation, these Secreting cells appear to have nothing to do with the operation of Reproduction. We have seen that they do not even regenerate themselves; all their energies being, as it were, concentrated upon their own growth; and the successive production of new generations being provided for by other means. But special Reproductive cells, destined to furnish the germs for the continuance of the race, are not wanting. These are developed within the tubuli of the Testicle ; where they appear to hold exactly the same relation to the membranous walls of those tubuli, as do the secreting cells to the tubes and follicles of the proper Glands. The contents of these repro- ductive cells are peculiarly granular; and the granules are at one time dif- fused through the entire cell. They are afterwards seen, however, to pre- sent a regular linear arrangement; forming a bundle of fibrous bodies, still comprehended, however, within the cell. After a time, however, the con- taining cells burst, and the fibrous bodies within separate and are set free. From the very peculiar motion which they possess, they were lono- regarded as distinct Animalcules, and received the designation of Spermatozoa. It is now generally admitted, however, that they have Formation of Spermatozoa within semi- nal cells ; a, the original nucleated cell; 6, the same enlarged, with the formation of the Spermatozoa in progress ; c, the Sper- matozoa nearly complete, but still enclosed within the cell. SIMPLE ISOLATED CELLS.—ABSORBENT CELLS. 155 no more claim to a distinct animal character, than have the ciliated - epithelia of mucous membrane, which will likewise continue in move- ment when separated from the body. The so-called Spermatozoa appear to be nothing else than cell-germs, furnished with a peculiar power of movement, by means of which they are enabled to make their way into the situation where they may be received, cherished, and developed,—as will be shown hereafter. (Chap. XI.) It is a curious fact that the seminal cells, in which the Spermatozoa are formed, are sometimes ejected from the gland, not only before they have burst and set free the Spermatozoa, but even long before the development of the Spermatozoa in their interior is completed;—thus affording a complete demonstration of their independent vitality. 241. We now proceed to a class of cells, which are equally independent of each other, which begin and end their lives as cells, without undergoing any transformation, but which form part of the substance of the fabric, instead of lying upon its free surfaces and being continually cast off from them. Still their individual history is much the same as that of the cells already noticed; and they differ chiefly in regard to the destination of their products.—The first group of this class deserving a separate notice, is that which effects the introduction of aliment into the body;—of those kinds of aliment, at least, which are not received in solution by any more direct means. Along the greater part of the intestinal tube, from the point at which the hepatic and pancreatic ducts enter it, to the rectum, we find the mucous membrane furnished with a vast number of minute tufts or folds, by which its free surface is vastly extended; these are termed villi. They may be compared to the ultimate root-fibres of trees, both in structure and function; for each of them gives origin to a minute lacteal or chyle-absorbing vessel, which occupies its centre; whilst it also contains a copious network of blood-vessels, (Fig. 8, p. 118,) which appears likewise to participate in the act of absorption, by taking up substances that are in complete solution. Now at the end of every villus, there may be seen, whilst the process of digestion and absorption is going on, a cluster of minute cells, in the midst of which the origin of the lacteal is lost. These cells, whose size varies from 1-1000th to l-2000th of an inch, are turgid with a milky fluid, which is evidently the same with that which is found in the lacteals; and there is good reason to believe, that it is by the growth and nutrition of these cells, that this milky fluid, the chyle, is selected from the contents of the digestive cavity. Their function, therefore, is precisely the converse of that of the secreting cells already described; whilst the history of their individual lives is the same. These absor- bent cells draw their materials from the fluid in the digestive cavity, instead of from the blood; and when they burst or liquefy, they set free their contents where they may be taken up by a lacteal and con- veyed into the circulating current, instead of pouring them into a cavity through which they will be shortly expelled. 242. In the intervals of the digestive process, the extremities of the 156 ABSORBENT ?ELLS. villi are comparatively flaccid ; and instead of cells, they show merely a collection of granular germs. These begin to develop themselves, as soon as the food has been dissolved in the stomach and transmitted to the intestine; and their development goes on, as long as the villi are surrounded with nutrient matter. The cells rapidly grow, select, absorb, and prepare the nutritious matter, by making it a part of themselves; and, when their work is accomplished, they deliver it to the lacteals by their own rupture or deliquescence. The accompany- ing diagrams represent the comparative condition of the Mucous Fig. 27. Fig. 28. Diagram of mucous membrane during diges- tion and absorption of chyle; a, a villus, turgid and erect; its protective epithelium cast off from its free extremity; its absorbent vesicles, its lac- teals, and its blood-vessels turgid; 6, a follicle discharging its secreting epithelial cells. Diagram of mucous membrane of jejunum, when Absorption is not going on; o, protec- tive epithelium of a villus; 6, secreting epi- thelium of a follicle; c, e, c, primary membrane, with its germinal spots or nuclei, d, d; e, ?erms of absorbent vesicles;/, vessels and acteals of villus. membrane, its villi, and its secreting follicles, during the time when absorption is going on, and in the intervals of the process. It will be seen that, in the former state, the epithelium-cells are not only being cast off from the free surface of the membrane, and from the interior of the follicles; but they are also detached from the surface of the villus, that they may offer no impediment to the process of absorption. During the intervals of digestion, the secreting epithe- lium of the follicles, and the protective epithelium of the villi, are alike renewed, from the germs supplied by the basement-membrane. 243. Although the mucous membrane of the intestinal tube is the only channel through which insoluble nutriment can be absorbed in the completely-formed Mammal, and the only situation, therefore, in which we meet with these absorbent cells, there are other situations in which similar cells perform analogous duties in the embryo. Thus the Chick derives its nutriment, whilst in the egg, from the substance of the yelk, by absorption through the blood-vessels spread out in the vascular layer of the germinal membrane surrounding the yelk; which vessels answer to the blood-vessels and lacteals of the permanent digestive cavity, and are raised into folds or villi as the contents of the yelk-bag are diminished. Now the ends of the vessels are sepa- rated from the fluid contents of the yelk-bag, by a layer of cells; ABSORBENT CELLS. 157 which seems to have for its object to select and prepare the materials supplied by the yelk, for being received into the absorbent vessels. 244. In like manner, the embryo of the Mammal is nourished, up to the time of its birth, through the medium of its umbilical vessels ; the ramifications of which form tufts, that dip down, as it were, into the maternal blood, and receive from it the materials destined to the nutrition of the fcetus; besides effecting the aeration of the blood of the latter, by exposing it to the more oxygenated blood of the mother. Now around the capillary loop of the fcetal tuft, there is a layer of cells, closely resembling the absorbent cells of the villi; and these are enclosed in a cap of basement-membrane, which completes the fcetal portion of the tuft, and renders it comparable in all essential respects to the intestinal villus. It is again surrounded, however, by another layer of membrane and of cells, belonging to the maternal system;—the derivation and arrangement of which will be explained hereafter. The maternal cells (b, Fig. 29) may be regarded as the Fig. 29. first selectors of nutriment from the circulating fluid of the parent: the materials, partially prepared by them, are poured into the cavity (d) surrounding the extremity of the tuft; and from this they are taken up by the fcetal cells (f), which further elaborate them, and impart them to the capillary loop (g) of the umbilical vessels. 245. Thus we see that the several functions of Selection, Absorp- tion, Assimilation, Respiration, Secretion, and Reproduction, are performed by the agency of cells in the Animal as in the Vegetable kingdom,—in the complex Human organism, as in the humblest Cryptogamic Plant; the only difference being, that in the latter there is a greater division of labour, different groups of cells being appro- priated to different functions, in the general economy, whilst the history of their own processes of nutrition and decay is everywhere essentially the same. Thus we have seen that the Absorbent cells, at the extremities of the intestinal or placental villi, select and draw into themselves, as the materials of their own growth, certain substances in their neighbourhood; which are still as much external to the tissues of the body, as are the fluids surrounding the roots of plants. Having come to their full term of life, they burst or dissolve away, and give up their contents to the absorbent vessels, which carry them into the general current of the circulation, where they are mingled wi,th the fluid previously assimilated,—the blood. Whilst passing through the vessels they are subjected to the action of another set of cells, (the Extremity of a placental villus:—o, external membrane of the villus, continuous with the lining membrane of the vascular system of the mother; 6, external cells of the villus, belonging to the pla- cental decidua; c, c, germinal centres of the external cells; d, the space between the maternal and fcetal portions of the villus; e, the internal membrane of the villus, continuous with the external membrane of the chorion;/, the internal cells of the villus, belong- ing to fhe-chorion; g, the loop of umbilical vessels. 158 SIMPLE ISOLATED CELLS. lymph and chyle-corpuscles, and the colourless corpuscles of the blood,) by which ffiey are gradually assimilated, or converted into a substance of a more directly organizable character ; these assimilating cells being developed from germs that float in the fluid, drawing into themselves the albuminous matter, converting it into fibrin, and then setting it free by their own dissolution. In the same fluid another set of cells, the red corpuscles of the blood, are observed to float, in the higher classes of animals; whose special function appears to be the conveyance of oxygen from the lungs to the tissues, and of car- bonic acid from the tissues to the lungs; in other words, that of Respiration: these cells do not appear to pass through their course of existence as rapidly as the preceding. Next we have various groups of cells, external to the vessels, on the free surfaces of the body; whose office it is to draw from the blood certain materials, which are destined for Secretion or separation from it; either for the sake of preserving that fluid in its requisite purity, or for answering some other purpose in the system. These cells grow at the expense of the substances, which they draw into themselves from the blood ; and on their dissolution, they cast forth their contents on the free surfaces communicating with the exterior of the body, to which they are in time conveyed. And, lastly, we have a special set of cells, destined to prepare the germs of new beings; which are, in like manner, set free by the rupture of the parent-cell, in a condition that enables them to be conveyed to a place appropriated for their further develop- ment, and thus to perform the essential part of the process of Repro- duction. 246. The cells which are thus the active instruments of the Organic functions, are produced and succeed one another with a rapidity pro- portional to the energy of those functions. The causes which influence their growth and decay are not always evident; thus we occasionally find an extraordinary tendency to the elaboration of Fibrin, as mani- fested in the increase in the proportion of that ingredient of the blood, and in the number of the Assimilating cells or white corpuscles that float in that fluid ; and as to the causes of this condition, which is one important part of the disordered state termed Inflammation, we are almost entirely in the dark. The development of the Absorbent cells appears to depend upon the supply of alimentary materials afforded by the contents of the digestive cavity; and also upon the supply of blood furnished by the capillaries of the villi, from which last the materials of the cell-walls are probably derived. The conditions of the development of the Secreting cells are not sufficiently understood ; it does not appear to depend solely upon the supply of their materials; for, as we shall see hereafter, these materials may accumulate unduly in the blood, through the insufficient production of the cells which are destined to separate them; whilst, on the other hand, the presence of certain substances in the blood appears to accelerate their develop- ment. Of these stimuli, Mercury is one of the most powerful; and we have continual opportunities of witnessing its effects, in giving an CELLS CONNECTED TOGETHER IN SOLID TISSUES. 159 increased activity to the secreting actions. There is probably not a gland in the body, which is not in some degree influenced by its pre- sence in the blood ; but the liver, the kidneys, the salivary glands, and the glandulae of the intestinal canal, appear to be those most affected by its stimulating powers. The action of the glands, in other words the development of the secreting cells, appears to be influenced by mental emotions; being sometimes accelerated, and sometimes retarded, through their agency. This is especially the case in regard to the secretion of Milk, Tears, Saliva, and Gastric juice. But we shall hereafter see that the influence thus manifested is probably exerted through the capillary circulation, which is known to be power- fully affected by mental emotions, as in the acts of blushing and erec- tion ; and that the increased production of the secretion is immediately due to the increased flow of blood to the gland. We have an example of this, in the " draught" (as it is termed) experienced by Nurses, when the child is applied to the breast; which is a perceptible rush of blood into the organ. 5. Of Cells connected together as permanent constituents of the Tissues. 247. We now pass on to consider those Cells which enter as com- ponent elements into the solid and permanent fabric of the body, and which do not take so active a part in its vital operations. These we shall find to be usually more or less closely connected together, either by a general enveloping membrane, or by an intercellular substance, which is interposed between their walls, and holds them together by its adhesive properties. Before entering upon the description of the tissues thus formed, it will be desirable to consider a little more fully the mode in which the component cells are developed, and the cha- racter of the transformations they may undergo. 248. We have seen that a minute isolated molecule, prepared by a parent-cell, and set free by its dissolution, may become the germ of a new cell; and that the assimilating cells which float in the animal fluids seem to have their origin, like the equally-simple cells of the Yeast- fungus, in such floating germs; whilst the epithelial and epidermic tissues arise from similar granules diffused through the substance of the basement-membrane, or aggregated in its germinal spots. But the usual mode of development of the cells of a higher and more perma- nent character, is somewhat different; for these are developed within the parent-cell, which, instead of dissolving away, may remain as a thin membrane around them;—all traces of it, however, at last disappearing, in consequence of the distension which it undergoes. Even whilst still evidently contained within the parent-cell, the secondary cells may themselves be developing a third generation within them. The rapidity of the process, and the number of cells thus developed, appear to bear a close relation with the transitory or permanent character of the structure. It is in Cancerous growths, that we meet with the most remarkable examples of rapid production ; 160 CELLS CONNECTED TOGETHER IN SOLID TISSUES. Fig. 30. Parent-cells, a, a, of cancer- ous structure, containing se- condary cells, 6, b, each having one, two, or three nuclei, c, c. a large number of secondary cells being developed within each pri- mary ; these secondary cells again becoming the parents, each one of an equally large generation; and so on. Here the whole energy seems concentrated upon the reproductive process; and we find that growths composed of such cells have a very rapid increase, but very little solidity or per- manence.—On the other hand we find that, in structures which are destined to undergo a higher development, and to possess a more permanent character, the number of cells developed within each parent is more limit- ed ; thus in the early development of the embryo of Mammalia it is limited to two; and the first pair of cells is thus progressively developed into four, eight, sixteen, and so on. The same tendency to a binary multiplication is apparent also in the cells of Cartilage (§ 267); and it probably exists also in other cellular structures of a permanent character. 249. It is most commonly to be observed in these cells, that the reproductive granules, instead of being diffused throughout the cavity of the cell,—as they are in the cells of the Cryptogamic Plants, the White Corpuscles of the blood of Animals, &c. &c,—are concen- trated in one spot, forming a nucleus (Figs. 20, 21); and it is from this nucleus that the new cells originate. The granules appear to undergo the same changes, when developed in this situation, as they do when isolated within the cell, or altogether set free; at first they show a simple enlargement, looking like little warts projecting into the cell; this enlargement continues, until the difference between the cell-wall and the cavity, the containing and the contained parts, becomes perceptible; and the character of the young cell is then obvious. 250. According to Dr. Barry's observations on the processes of cell-growth in the germinal vesicle and early embryo of the Rabbit, it is the outer circle of granules forming the nucleus, which is first developed into young cells; the next then commences, and pushes outwards the ring of cells previously formed; and by the continuance of the same process, the parent-cell may be completely filled with a new generation. Of these, however, the greater part may be destined to liquefy or dissolve away; their office having apparently been, to assimilate or prepare the materials that are destined for the nutrition of the permanent offspring, which are the cells latest formed in the centre of the nucleus. A pellucid spot, which is frequently seen in the centre of the nucleus, has received the name of nucleolus (Fig. 20, c); sometimes two or even three nucleoli may be seen in a single nucleus (Fig. 21, a). The cause of this appearance is not precisely understood ; but it seems to be of a transitory character, indicating a certain stage in the conversion of the nucleus into new cells. CELLS CONNECTED TOGETHER IN SOLID TISSUES. 161 251. The function of the nucleus in the development of new cells, is thus evidently identical with that of the "germinal spots" already described as existing at the extremity of the secreting follicles (§ 238), or in the substance of the basement-membrane. In fact we are pro- bably to regard each secreting follicle as a large parent-cell; of which the functions are permanent, instead of transitory; and which, having opened into a neighbouring duct, instead of remaining closed, con- tinues to develop new secondary or secreting cells, from the nucleus or germinal spot at its opposite extremity, to an unlimited extent. And it is probable, also, that the " germinal spots" in the substance of the basement-membrane are really the nuclei of cells, by the coales- cence of which it is formed, in the manner to be presently noticed. 252. Now if the walls of the parent-cells, instead of liquefying or thinning away, are thickened or strengthened by additional nutrition, they may remain as permanent vesicles, enclosing arid holding together numerous secondary cells; and this appears to be the case in Adipose tissue, and also in tumours of various kinds. 253. Where such enveloping membranes are wanting, we fre- quently find the component cells of the permanent tissues of Animals (like those of the higher plants) held together by an intercellular sub- stance ; which generally presents no distinct traces of organization; and which usually consists of Gelatin, or of a substance allied to it in composition. The proportion of this substance to the cells may vary in different cases ; and very different characters may thus be presented by a tissue made up of the same elements. Thus the subjoined figure represents a portion of one of the animal layers included between the calcareous laminae of a bivalve shell; in which we see on the one side a number of nuclei or incipient cells, scattered through a bed of homo- geneous intercellular substance, and bearing but a very small propor- tion to it; whilst the opposite end exhibits a set of polygonal cells, in close contact with each other, the intercellular substance being only represented by the thick dark lines, which mark the boundaries of the cells, and which are rather thicker at the angles of the latter. Between these two extremes, we observe every stage of transition. 254. The presence of a very large amount of intercellular substance, through which minute cells are scattered at considerable intervals, (Fig. 31, a,) is characteristic of various forms of Cartilage; and more particularly of that soft semi-cartilaginous structure, of which the Jelly-fish are for the most part composed. In other forms of cartilage, we find the cells more developed, and in closer proximity to each other, the proportion of the intercellular substance being at the same time diminished (as seen at b and c, Fig. 31); but it is not often, save in embryonic structures, that we find the cells in such close proximity, and the intercellular substance so nearly wanting, as at d. Such examples do occasionally present themselves, however, even in the soft tissues. Thus the chorda dorsalis, which replaces the verte- bral column in the lowest Fishes, and of which the analogue is found in the embryos of the higher Vertebrata, is made up of a structure of 11 162 CELLS CONNECTED TOGETHER IN SOLID TISSUES. this kind. The true Skin, in the Short Sun-fish, is replaced by a similar layer of cellular tissue, which extends over the whole body, varying in thickness from one-fourth of an inch to six inches. And Fig. 31. Portion of shell-membrane, showing the origin of cells in the midst of horny intercellular substance; a, nuclei; 6, incipient cells; e, the same further advanced, but separated by intercellular substance d, the cells become polygonal by mutual pressure. in the Lancelot (a little fish which is destitute of so many of the characters of a Vertebrated animal, that its right to a place in that division has been doubted), a considerable portion of the fabric is made up of a similar parenchyma. 255. Now we shall find that one method, by which the requisite firmness and solidity are given to the animal fabric, consists in the deposition of earthy substances in the interior of such cells, by a peculiar secreting action of their own. Thus in Shell, we find them completely filled up with carbonate of lime; and in Bones and Teeth with car- bonate and phosphate of lime. When this is the case, there is a tendency to an apparent coalescence of the cells, by the obliteration of their partitions; or rather, perhaps, by the removal of the whole intercellular substance from between them, the actual cell-walls being so very thin, that they are not distinguishable. The incipient stages of this coalescence, as seen in another portion of the same membrane as that represented in the last figure, are shown in Fig. 32. At a, Fig. 32. Portion of shell-membrane, showing the gradual coalescence of distinct cells; at o, the cells sepa- rated by intercellular substance J at b, the partitions are thinner; and at e, tuey almost disappear. COALESCENCE AND METAMORPHOSIS OF CELLS. 163 the nucleated cells are very distinct; and are separated by a large quantity of intercellular substance. At b, they approach each other more closely, the amount of intercellular substance being less; the widest intervals are seen at the angles of the cells. At c, the approxi- mation is much closer; and the cell-walls are scarcely distinguishable at the points where they come into immediate contact. Proceeding further, we observe that the partitions are much less complete; so that the originally distinct cellular character of the membrane is chiefly indicated by the bright nuclei, which are regularly dispersed through it, and by the triangular dark spots, which show the remains of the intercellular substance at the angles where three cells join each other. The coalescence may be traced further than it is shown to do in the figure ; so that, if it were not for the evidence afforded by the transi- tion-stages here represented, it would be difficult to prove that the membranous layer had its origin in cells. 256. These facts, respecting the gradual coalescence of cells, ex- plain not merely the appearances presented in Tooth, Shell, &c. (hereafter to be described); but also those which are exhibited by the Basement-membrane, as already detailed (§ 206.) 257. There is no evidence, in the preceding case, that the cavities of the cells coalesce; and there is no reason why they should do so. But we often find such an union, where the production of a continuous tube is required. The long straight open ducts, through which the sap of Plants rises in the stem, are unquestionably formed by a coalescence of the cavities of cells of a cylindrical form, placed regularly end to end; and it seems probable that the network of anastomosing vessels, through which the elaborated sap finds its way to the various parts of the vegetable fabric, is formed, in like manner, by the coalescence of cells, arranged obliquely and transversely in regard to one another. In like manner, the capillary Blood-vessels of Animals are usually believed to originate in rows of cells, the cavities of which have run together by the obliteration of the transverse partitions; as the per- sistent nuclei of such cells may be occasionally brought into view in the walls of the capillaries. And the same appears to be the origin of the tubular fibres of Muscular and Nervous tissue, which contain the elements characteristic of those tissues; these elements,—the fibrillae of muscle, and the granular pith of the nerve-tube,—being evidently the secondary products of parent-cells, which seem to remain as their investing tubuli, in the walls of which the original nuclei are often to be seen (§§ 338 and 388). 258. Besides these changes, the original cells may often undergo marked alterations of form; and this quite independently of any pres- sure to which they may be subject. Thus the pigment-cells, as already mentioned (§ 229,) frequently exhibit a curious stellate form; arising from the development of radiating prolongations, which are put forth from the original spheroid. A form which is frequently assumed by the cells that are developed in fibrinous or plastic exu- dations, and which is also met with in the cells of tumours, both 164 FUSIFORM CELLS.—ADIPOSE TISSUE. malignant (or Cancerous) and non-malignant, is that which has re- ceived the designation offusiform or spindle-like, from its prolonged shape and pointed extremities. The various stages of transition, which may be observed between the simple rounded cell and the fusiform cell, are shown in Fig. 33; and it is there seen that, when the transformation has gone to its utmost extent, the nucleus of the cell is no longer visible, so that it bears a close resemblance to a simple fibre. Such cells are found amongst the simple fibrous tissues; and, in the opinion of many, they give origin to them.—The appear- ance of tissue, composed of fusiform cells, is shown in Fig. 34; this is seldom met with as a permanent part of the normal fabric ; but it is a frequent product of morbid action. Fig. 33. Fig. 34. Fig. 35. Transition from cellular to Fusiform tissue of plastic Areolar and Adipose tissue ; fusiform tissue; a, circular or exudations; a, fusiform bo- a, a, fat-cells; b, b, fibres of are- oval cells; 6, the same becoming dies without nuclei; 6, nu- olar tissue. pointed; c, fusiform cells con- cleated fusiform cells; c, taining nuclei; d, fusiform cells granular intercellular sub- more elongated, and destitute stance. of nuclei. 259. We now proceed with the description of the various tissues in the Human body, which are composed of cells united or trans- formed in the foregoing manner; and we shall commence with Adipose or Fatty tissue, which may be considered as a sort of link, connect- ing the permanent tissues with those which are more actively con- cerned in the processes of Nutrition, Secretion, &c. The Adipose tissue is composed of isolated cells, which have the power of appro- priating fatty matter from the blood, precisely in the same manner as the secreting cells appropriate the elements of bile, milk, &c. These cells are sometimes dispersed in the interspaces of the Areolar tissue ; whilst in other cases they are aggregated in distinct masses,—consti- tuting the proper Adipose tissue. In the former case they are held in their places by fibres, that traverse the areolae in different directions; whilst in the latter, each small cluster of fat cells is included in a common envelop, on the exterior of which the blood-vessels ramify ; and these sacculi are held together by areolar tissue. We are thus probably to regard each fatty mass in the light of a gland, or assem- ADIPOSE TISSUE. 165 blage of secreting cells, penetrated by blood-vessels, and bound together by fibrous tissue ; but having its follicles closed instead of open, (which, as just now stated, appears to be the early conditions of the follicles of all glands, § 251;) and consequently retaining its secretion within itself, instead of pouring it forth into a channel for excretion. 260. The individual fat-cells always present a nearly spherical or spheroidal form; sometimes, however, when they are closely pressed together, they become somewhat polyhedral, from the flattening of their walls against each other. Their intervals are traversed by a minute network of blood-vessels, from which they derive their secretion; and it is probably by the constant moistening of their walls with a watery fluid, that their contents are retained without the least transudation, although they are quite fluid at the temperature of the living body. If the watery fluid of the cell-walls of a mass of Fat be allowed to dry up, and it be kept at a temperature of 100°, the escape Of Capillary network around Fat-cells. the contained oily matter is soon percep- tible.—By this provision, the fatty matter is altogether prevented from escaping from the cells of the living tissues, by gravitation or pressure ; and as it is not itself liable to undergo change when secluded from the air, it may remain stored up, apparently unaltered, for an almost unlimited period. 261. The consistency, as well as the Chemical constitution, of the fatty matter contained in the Adipose cells, varies in different animals, according to the relative proportions of three component substances, which may be distinguished in it—Stearine, Margarine, and Oleine. The two former are solid when isolated, and the latter is fluid; but at the ordinary temperature of the warm-blooded animal, they are dissolved in it. Of these, Stearine is the most solid ; and it is most largely present, therefore, in the hardest fatty matter, such as mutton- suet. It is crystaline like spermaceti; it is not at all greasy between the fingers, and it melts at 143°. It is insoluble in water, and in cold alcohol and ether; but it dissolves in boiling alcohol or ether, crystalizing as it cools. The substance termed Margarine exists alono- with stearine in most fats; but it is the principal solid con- stituent of Human fat, and also of Olive oil. It corresponds with Stearine in many of its properties, and is nearly allied to it in Chemi- cal composition; but it is much more soluble in alcohol and ether, and it melts at 118°. On the other hand, Oleine, when pure, remains fluid at the zero of Fahrenheit's thermometer; and it is soluble in cold ether, from which it can only be separated by the evaporation of the latter. It exists in small quantity in the various solid fats ; but it constitutes the great mass of the liquid fixed oils. The tendency of these to solidification by cold, depends upon the proportion of 166 ADIPOSE TISSUE. stearine or margarine they may contain. All these substances are neutral compounds, formed by the union of Stearic, Margaric, and Oleic acids, respectively, wTith a base termed Glycerine; this base may be obtained from any fatty matter, by treating it with an alkali, which unites with the acid and forms a soap, setting free the Glycerine. They contain no Nitrogen ; and their proportion of Oxygen is ex- tremely small in regard to their amount of the Carbon and Hydrogen : thus Stearine has 142 Carbon and 141 Hydrogen to 17 Oxygen; and in the other substances the proportions are similar. The fatty bodies appear to be mutually convertible; thus margaric acid may be pro- cured from stearic acid, by subjecting it to dry distillation; and there is ample evidence that animals supplied with one of them may pro- duce the others from it. 262. Since these fatty matters are abundantly supplied by the Vegetable kingdom, and are found to exist largely in substances which were not previously supposed to contain them, it is not requisite to suppose, that Animals usually elaborate them by any transforming process from the elements of their ordinary food. The mode in which they are taken into the blood, and the uses to which they are subserv- ient, will be hereafter investigated; but it may be here remarked, that the portion separated from the circulating fluid to form the Adi- pose tissue, is only that which can be spared from the other purposes, to which the fatty matters have to be applied. Hence the production of this tissue depends in part upon the amount of Fatty matter taken in as food; but this is not entirely the case, as some have main- tained ; for there is sufficient evidence that animals may produce fatty matter by a process of chemical transformation, from the starch or sugar of their food, when there is an unusual deficiency of it in their aliment. 263. The development of Adipose tissue in the body appears to answer several distinct purposes. It fills up interstices, and forms a kind of pad or cushion for the support of movable parts; and so necessary does it seem for this purpose, that, even in cases of great emaciation, some fat is always found to remain, especially at the base of the heart around the origin of the great vessels, and in the orbit of the eye. It also assists in the retention of the animal temperature by its non-conducting power; and wre accordingly find a thick layer of it, in those warm-blooded mammals that inhabit the seas,—either immediately beneath their skin, or incorporated with its substance. And it also serves as a reservoir of combustible matter, at the expense of which the respiration may be maintained when other materials are deficient; thus we find that the respiration of hybernating animals is kept up, during the period when they cease taking food (§ 121), by the consumption of the store of fat which was laid up in their bodies, previously to their passing into that state; and it is also to be noticed that herbivorous animals, whose food is scanty during the winter, usually exhibit a strong tendency to such an accumulation, during the latter part of the summer, when their food is most rich and abundant, ADIPOSE TISSUE.—CARTILAGE. 167 in order to supply the increased demand created by the low external temperature of the winter season. Other circumstances being the same, it appears that the length of time during wdiich a warm-blooded animal can live without food, depends upon the quantity of fat in its body; for the rapid lowering of its temperature, which is the imme- diate cause of its death (§ 117,) takes place as soon as the whole of this store has been exhausted. Of the means by which the fatty secretion is taken back again into the current of the circulation, when it is required for use in the system, we know' nothing whatever. 264. In the simpler forms of Cartilage, we have an example of a tissue of remarkable permanence, composed entirely of cells scattered through an intercellular substance. This substance has a close resemblance to Gelatin, in composition and properties; but is not identical with it; and has received the distinguishing appellation of Chondrine, which marks it as the solidifying ingredient of Cartilage. It requires longer boiling than Gelatin for its solution in wTater ; but the solution fixes into a jelly in cooling, and dries by evaporation into a glue that cannot be distinguished from that of gelatin. Chondrine is not precipitated, however, by tannic acid, but, on the other hand, it gives precipitates with acetic acid, alum, acetate of lead, and proto- sulphate of iron, which do not disturb a solution of gelatin. That the Chondrine obtained by boiling cartilage is an actual component of that tissue, and is not a product of the operation, appears from the fact that its elementary composition agrees with that of pure cartilage, when analyzed by combustion. According to Mulder, the propor- tions of the elements are as follows: 32 Carbon, 26 Hydrogen, 4 Nitrogen, 14 Oxygen; with which one-tenth of an equivalent of sul- phur is combined. This formula is deduced from the definite com- pound which Chondrine forms with Chlorine. 265. Now it is a very curious fact, that all the Cartilages of the fcetus,—those which are to be converted into bone, as well as those which are to remain unossified,—are composed of Chondrine; and yet, as soon as the process of ossification commences, the chondrine is replaced by gelatin, which is the sole organic constituent of the intercellular substance of bones. The permanent cartilages, however, still contain only Chondrine; but if accidental bony deposits should take place in them, (as frequently happens in old persons, especially in the cartilages of the ribs,) the Chondrine gives place to Gelatin. There can be little doubt that, in these cases, there is an actual con- version of the Chondrine into Gelatin; but the mode in which this is effected, is not in the least understood. As Chondrine agrees more nearly with Proteine, in its elementary composition, than Gelatin does, it may be surmised that it is a sort of intermediate stage in the conversion of Proteine into Gelatin ; but it must be kept in mind, that no such substance is met with in any other of the gelatinous tissues,—Chondrine being restricted to pure cellular cartilage. Those in which the intercellular substance has the characters of the white fibrous tissue (§ 189), yield gelatin on boiling, in the manner of the 168 CARTILAGE. ligaments and tendons ; whilst those which contain much of the yel- low or elastic tissue, undergo very little change by boiling, and only yield, after several days, a small quantity of an extract which does not form a jelly, but which has the other chemical properties of Chondrine. 266. Besides the organic compounds already described, most Car- tilages contain a certain amount of mineral matter, which forms an ash when they are calcined. This ash contains a large proportion of carbonate and sulphate of soda, together with carbonate of lime, and a small quantity of phosphate of lime; as age advances, the propor- tion of the soluble compounds diminishes, and the phosphate of lime predominates. This is especially the case in the costal cartilages, which almost invariably become converted into a semi-ossified sub- stance, in old persons; and it is remarkable that, even before they have themselves become thus condensed, they are united by ossific matter, when they have undergone fracture. 267. When a pure Cellular Cartilage is examined microscopically, its cells are seen to lie, sometimes singly, and sometimes in clusters of two, three, or four, in cavities excavated in the intercellular sub- stance; and these occur at very variable distances. From the various appearances which may be observed in the same cartilage, at different stages of its growth, it would appear that the component cells multiply by the doubling process already described (§ 248); that they then separate from one another, each of them drawing towards itself (as it were) an envelop of intercellular substance; and that, by the repeti- tion of the same process, the number of cells in the cartilage may be indefinitely multiplied. Various stages of this history are shown in the accompanying figure, which is taken from a section of the cartilaginous branchial ray of the larva or tadpole of the Rana esculenta, or Edible Frog. In the centre of the figure are shown three separate cells, which have evidently been at one time in closer proximity with each other. In one of these cells, the nucleus is seen to be developing two new cells in its interior; and a continua- tion of this process would give rise to the appearance shown at b, where two cells are shown in close contact, being evi- dently the offspring of the same parent. Now if each of these cells in like manner develops two others within itself, a cluster of four will be developed, as shown at a; and after a time, intercellular substance being accumulated around Section of the Branchial cartilage of Tadpole; a, group of four cells, separating from each other; b, pair of cells in apposition; c, c. nuclei of cartiluge-cells; d, cavity containing three cells. CARTiLAGE. 169 each, their walls will separate, and they will acquire the character of distinct cells. It would seem as if, in other cases, one of the first pair of cells develops another pair in its interior, whilst the other (from some unknown cause) does not at once proceed to do so; and thus only three cartilage-cells instead of four are clustered together in the cavity, as seen at d. 268. The primitive cellular organization now described is retained in some Cartilages through the whole duration of their existence. This is the case, for example, in most of the articular cartilages of joints; in the cartilaginous portion of the septum narium, in the car- tilages of the alae and point of the nose, in the semilunar cartilages of the eyelids ; in the cartilages of the larynx, (with the exception of the epiglottis,) the cartilages of the trachea and bronchial tubes, the car- tilages of the ribs, and the ensiform cartilage of the sternum. When partial ossific deposits take place, it is usually in the substance of cellular, rather than in that offibrous cartilage. 269. When the intercellular substance, instead of being homo- geneous, has a fibrous character, the tissue called Fibro- Cartilage is produced ; and this may be either elastic or non-elastic, according as the yellow or the white form of fibrous structure prevails. In some instances, the fibrous structure is so predominant over the cellular, that the tissue has rather the character of a ligament than of a car- tilage. The white fibrous structure is seen in all those cartilages, which unite the bones by synchondrosis, and which are destined not merely to sustain pressure, but also to resist tension. This is the case especially in the substances, which intervene between the verte- brae, and which connect the bones of the pelvis; these in adult Man are destitute of cartilage-corpuscles, except in and near their centres ; but in the lower Vertebrata, and in the early condition of the higher, the fibrous structure is confined to the exterior, and the whole interior is occupied by the ordinary cartilage-corpuscles. The yellow-fibrous or reticulated structure is best seen in the epiglottis, and in the concha of the ear ; in the former of these, scarcely any trace of cartilage-cor- puscles remains; and in the latter, the cellular structure is only to be met with towards the tip. 270. We have seen that the elements of the cellular tissues hitherto described, do not come into direct contact with the blood-vessels. The Epidermic and Epithelial cells are separated from them, by the continuous layer of basement-membrane, which forms the surface of the true skin, the mucous membranes, the glandular follicles pro- duced from them, &c. &c. In like manner, the cells of Adipose tissue are formed within membranous bags ; on the outside of which the blood-vessels form a minute network. The cells of Cartilage are not nourished in anymore direct manner; and are sometimes at a considerable distance from the nearest vessels. It is certain that the substance of the permanent cellular Cartilages is not permeated, in a state of health, even by the minutest nutrient vessels; none such being brought into view under the highest magnifying power. They 170 CELLS CONNECTED TOGETHER.—CARTILAGE. are, however, surrounded by vessels, which form large ampullce or varicose dilatations at their edges, or spread over their surfaces; and it is by the fluid which is drawn from them by the Cartilage-cells, that the latter are nourished. The nutrition of a mass of Cartilage thus seems to bear a strong resemblance to that of the thick fleshy Sea-weeds, which are in like manner composed entirely of cells, with Fig. 38. Vessels situated between the attached synovial membrane, and the articular cartilage, at the point where the ligamemtum teres is inse rted in the head of the os femoris of the human subject, between the third and fourth months of foetal life;—a, the surface of the articular cartilage; 6, the vessels between the articular cartilage and the synovial membrane; c, the surface to which the ligamentum teres was attached; d, the vein; e, the artery. intercellular substance disposed between them in greater or smaller amount. The cells in nearest proximity to the nutrient fluid, draw from it the requisite materials, and transmit these to the cells in the interior of the mass, receiving a fresh supply in their turn from the source in their own neighbourhood. When the Articular or other cellular Cartilages are inflamed, however, we find vessels passing into their substance; but these vessels are formed in an entirely new tissue, which is the product of the inflammatory process, and cannot be said to belong to the Cartilage itself. 271. The temporary Cartilages, which have a like cellular struc- ture, but which are destined to undergo metamorphosis into Bone, are equally destitute of vessels when their mass is small; but if their thickness exceed an eighth of an inch, they are permeated by canals for the transmission of vessels. Still these vessels do not ramify with any minuteness in the tissue ; and they leave large islets, in which the nutritive process must take place on the plan just described. 272. The Fibro-Cartilages, formed as it were by the intermingling of two distinct elementary structures, have a degree of vascularity proportioned to the amount of the fibrous tissue which, they contain ; but these vessels do not penetrate the cellular portions, where such are distinct from the mixed structure. 273. The Cartilaginous tissue appears to be more removed than almost any other in the body, from the general tide of nutritive action. Its properties are simply of a physical character; and they are not impaired for a long time after the death of the tissue, its tendency to CORNEA.—CRYSTALINE LENS. 171 Fig. 39. decomposition being very slight, so long as it is exposed to ordinary temperatures. It is protected by its toughness and elasticity from those mechanical injuries, to which softer or more brittle tissues are liable ; and consequently it has little need of any active power of reparation. It seems doubtful whether, when loss of substance occurs as a result of disease or accident, this is repaired by real cartilaginous tissue. In the process of ulceration, as observed by Mr. J. Goodsir, it appears that the formation of depressions in the surface is due, not so npich to any change originating in the substance of the Cartilage, as to the eroding action of the cells of the false membrane, which is the product of inflammatory action upon its surface ; and it is in this false membrane, that the new vessels are formed, which dip down into nipple-like prolongations of the membrane, that enter corre- sponding hollows excavated in the cartilage. 274. The Cornea of the Eye bears a close resemblance to Cartilage, both in structure and composition ; and it corresponds rather with the cellular than with the fibrous form of that tissue. The cells are not so numerous as those of the articular cartilages; and they are sur- rounded by a plexus of bright fibres, loosely connected together, so as to resemble areolar tissue. Two sets of vessels, a superficial and a deep-seated, surround the margin of the cornea. The former (Fig. 39, a.) belong rather to the Conjunctival mem- brane, which forms the outer layer of the cornea; and they are prolonged to the distance of one-eighth or half a line from its margin, then returning as veins. The latter (b) do not pass into the true Cornea, but terminate in dilatations from which veins arise, just where it becomes con- tinuous with the sclerotic. In diseased conditions of the Cornea, however, both sets of vessels extend themselves through it. Notwithstanding the absence of ves- sels in the healthy condition of the corneal tissue, incised wounds of its substance commonly heal very readily, as is well seen after the operation for Cataract; but there is a danger in carry- ing the incision around a large propor- tion of its margin, lest the tissue should be too much cut off from the supply of nutriment afforded by the am- pullae of the vessels that surround it. 275. The Crystaline Lens of the Eye approaches Cartilage, in its structure and mode of nutrition, more nearly than any other tissue. It may be separated into numerous laminae; which are composed of fibres that lock into one another, by their delicately-toothed margins. Each of these fibres appears to be made up of a series of cells, linearly Nutrient Vessels of the Cornea, a. Superficial vessels belonging to the Conjunctival membrane, and continued over the margin of the Cornea; b. ves- sels of the Sclerotic, returning at the margin of the Cornea. 172 CRYSTALINE AND VITREOUS BODIES.—BONE. arranged, which coalesce at an early period. The lens is not per- meated by blood-vessels; at least after it has been completely formed ; these being confined to the capsule. During the early part of fcetal life, and in inflammatory conditions of the Capsular membrane, both its anterior and its posterior portions are distinctly vascular; but at a later period, only the posterior half of the Capsule has vessels distri- buted upon its surface. It has been shown by optical experiments devised for the purpose, that a moderate vascularity of the posterior capsule does not interfere with distinct vision; whilst if the anterior capsule were traversed by vessels, the picture on the retina would be no longer clear. The substance of which the lens is composed ap- pears to be soluble Albumen, or perhaps more closely resembles the Globulin of the blood. 276. The Vitreous body, which fills the greater part of the globe of the eye, also seems to possess a cellular structure; the cells contain- ing a fluid, which is little else than water holding in solution a small quantity of albumen and saline matter; and the membrane which forms their walls being so pellucid as to be scarcely distinguishable. Indeed, the cellular character of this substance is chiefly inferred from the faet, that when its capsule or enveloping membrane is punctured, even in several places, the contained fluid does not speedily drain away,—as it would do if it were merely contained in the interstices of an areolar tissue. The blood-vessels which traverse the Vitreous body do not send branches into its substance; and it must derive its nutriment from those which are distributed minutely upon its general envelop, and probably also from the large plexiform vessels of the ciliary processes of the Choroid coat. 277. We next proceed to examine the nature of the tissues, which have a cellular structure at their original basis; but which have undergone a metamorphosis in regard to the arrangement of their ele- mentary parts; and which have received an additional consolidation, by the deposition of earthy matter in their substance. These tissues are the Osseous and the Dental,—Bones and Teeth. The structure of both of them is well adapted to demonstrate the distinction between the tissues themselves, and those subsidiary parts, by which they are connected with the rest of the structure. We have seen that Car- tilage is essentially non-vascular; that is, even when it exists in a considerable mass, it is not traversed by vessels, but is nourished by absorption from the fluids contained in the vessels distributed on its exterior. Now every mass of Bone is penetrated by vessels; never- theless these do not penetrate its ultimate substance, and may be easily separated from it, leaving the bone itself as it was. In fact, as Mr. J. Goodsir observes, " a well macerated bone is one of the most easily made, and, at the same time one of the most curious anatomical preparations. It is a perfect example of a texture completely isolated ; the vessels, nerves, membranes, and fat, are all separated, and nothing is left but the non-vascular osseous substance." Precisely the same may be said of the substance of a Tooth, from which the vascular STRUCTURE OF BONE. 173 lining of the pulp-cavity has been removed; for it then possesses neither vessels, nerves, nor lymphatics ; and yet, as we shall presently see, it has a highly-organized structure, peculiar to itself. 278. The general characters of Osseous texture vary according to the shape of the Bone, and the part of it examined. Thus in the long bones, we find the shaft pierced by a central canal, which runs con- tinuously from one extremity to the other; and the hollow cylinder which surrounds this is very compact in its structure. On the other hand, the dilated ends of the bone are not pene- trated by the large central canal; nor are t-hey composed of solid osseous substance. They are made up of cancellated structure, as it is termed ; that is, of osseous lamellae and fibres interwoven together (like those of areolar tissue, on a larger scale) so as to form a multitude of minute cham- bers or cancelli, freely communicating with each other, and with the cavity of the shaft; whilst the whole is capped with a thin layer of solid bone. Again, in the thin flat bones, as the scapula, we find the two surfaces composed of solid osseous texture, with more or less of cancellated structure Extremity of os fe- interposed between the layers. And in the thicker K'.fS^Sn flat bones, as the parietal, frontal, &c, this can- layer of bone, in contact ' r. ' ' j- ,• . i with the articular car- cellated structure becomes very distinct, ana tiiage; &, cancem. forms the diploe; this, however, is sometimes deficient, leaving a cavity analogous to the canal of the long bones; whilst the plates which form the surfaces of the bone (the external and internal tables of the skull), resemble in their thickness and soli- dity, as well as in the intimate structure presently to be described, the shaft or hollow cylinder of those bones. Finally, we frequently meet (especially in the Ethmoid and Sphenoid bones), with thin lamellae of osseous substance, resembling those which elsewhere form the boundaries of the cancelli; these consist of but one layer of bony matter, and show none of the varieties previously adverted to; they are not penetrated by vessels, but are nourished only by their sur- faces ; and they consequently exhibit to us the elements of the osseous structure in their simplest form. It will be desirable, therefore, to commence with the description of these. 279. When a thin natural lamella of this kind is examined, it is found to be chiefly made up of a substance which is apparently homo- geneous, but which may be seen (especially after prolonged boiling) to consist of minute granules, varying in size from l-6000th to l-14000th of an inch ; these are more or less angular in shape, and seem to cohere by the medium of some second substance, which is dissolved by the boiling. They are composed of Calcareous salts, apparently in chemical union with the Gelatin that forms the basis of the osseous substance. In the midst of this granular substance a number of dark spots are to be observed, the form of which is very 174 STRUCTURE OF BONE. Lacunae of Osseous substance, magnified 500 diameters;—o, central cavity; 6, its ramifications. peculiar. In their general outline they are usually somewhat oval; but they send forth numerous radiating prolongations of extreme minuteness, which maybe frequently traced to a considerable dis- tance. These spots, known as the osseous corpuscles, (sometimes termed the Purkinjean corpuscles, after the name of their discoverer,) are highly characteristic of the true bony structure, being never deficient in the minutest parts of the bones of the higher animals, although those of Fishes are frequently destitute of them. These corpuscles were formerly sup- Fig. 4i. posed, from their dark appearance, to be opaque, and to consist of ag- gregations of calcareous matter which would not transmit the light: but it is now quite certain, that they are lacunce or open spaces; and that the radiating pro- longations from them, which are far smaller than the minutest capil- lary vessel, are canaliculi or deli- cate tubes. Of these canaliculi, some may be seen to interlace freely with each other, whilst others proceed towards the surface of the bony lamella; and thus a system of passages, not by any means wide enough to admit the blood-corpuscles, but capable of transmitting the fluid elements of the blood, or matters selected from them, is estab- lished through the whole substance of the lamella. The lacunae have an average length of 1-I800th of an inch ; and they are usually about half as wide, and one-third as thick. The diameter of the canaliculi is from l-12000th to l-20000th of an inch. The succeeding figure represents the arrange- ment of these lacunae and canaliculi in the bony scale of a Fish (the Lepi- dosteus); which is al- most the only existing representative of a large class of bony-scaled Fishes, that formerly tenanted the seas. This subject is selected on account of the peculiar distinctness with which these elementary parts are shown; and the entire absence of any of that more complex arrangement, caused by the penetration of blood- vessels which we shall presently have to describe. 280' The lacunae of the solid osseous texture are not unoccupied, however in the living Bone. They are filled with a minute granular substance; which is probably to be regarded (as first pointed out by Mr J Goodsir) in the light of a germinal spot or nutritive centre, Section of the bony scale of Lepidosteus;—o, showing the regular distribution of the lacunae and of the connecting canali- culi ; b, small portion more highly magnified. STRUCTURE OF BONE. 175 that has the power of drawing to itself, through its own system of canaliculi, the nutritive materials supplied by the blood-vessels on the nearest surface, and of diffusing these through the surrounding substance. Between the blood-vessels and the surface of the bony lamella, however, there is a layer of cells; which are probably the immediate agents in the selection and elaboration of the nutritive matter, and which then deliver it to be taken up by the canaliculi.— Thus the nutrition of the ultimate osseous texture is carried on upon the same plan with that of Cartilage; being effected by the imbibition of nutrient matter from the surface, through the agency of cells. But it differs in this;—that there is a provision in Bone for the ready transmission of nutrient matter through its texture, by means of minute channels, which does not exist in Cartilage;—a difference obviously required by the greater solidity of the substance of the former, which does not allow of the diffused imbibition, that is permitted by the softer and moister nature of the latter. We shall presently find that they are only formed at a late stage of the development of bone, when the remaining tissue has acquired its completest consolidation. 281. Now, as already remarked, the simple structure just described is found, not merely in the delicate plates which form the thinnest part of certain bones in Man; but also in those lamellae, which form the walls of the cancelli of the larger and thicker bones. Every one of these lamellae repeats, in fact, the same history. The cancelli are lined by a membrane derived from that of the cavity of the shaft, over which blood-vessels are minutely distributed; between these blood-vessels and the osseous texture, is a layer of cells; and from the materials selected and communicated by these, each lamella is nourished, through its system of radiating canaliculi and nutritive centres. The cancelli, at the time of their formation in the fcetal bone, are entirely filled with such cells; which appear (as will be presently explained) to be the descendants of the cells of the original cartilage ; but in the adult bone, a large proportion of them is filled with fatty matter, which they secrete into their cavities.—The vessels of the cancellated structure at the extremities of the long bones, are derived from those of the medullary cavity, which is penetrated by large trunks from the exterior; and in the flat bones, they form a system of their own, connected with the vessels of the exterior by several smaller trunks. 282. The solid osseous texture, which forms the cylindrical shafts of the long bones, and the thick external plates of the denser flat bones, is not cut off from nutritive action in the degree in which it might seem to be; for it is penetrated by a series of large canals, termed the Haversian, (after Clopton Havers, their discoverer,) which form a network in its interior, and which serve for the trans- mission of blood-vessels into its substance. These canals, in the long bones, run for the most part in a direction parallel to the central cavity; and they communicate with this, with the external surface, with the cancelli, and with each other, by frequent transverse 176 STRUCTURE OF BONE. branches; so that the whole system forms an irregular network, per- vading every part of the solid texture, and adapted for the establish- ment of vascular communications through- out. The diameter of the Haversian canals varies from 1-2500th to l-200th of an inch, or more; the smallest being only of sufficient size to admit the passage of a single capil- lary vessel; whilst the largest receives a plexus of minute blood-vessels. Their ave- rage diameter maybe stated at about l-500th of an inch. They are lined by a membrane, which is continuous with that of the external surface, and which carries this inwards (so to speak) to form the lining membrane of the central cavity, and of the cancelli. On this membrane, a plexus of blood-vessels is dis- tributed, where the size of the canal admits it;—otherwise, the tube encloses a single twig of an artery or vein. Thus we may consider the whole Osseous texture as in- closed in a membranous bag; on which blood-vessels are minutely distributed ; and which is so carried into the bone by invo- lutions and prolongations, that no part of the latter is ever far removed from a vascu- lar surface. 283. Between the vascular lining of the Haversian canals, and their bony walls, there is a layer of cells ; as in the corresponding situation in the cancelli: so that it may be stated as a general fact, that these everywhere intervene between the blood-vessels and the osseous substance. In the adult bone, the cells which fill the remain- ing cavity of these canals secrete fatty matter; this is particularly evident in the case of the central cavity, where they constitute the medulla or marrow. It does not appear that these take any active part in the nutrition of the bone ; indeed in the bones of Birds, the shaft is entirely hollow, and air is admitted into it from the lungs, so that its lining membrane is rendered subservient to the aeration of the blood. 284. The arrangement of the elementary parts of the osseous sub- stance around the Haversian canals, is very interesting and beautiful. When a transverse section of a long bone is made, the open orifices of the longitudinal canals present themselves at intervals, sometimes connected by a transverse canal, where the section happens to traverse this. Around these orifices, we see the osseous matter arranged in the form of cylinders, which appears to be marked by concentric circles. Now when one of these circles is minutely examined, it is found to be made up of a series of lacunae, analogous to those already described • these, however, are seldom or never so continuous as to Haversian Canal, seen on a longitudinal section of the com- pact tissue of the shaft of one of the long bones; 1, arterial canal; 2, venous canal; 3, dilatation of another venous canal. STRUCTURE OF BONE. 177 form a complete circle. The long sides of the lacunae are directed, the one towards the Haversian canal in the centre, the other towards the circular row next beyond it. And when the course of the cana- liculi is traced, it is found that these converge on the inner side towards the central canal, inosculating with those of the series next within, whilst those of the .outer side pass outwards in a radiating or diverging direction, to inosculate with those of the series next ex- ternal. Thus a complete communication is formed, by means of this system of radiating canaliculi, and intervening lacunae, between the central canal, and the outermost cylindrical lamella of bony matter; and each of these lamellae derives its nourishment from the vessels of the central canal, through the lamellae which intervene between itself and the vascular membrane lining that tube. 285. Thus every one of the Haversian canals is the centre of a cylindrical ossicle; which is complete in itself, as far as its elementary structure is concerned ; and which has no dependence on, or con- nection with, other similar ossicles. These are arranged, however, side by side, like sticks in a faggot; they are bound together by a thin cylinder of bone, on the exterior of all, which derives its nourish- ment from the periosteum or enveloping membrane; in like manner, the hollow bundle is lined by a similar cylinder, which surrounds the great medullary cavity, and is nourished by its vascular membrane; and the spaces that here and there intervene between the ossicles, are filled up with laminae, which are parallel to those of the external and internal cylinders, and which seem to derive their nutriment from them (Fig. 44, 4). In this manner, the whole structure acquires great Fig. 44. Minute structure of bone, drawn with' the microscope from nature. Magnified 300 diameters. 1. One of the Haversian canals surrounded by its concentric la- mellae. The corpuscles are seen between, the lamellae; but the calcigerous tubuli are omitted. 2. An Haversian canal with ita concentric lamellae, Purkinjean corpuscle* and tubuli. 3. The area of one of the canals. 4, 4. Direction of the lamellae of the great medullary canal. Between the lamellae at the upper part of the figure, several very long corpuscles with their tubuli are seen. In the lower part of the figure, the outlines of three other canals are given, in order to show their form and mode of arrangement in the entire bone. density and solidity.—The structure of the outer and inner tables of the skull, and of other thick solid layers of bone, is precisely similar; 12 178 STRUCTURE AND COMPOSITION OF BONE. except that the Haversian canals have no such definite directions, and form an irregular network. 286. Thus we see that each of the lamellae of bone, surrounding an Haversian canal, or bounding the cancelli, may be regarded as a repetition of the simple bony plate, which draws its nourishment direct from the vascular membrane covering its surface, by means of its system of lacunae and canaliculi. The membrane lining the Haversian canals, cancelli, and central medullary cavity, is an internal prolongation of that which clothes the exterior;—just as the mucous membranes, with their extensions into glandular structures, are in- ternal prolongations of the true skin. Every Haversian canal and every cancellus are repetitions of each other in all essential particulars ; their form alone being different. The central medullary canal is but an enlarged Haversian canal or cancellus. And the whole cylin- drical shaft is a collection of ossicles, each of which is a miniature representation of itself, being a hollow cylinder, with a central vas- cular cavity. 287. The principal features of the Chemical constitution of bone are easily made evident. After all the accessory parts have been removed, and nothing remains but the real osseous texture, this may be separated, by simple processes, into its two grand constituents,— the animal basis, and the calcareous matter. The latter may be entirely dissolved away, by maceration of the bone in dilute Muriatic ©r Nitric acid; and a substance of cartilaginous appearance is then left, which, when submitted to the action of boiling water for a short time, is almost entirely dissolved away, and the solution forms a dense jelly on cooling. The same substance, Gelatin, may be obtained by long boiling under pressure, from previously unaltered bone ; and the calcareous matter is then left in a friable condition. By submitting a bone to a heat sufficient to decompose the animal matter, without dissipating any of the earthy particles, we may obtain the whole calcareous matter in situ; but the slightest violence is sufficient to disintegrate it. The bones of persons long buried are often found in this condition; their form and position being retained, until they are exposed to the air, or are a little shaken ; when they crumble to dust. —The proportion of the earthy matter of Bones to the animal basis may be differently stated ; according as we include in our estimate of the latter, the contents of the medullary cavity, the Haversian canals, and the cancelli; or confine ourselves to that portion only of the ani- mal matter, which is united with the calcareous element in the proper osseous tissue. According to the recent experiments of Dr. Stark,* the relative amount of the two elements, in the latter estimate, is subject to very little variation, either in the different classes of ani- mals or in the same species at different ages; the animal matter composing about one-third, or 331 per cent.; and the mineral matter two-thirds, or 66| per cent. The degree of hardness of bone does * Edinburgh Medical and Surgical Journal, April, 1845. COMPOSITION OF BONE. 179 not altogether depend, therefore, on the amount of earthy matter they may contain; for the flexible, semi-transparent, easily-divided bones of Fish contain as large an amount of animal matter, as the ivory-like leg-bones of the deer or sheep. The usual analyses of Bone, how- ever, have been made upon the former kind of estimate; and they show that the proportion of the earthy matter to the whole of the animal substance contained in bone, varies much in different animals, in the same animal at different ages, and even in different parts of the same skeleton. The reason of this wrill be apparent, when the history of the growth of Bone has been explained ; since there is a gradual filling-up of all the cavities at first occupied by fat-cells, vessels, &c, which does not cease with adult age, but which continues during the whole of life. In this manner, the bones of old persons acquire a high degree of solidity, but they become brittle in proportion to their hardness. From the same cause, the more solid bones contain a larger proportion of bone-earth than those of a spongy or cancellated texture; the temporal bone, for example, containing 63^ per cent., whilst the scapula possesses only 54 per cent. In the former of these bones, the proportion is nearly the same as that which exists in pure osseous tissue, the amount of the remaining tissues which it includes being very small, on account of the solidity of the bone: but the latter contains in its cancelli a large quantity of blood-vessels, fat-cells, &c, which swell the proportion of the animal matter from 33^ to 46 per cent. 288. The Lime of bones is, for the most part, in a state of Phos- phate, especially among the higher animals ; the remainder is a Car- bonate. In Human bones, the proportion of the latter seems to be about one-sixth or one-seventh of the whole amount of bone-earth. In the bones of the lower animals, however, the proportion of Car- bonate is greater ; and it is curious that in callus, exostosis, and other irregular osseous formations in the higher animals, the proportion of the Carbonate should be much greater than in the sound bone. In caries, however, the proportion of the Carbonate is less than usual. The composition of the Phosphate of Lime in Bones, is somewhat peculiar; eight proportions of the base being united with three of the acid. According to Professor Graham, it is to be regarded as a com- pound of two tribasic phosphates; one atom of the neutral phosphate (in which one proportional of the acid is united with two of lime and one of water), being united with two proportionals of the alkaline phosphate, (in which one part of acid is united wTith three of the base,) together with an atom of water, which is driven off by calcination. Besides these components, some Chemists assert that a small quantity of Fluoride of calcium is present in Bone ; but this is rather doubtful, since it has been shown by Dr. G. 0. Rees that the solvent action upon glass, which has been supposed to be characteristic of fluoric acid, may be imitated by phosphoric acid in combination with water, which, if heated upon glass of inferior quality until it volatilizes, will act upon it with considerable energy.—Other saline matters, such as 180 SKELETONS OF INVERTEBRATA. phosphate of magnesia, oxides of iron and manganese, and chloride of sodium, are found in bones in small amount. 289. The purpose of Bone in the Animal economy is obviously mechanical, and that only; its use being, to afford support and pro- tection to the softer textures, and to form inflexible levers, by the action of the muscles upon which, motion may be given to the dif- ferent parts of the fabric. A slight comparison of the characters and offices of the Bones of Vertebrated Animals, with the structures which form the solid skeletons of the lower classes, will afford many points of interest; and will aid in the comprehension of the purpose of the highly-elaborate structure we have been considering. Commencing with the Polypifera, or Coral-forming animals, we observe that their strong axes or sheaths are destined only to give support to their softer structures, and that the parts once consolidated undergo no subse- quent change. It was formerly imagined, that the stony Corals were built up by the animals which form them, somewhat in the same manner as a Bee constructs its cell. But it is now fully demonstrated, that the calcareous matter (which here consists solely of the Carbonate of Lime) is deposited in the cells of the living tissue, by a secreting action of their own ; and that the most solid mass of Coral thus has an organized basis, as complete as that of Bone. The proportion of earthy to animal matter, however, is so great in the former structures, that very little, if any, nutrient changes can take place in their tissue, when once it has become consolidated. Such changes are not, how- ever, required. The substance thus developed by the wonderful secreting powers of the lime-secreting cells, which draw into them- selves the small quantity of calcareous matter dissolved in the sur- rounding water, is so little disposed to undergo change, that it will maintain its solidity for centuries; and even when acted on by water or by heat, it does not undergo disintegration, for its calcareous par- ticles arrange themselves in a new method, and become converted into a solid crystaline rock. Such rocks, the product of the meta- morphosis of ancient coral-formations, make up a large proportion of the external crust of the earth. The solid stem or sheath, once consolidated, appears to undergo no further change in the living Coral-structure ; for its increase takes place, not by interstitial but by superficial deposit,—that is, not by the diffusion of new matter through its whole substance, separating the parts formerly deposited from each other, but by the mere addition of particles to its surface and extremities. In this manner the growth of a solid Coral-structure may go on to an enormous extent; the surface at which the consoli- dating action is going on, being the only part alive, that is exhibiting any vital change; and all the rest of the mass being henceforth per- fectly inert. 290. In the class of Echinodermata, which includes the Star-fish, Sea-Urchin, &c, we find the calcareous structure presenting a very elaborate organization ; as an example of this, we shall select the shell of the Echinus, commonly known as the Sea-Egg. This shell is made SHELL OF ECHINODERMATA. 181 up of a number of plates, more or less regularly hexagonal, and fitted together so as completely to enclose the animal, except at two points, one of which is left open for the mouth, the other for the anus. On the surface of these plates are little tubercles, for the articulation of the spines, which serve as instruments of defence and of locomotion. The substance of the shell and of the spines is exactly alike; being a sort of areolar tissue, consolidated by the deposition of calcareous matter, and having an innumerable series of interspaces or minute cancelli, freely communicating with each other. The arrangement of this calcareous network in the spines, is most varied and elaborate; and causes thin sections of them to be among the most beautiful of all microscopic objects. The external and internal surface of each plate, in the shell of the living Echinus, is covered with a membrane, Fig. 45. Portion of the shell of the Echinus, showing at a the constituent plates with the calcified areolar tissue, of which they are composed, at b. from which its nutrition is derived; this membrane dips down into the spaces between the adjacent plates; but it does not penetrate the substance of the plates themselves, nor does it transmit vessels to their interior. A similar membrane covers and encircles the spines; and it also connects these with the shell, being continuous with the mem- brane that envelops the latter. Thus each plate and spine is itself completely extra-vascular; but it is enclosed in a soft membrane, which furnishes (whether by vessels or otherwise, has not yet been ascer- tained), the elements of its nutrition. 291. But we do not here find any evidence of interstitial growth; nor is there any reason why such should be required. For the tissue of which it is composed, although of such extreme delicacy, is of great permanence, and does not exhibit the slightest tendency to decay, however long it is preserved; so that, when once consolidated, it appears to undergo no further change in the living animal. The growth of the animal, however, requires a corresponding enlargement of its enveloping shell; and this is provided for by the simple process of superficial deposit, through the subdivision of the whole shell into component plates. For by the addition of new matter at the edge of 182 SHELLS OF MOLLUSCA. each plate, by the consolidation of a portion of the soft membrane that intervenes between the adjacent plates, the whole shell is enlarged, without losing its globular form. At the same time it is strengthened in a corresponding degree, by the consolidation of the soft tissue at the surface of each plate. And, in like manner, the spines are en- larged and lengthened by the progressive formation of new layers, each on the exterior of the preceding; so that a transverse section exhibits a number of concentric rings, like those of an Exogenous tree.—Thus even in the growth of this complex and elaborate struc- ture, we recognize the principle of superficial deposit, which we shall find to be universal amongst the hard parts of the Invertebrata; not- withstanding that, at first sight, it would have appeared impossible to provide on this plan for the gradual enlargement of a globular shell, completely enclosing the animal, and therefore required to keep pace with the latter in its rate of increase. 292. Among the Mollusca, we find the body sometimes altogether destitute of solid organs of support, protection, or locomotion,—as is the case, for example, in the Slug; and the movements are feeble and the habits inert, the muscle having no fixed points for their attach- ment, and acting without any of the advantages of leverage. In other cases, we find the body more or less completely protected by a Shell; which is sufficiently large in some instances to cover the body com- pletely ; whilst in others, it affords only a partial investment. The plan on which this shell is formed, however, is very different %om that which has just been described; being much less complex. The Univalve shells, or those formed in one piece, are always of a conical form ; the cone being sometimes simple, as in the Limpit; in other cases being spirally coiled, as in the Snail. Now the base of this cone is open ; and through this, the animal can project its movable parts. When its increasing size requires additional accommodation, it is obvious that an addition to the large end of the cone will increase its diameter and its length at the same time; so as to afford the re- quired space, without any alteration in the form or dimensions of the older and smaller portions of the cone. This last, indeed, is fre- quently quitted by the animal, and remains empty; being sometimes separated from the latter portions, by one or more partitions thrown across by the animal,—as is seen especially in the Nautilus and other chambered shells.* Besides the new matter added to the mouth of the shell, a thin layer is usually formed over its whole interior sur- face; so that the lining of the new part is continuous with that of the old.—In the Bivalve shells, we trace this mode of increase without any difficulty; especially in such shells as that of the Oyster, in which the successive laminae remain distinct. Each lamina is interior to the preceding, being formed on the living surface of the animal; but it also projects beyond it, so as to enlarge the capacity of the shell; and as the separation of the valves affords free exit to those parts of the animal, which are capable of being projected beyond the shell, there is obviously no need of any other provision to maintain the shell in SHELLS OF MOLLUSCA. 183 its natural form.—Thus in the shells of the Mollusca, increase takes place at the surfaces and edges only. 293. The proportion of organic and calcareous matter in Shell differs considerably in the various tribes. The former is sometimes present in such small amount, that it can scarcely be detected; and the condition of the calcareous matter then obviously approaches that of a crystaline deposit. But in other instances, the animal basis is very obvious; remaining as a thick consistent membrane, after all the calcareous matter has been dissolved away by an acid. This mem- brane is formed of an aggregation of cells arranged with great regu- larity (Fig. 46, a;) the cavities of which are filled with carbonate of Fig. 46. Prismatic cellular structure of shell of Pinna; a, surface of lamina; 6, vertical section. lime in a crystaline state. The form of the cells approaches the hexagonal; their diameter varies in different shells from 1-I00th to l-2800th of an inch; their thickness also is extremely variable, even in different parts of the same shell. Thus we sometimes meet with a lamina of such tenuity, as not to measure l-100th of an inch in thick- ness; whilst in other instances, a single layer may have a thickness of half an inch, or even (in certain large fossil species) of an inch or more. In this case, the cells, instead of being thin flat scales like the pavement-epithelium (§ 233), are long prisms, somewhat like the cells of the cylinder-epithelium (Fig. 22), with their walls flattened against each other. The appearance which is then presented by a vertical section of them, is represented in Fig. 46, b. The long prismatic cells are there seen to be marked by delicate transverse striae; and these, taken in connection with other indications, appear to show, that every such prism is in reality formed by the coalescence of a pile of flat cells, resembling those which are seen in the very thin laminae just described ; so that the thickness of the layer depends upon the number of the cellular laminae, which have coalesced to form its component prisms. This character is of interest, as representing on a magnified scale a corresponding appearance in the enamel of human 184 SKELETON OF ARTICULATA. Tooth, which we shall presently find to be formed upon the very same plan. 294. We are to regard this kind of shell-substance, therefore, as formed by the secreting action of the epithelial-cells covering the mantle of the animal,—which membrane, though it answers in posi- tion to the skin, has the soft, spongy, glandular character of a mucous membrane. These draw calcareous matter into their cavities, as a part of their own process of growth; this matter being supplied from the fluids of the vascular surface beneath. Now when these cal- cigerous cells are separated by intercellular substance, they remain distinct through the whole of their lives, and they form by their cohesion a tenacious membrane, that retains its consistency after the removal of the calcareous matter. But this is only the case in certain groups of shells, chiefly belonging to the bivalve division. When the inter- cellular substance is wanting, and the cells come into close contact, their partitions become indistinct on account of their extreme tenuity; and not unfrequently a fusion of the whole substance appears to take place, by the dissolution of the original cell-walls, so that it becomes more or less homogeneous,—traces of the original cellular structure being here and there distinguishable (§ 255). 295. Sometimes where this fusion has taken place, so as to oblite- rate the original cell-structure, we find the almost homogeneous sub- stance traversed by a series of tubuli, not arranged, however, in any very definite direction, but forming an irregular network. These tubes vary in size from l-2000th to l-20,000th of an inch ; but their general diameter, in the shells in which they most abound, is l-4500th of an inch. In the larger tubuli, something of a bead-like structure may occa- sionally be seen ; as if their in- terior were occupied by rounded granules arranged in a linear t„k„io, d,„n .♦ . f a • direction. Although it might be 1 ubular shell-structure, from Anomia. o o supposed that this structure is destined to convey nutrient fluid into the substance of the shell, yet there is no evidence that such is the fact; and, on the contrary, there is ample evidence, that, even in shells most copiously traversed by these tubuli, no processes of interstitial growth or renewal take place. The permanent character of the substance of all Shells, when once it is fully formed, is as remarkable as that of Coral; and as the adapta- tion of their size, to that of the animals to which they belong, is en- tirely effected by additions to their surfaces and edges, no interstitial deposit can have a share in producing it. 296. Among the Articulated classes, we still find that the skeleton is altogether external, and belongs therefore to the cutaneous system ; but it is formed upon a very different plan from the shells of the Mol- MOULTING.—SHELLS OF CRUSTACEA. 185 lusca, being closely fitted to the body, and enveloping every part of it; consequently it must increase in capacity, with the advancing,^ growth of the contained structures. / Moreover it is destined not merely to afford support and protection to these, but to serve for the attachment of the muscles by wThich the body and limbs are moved; and the hard envelops of the latter serve, like the bones of the Verte- brata, as levers by which the motor powers of the muscles are more advantageously employed. Again, the hard envelops of the body and limbs are not formed of distinct plates, like those of the Echinus-shell, but are only divided by sutures at the joints, for the purpose of per- mitting the requisite freedom of motion. It might have been thought that here, if anywhere, a process of interstitial growth would have existed, to adapt the capacity of the envelops to the dimensions of the contained parts, as the latter increase with the growth of the animal; but, true to the general principle, that epidermic structures are not only extra-vascular, but that they undergo no change when they are once fully formed, we find that the hard envelops of Articu- lated animals are thrown off, or exuviated, when the contained parts require an increase of room ; and that a new covering is formed from their surface, adapted to their enlarged dimensions. 297. This is well known to occur at certain intervals in Crabs, Lobsters, and other Crustacea; which thus exuviate not merely the outer shell, with the continuation of the epidermis over the eyes, but also its internal reflexion, which forms the lining of the oesophagus and stomach, and the tendinous plates by which the muscles are attached to the lining of the shell. A similar moulting may be ob- served to occur in some of the minute Entomostracous Crustacea of our pools, every two or three days, even after the animals seem to be full grown. During the early growth of Insects, Spiders, Centipedes, &c, a similar moult is frequently repeated at short intervals ; but after these animals have attained their full growth, which is the case with Insects at their last change, no further moulting takes place, the necessity for it having ceased.—This moulting is precisely analogous to the exfoliation and new formation of the Epidermis, in Man and most other Vertebrata; differing from it only in this, that the latter is constantly taking place to a small extent, whilst the former is com- pletely effected at certain intervals, and then ceases. We have ex- amples of a periodical complete moult in Vertebrata, however, among Serpents and Frogs. 298. The structure of the hard envelops of Articulated animals corresponds writh that of the Epidermis and its appendages in Man. The firm casings of Beetles, for example, are formed of layers of epidermic cells, united together, and having their cavities filled by a horny secretion. The densest structure is found in the calcareous shells of the Crustacea ; which consist of a substance precisely analo- gous to the Dentine of Teeth (§311); covered on the exterior with a layer of pigment-cells. The calcareous matter consist chiefly of car- bonate of lime; but traces of the phosphate are also found. The 186 BONE OF VERTEBRATA. animal basis has a firm consistent structure, resembling that of teeth. A thin vertical section shows the tubuli running nearly parallel, but with occasional undulations, from the internal surface towards the external; but no traces of the original calcigerous cells can be detected in the fully-formed shell, the process of fusion having gone so far as to obliterate them. The manner in which these tubuli are formed, will be presently considered, under the head of Dental substance. 299. Now the condition of the osseous skeleton of Vertebrated animals is altogether different. It forms a part of the internal sub- stance of their bodies; and as these grow in every part, and not merely by addition to this or that portion, so must the Bones also, in order to keep pace with the rest of the structure. Hence we find them so formed, that the processes of interstitial deposition may be continually going on in their fabric, as in that of the softer tissues ; and the changes in their substance do not cease, even when they have acquired their full size. The continuance of these changes appears destined, not so much to repair any waste occasioned by decomposi- tion,—for this must be very trifling in a tissue of such solidity,—as to keep the fabric in a condition, in which it may repair the injuries in its substance occasioned by accident or disease. The degree of this reparative power is proportional, as we shall presently see, to the activity of the normal changes, which are continually taking place in the bone ; and is thus much greater in youth than in middle life, and greater in the vigour of manhood than in old age. 300. We shall commence the history of the development of Bone, with the period in which its condition resembles that of the permanent Cartilages. As already mentioned, there is no essential difference between the temporary and permanent Cartilages, in regard to their ultimate structure ; the former, however, are more commonly traversed by vessels, especially when their mass is considerable. These ves- sels, however, do not pass at once from the exterior of the cartilage into its substance; but they are conveyed inwards along canals, which are lined by an extension of the perichondrium or investing mem- brane, and which may thus be regarded as so many involutions of the outer surface of the cartilage. These canals are especially de- veloped at certain points, which are to be the centres of the ossifying process ; of these puncta ossificationis, we usually find one in the centre of the shaft of a long bone, and one in each of its epiphyses; in the flat bones there is one in the middle of the surface, and one in each of the principal processes. Up to a late stage of the ossifying process, the parts which contain distinct centres are not connected by bony union, so that they fall apart by maceration ; and even when they should normally unite, they sometimes remain separate,—as in the case of the Frontal bone, in which we frequently meet with a continuation of the sagittal suture down the middle, dividing it into two equal halves, which have originated in two distinct centres of ossification. It is interesting to remark that, in the two lowest classes of Vertebrata,—Fishes and Reptiles,—we find the several parts of CONVERSION OF CARTILAGE INTO BONE. 187 the osseous system presenting, in a permanent form, many of the conditions which are transitory in the higher; thus the different por- tions of each vertebra, the body, lateral arches, spinous and transverse processes, &c, which have their distinct centres of ossification, but which early unite in Man, remain permanently distinct in the lower Fishes ; the division of the frontal bone, just adverted to, is constant amongst Reptiles; and in that class we meet with a permanent sepa- ration of the parts of the occipital and temporal bones, which, being formed from distinct centres of ossification, are at first distinct in the higher animals. 301. During the formation of the punctum ossificationis, and the spread of the vessels into the cartilaginous matrix, certain changes are taking place in the substance of the latter, preparatory to its con- version into bone. Instead of single isolated cells, or groups of two, three, or four, such as we have seen to be characteristic of ordinary Cartilage, (§ 267,) we find, as we approach the ossifying centre, clusters made up of a larger number, wThich appear to be formed by a continuance of the same multiplying process as Tls- 48- that already described. And when we pass still nearer we see that these clusters are composed of a yet greater number of cells, which are ar- ranged in long rows, whose direction corre- sponds with the longitu- dinal axis of the bone ; these clusters are still separated by intercellu- lar substance, and it is in this, that the ossific matter is first deposited. Section of Cartilage, near the seat of ossification; each single cell having given birth to four, five or six cells, which form clus- ters. These clusters become larger towards the right of the figure, and their cells more numerous and larger; their long di- ameter being l-1500th of an inch. Thus if we separate Fig. 49. The same cartilage at the seat of ossification; the clusters of cells are arranged in columns; the in- tercellular spaces between the columns being l-3250th of an inch in breadth. To the right of the figure, osseous fibres are seen occupying the intercellular spaces at first bounding the clusters laterally, then splitting them longitudinally and encircling each separate cell. The greater opacity of the right hand border is due to a threefold cause, the increase of osseous fibres, the opacity of the contents of the cells, and the multiplication of oil-globules. 188 PRODUCTION AND GROWTH OF BONE. the cartilaginous and the osseous substance at this period, we find that the ends of the rows of cartilage-cells are received into deep narrowT cups of bone, formed by the transformation of the intercellu- lar substance between them. Immediately upon the ossifying surface, the nuclei, which were before closely compressed, separate considera- bly from one another, by the increase of material within the cells; and the nuclei themselves become larger and more transparent. These changes constitute the first stage of the process of ossification, which extends only to the calcification of the intercellular substance; in this stage there are no blood-vessels directly concerned. The bony lamellae thus formed, mark out the boundaries of the cancelli and Haversian canals; which are afterwards to occupy a part of the space that is hitherto filled by the rows of cartilage-corpuscles. 302. The second stage of the ossifying process consists in the fur- ther transformation of the original cartilage-cells. These seem to be- come flattened against the osseous layers already formed, and then to become themselves consolidated by the secretion of calcareous mat- ter into their interior,—at the same time coalescing to such a degree, that the original boundaries of the cells can no longer be traced. The consolidation, however, does not extend to the nuclei of the cells; which retain their granular condition, and, being surrounded by calcareous matter, are enclosed in cavities which take their own shape. These cavities are the subsequent lacunce of the bony struc- ture ; and the branching canaliculi proceeding from them become more and more distinct, as the consolidation of the surrounding struc- ture is completed. By the continuation of this process, one layer of cells after another is converted into bony matter; and the canals, which at first occupied the whole diameter of the cylindrical ossicles shown in Fig. 44, become gradually contracted by these deposits upon their walls. The cause of the concentric lamination of the osseous matter in each of the ossicles, of which the permanent Haversian canal is the centre, is thus apparent. 303. As the calcification of the original cartilage-cell goes on, a new substance appears in the cavities of the cancelli and canals; this is a cellular mass resembling that in which all new tissues origi- nate ; and it seems to be from this, that the vascular lining is formed, which gradually extends itself into the cancelli and canals, and which is to become from henceforth the principal source of the growth and nutrition of Bone. From the central part of this blastema, the fat- cells, constituting the medulla, must be generated. But, as already stated (§ 283), a layer of cells resembling the originals constantly intervenes between the bony walls of the canals and cancelli, and their vascular lining; apparently for the purpose of serving as the immediate agent in the nutrition of the osseous tissue. 304. When the complete Ossification of the temporary Cartilage has thus been effected, the Bone has still to be enlarged, in conformity with the increasing size of the surrounding parts; and this enlarge- ment is due in part to superficial, in part to interstitial addition. The GROWTH OF BONE. 189 superficial addition is due to the progressive formation and conversion of new cartilage at the edges and surfaces of the bone, or at the im- perfectly-consolidated part that intervenes between the separately- ossified portions of the same bone. Thus it was long since proved by the experiments of Hales and Hunter, that the growth of a long bone takes place chiefly towards the extremities ; for they found that, when metallic substances were inserted in the shaft of a growing bone of a young animal, the distance between them was but little altered after a long interval, whilst space between the extremities of the bone had greatly increased. And it seems that, at a later period, when the epiphyses have become completely united to the shaft, an elongation continues to take place, by the slow ossification of the articular carti- lage.—Again, the bone is progressively increased in thickness, by the gradual production of new osseous matter upon its surface; this production taking place exactly upon the same plan with the original process, and involving the formation of new Haversian canals and concentric lamellae, so that no distinction can be traced between the new and the older layers. 305. If this were the whole history of the growth of Bone, there would be no essential difference between the character of its nutri- tion, and that of the skeletons of the Invertebrata. But it is unques- tionable, that bone is also susceptible of an interstitial change, though this is of a slow and gradual nature. The layers first deposited on the inner surface of the early cancelli, are pushed outwards by the succeeding ones, and gradually acquire an increased diameter; so that the ultimate dimensions of the cancelli and cylindrical ossicles far exceed those of the primitive cavities marked out by the calcifi- cation of the intercellular substance ; and this last, also, must be greatly extended to permit such an increase. This process could not go on beyond a certain point, however, without removing the outer laminae too far from the vascular lining of the Haversian canals ; and we consequently find that the increase of the bone takes place by superficial addition, wherever this is admissible.—A very remarkable change takes place in the interior of the long bones of young animals, for the production of the central medullary cavity. At an early pe- riod, no such cavity exists, and its place is occupied by small can- celli ; this is the permanent condition of the bones in most Reptiles. The cancelli gradually enlarge, however; and those within the shaft coalesce with one another until a continuous tube is formed, around which the cancelli are large, open, and irregular. At the same time, the diameter of the surrounding shaft is increasing by the process of interstitial growth just described ; so that the size of the medullary cavity at last becomes greater than that of the whole shaft when its formation commenced. The aggregation of the osseous matter in a hollow cylinder, instead of a solid one, is the form most favourable to strength, as may be easily proved upon mechanical principles. The same arrangement is adopted in the arts, wherever it is desired to obtain the greatest strength with a limited amount of material. 190 GROWTH AND REGENERATION OF BONE. 306. The difference in the relations of the Osseous substance to the vascular network, at different ages,—accounting for the varia- tions in the rapidity of its nutrition and reparation,—is well displayed in the effects of Madder. This substance has a peculiar affinity for Phosphate of Lime; so that when the latter is formed by precipita- tion in a fluid tinged with madder, it attracts colour to it in its de- scent, and falls to the bottom richly tinted. Now when animals are fed with this substance, it is found that their bones become tinged with it; the period required being in the inverse proportion to their age. Thus in a very young animal a single day suffices to colour the entire skeleton, for in them there is no osseous matter far from the vascular surfaces ; when sections are made, however, of the bones thus tinged, it is found that the colour is confined to the immediate neighbourhood of the Haversian canals, each of which is encircled by a crimson ring. In full-grown animals, the bones are very slowly tinged ; because the osseous texture is much more consolidated and less permeable to fluid than in earlier life ; and because, owing to the formation of new concentric lamellae, the outer and older ones are pushed to a greater distance from the Haversian canals, the di- ameter of which is contracted. In the bones of half-grown animals, a part of the bone is nearly in the perfect condition, while a part is new and easily coloured ; so that the action of this substance enables us to distinguish the new from the old. 307. The Regeneration of Bone, after loss of its substance by dis- ease or injury is extremely complete ; in fact there is no other struc- ture of so complex a nature, which is capable of being so thoroughly repaired. Although the regenerative power appears to be so much less in Vertebrated animals, than it is in the lower Invertebrata, yet it is probably not at all lower in reality,—the new structures actually formed being as complex in the one case as in the other. It is no- where, perhaps, more remarkably manifested, than in the reformation of nearly an entire bone, when the original one has been lost by dis- ease ; all the attachments of muscles and ligaments, as well as the external form and internal structure, being ultimately found as com- plete in the new bone, as they originally were in that which it has replaced. Much discussion has taken place in regard to the degree, in which the different membranous structures, that surround bone and penetrate its substance, participate in its regeneration ; some having supposed the periosteum to have the power of itself forming new bone, others attributing the same power to the membrane lining the medullary cavities. It appears next to certain, however, that new osseous tissue can only be formed in continuity with that which pre- viously existed ; and that it may be generated, by the mediation of even very minute fragments of such tissue, from any of the vascular membranes that happen to supply it. Thus when the portion of the shaft of a bone is entirely removed, but the periosteum is left, the space is filled up with new bony matter in the course of a few weeks; though, if the periosteum be also removed, the formation of new mat- REPARATION OF BONE. 191 ter will be confined to a small addition in a conical form to the two extremities, a large interspace intervening between them. This expe- riment might seem to indicate, that the periosteum itself forms the bone ; but the real production of new tissue,—as in cases where the periosteum has been detached by disease, and remains alive whilst the shaft dies,—is in continuity with minute spicula of the original bone, which still adhere to the periosteum.* Again, we find that in comminuted fractures, every portion of the shattered bone that remains connected with the vascular membranes, whether these be the internal or the external, becomes the centre of a new formation ; and that the loss of substance is filled up the more rapidly, in proportion to the number of such centres. 308. The reparation of Bone, after disease or injury, takes place exactly upon the same plan as its first formation. A plastic or organ- izable exudation is first poured out from the neighbouring blood-ves- sels, and this forms a sort of bed or matrix, in which the subsequent processes take place. Next, a cartilaginous substance is formed, as in the embryo, by the attraction of gelatinous intercellular substance to the exterior of certain cells, and this is gradually converted into Bone, by the regular process of ossification. When the shaft of a long bone has been fractured through, and the extremities have been brought evenly together, it is found that the new matter first ossified is that which occupies the central portion of the deposit, and which thus connects the medullary cavities of the broken ends, forming a kind of plug that enters each. This was termed by Dupuytren, by whom it was first distinctly described, the provisional callus. This is usually formed in the course of five or six weeks, or less in young persons; but at that period the contiguous surfaces of the bone itself are not cemented by bony union ; and the formation of the permanent callus occupies some months, during which the provisional callus is gradually absorbed, and the continuity of the medullary canal restored, in the same manner as it was at first established. The permanent callus has all the characters of true bone. 309. The most extensive reparation is seen, when the shaft of a long bone is destroyed by disease. If violent inflammation occur in its tissue, the death of the fabric is frequently the consequence,—appa- rently through the blocking-up of the canals with the products of the inflammatory action, and the consequent cessation of the supply of nutriment. It is not often that the whole thickness of the bone becomes necrosed at once; more commonly this result is confined to its outer or its inner layers. When this is the case, the new formation takes place from the part that remains sound ; the external layers, which receive their vascular supply from the periosteum and from the Haversian canals continued inwards from it, throwing out new matter on their interior, which is gradually converted into bone; whilst the internal layers, if they should be the parts remaining uninjured, do * See Mr. J. Goodsir on the Reproduction of Bone, in his Anatomical and Patho- logical Researches. 192 FORMATION OF TEETH.—DENTINE. the same on their exterior, deriving their materials from the medul- lary membrane and. its prolongations into their Haversian canals. But it sometimes happens that the whole shaft suffers necrosis ; and as the medullary membrane and the entire system of Haversian canals have lost their vitality, reparation can only take place from the splin- ters of bone which remain attached to the periosteum, and from the living bone at the two extremities. This is consequently a very slow process; more especially as the epiphyses, having been originally formed as distinct parts from the shaft, do not seem able to contribute much to the regeneration of the latter.* 310. We next proceed to the Teeth, which are organs of mechanical attrition, developed in the first part of the alimentary canal, for the purpose of comminuting the food con- veyed into it. Their place of origin is altogether different from that of bone, as they commence in little papillary eleva- tions of the mucous membrane covering the jaw; but the substance from which they are formed is the same primitive cellular tissue, as that in which Cartilage itself originates. We may best under- stand the structure and development of the Teeth in Man, by first inquiring into the characters presented by those of some of the lower animals, and the history of their evolution. In the fcetal Shark, the first appearance of the tooth is in the form of a minute papilla on the mucous mem- brane covering the jaws; the substance of this papilla is composed of spherical cells, which are imbedded in a kind of gela- tinous substance resembling that of inci- pient cartilage ; whilst its exterior is com- posed of a dense, structureless, pellucid membrane. The cellular mass is not at first permeated by vessels; but a small arterial branch is distributed to each papilla, and spreads out into a tuft of capillaries at its base (Fig. 50). The papilla gradually enlarges, by the formation of new cells at the part immediately adjacent to the blood-vessels, which supply the material requisite for their develop- ment ; and when it has acquired its full size, the process of calcifica- tion takes place, by which it is converted into Dentine,—the substance most characteristic of teeth. 311. This Dentine, which in the Elephant's tusk is known as Ivory, * For many parts of the foregoing account of the structure and development of Bone, the Author is indebted to the Chapter on that subject in Messrs. Todd and Bowman's Physiological Anatomy, as well as to the papers of Mr. Goodsir already referred to. Vessels of Dental Papilla. STRUCTURE OF DENTINE. 193 Fig. 51. Oblique section of Dentine of hu- man tooth, highly magnified, show- ing the calcigerous tubuli, and the outlines of the original cells. is a firm substance, in which mineral matter predominates to a greater extent than in bone; but which still has a definite animal basis, that retains its form when the calcareous matter has been removed by maceration in acid. In every 100 parts, the animal matter forms about 28 ; and of the mineral portion phosphate of lime constitutes about 64^ parts, carbonate of lime 5| parts, and phosphates of mag- nesia and soda, with chloride of sodium, about 2^ parts. When it is fractured, it seems to possess a fibrous appearance; the fibres radiating from the centre of the tooth towards its circumfe- rence. But when a thin section of it is submitted to the microscope, it is seen that this fibrous appearance is due to a peculiar structure in the dentine, which the unaided eye cannot discover. The dentinal sub- stance is itself very transparent; but it is traversed by minute tubuli, which appear as dark lines, generally in very close ap- proximation, running from the internal por- tion of the tooth towards the surface, and exhibiting numerous minute undulations, and sometimes more decided curvatures, in their course. They occasionally divide into two branches, which continue to run at a little distance from one another in the same direction; and they also frequently give off small lateral branches, which again send off smaller ones. In some animals, the tubuli may be traced at their extremities into minute cells, or cavities, analogous to the lacunae of bone; and the lateral branchlets occasionally terminate in such cavities, which are called the intertubular cells. The diameter of the tubuli at their largest part averages l-10,000th of an inch ; their smallest branches are immeasurably fine. It is impossible that even the largest of them can receive blood, as their diameter is far less than that of the blood-discs; but it is probable that, like the canali- culi of bone, they may absorb nutrient matter from the vascular sur- face, with which their internal extremities are in relation. 312. In the Teeth of Man and of most Mammalia, we find the central portion hollow ; and lined, in the living tooth, by a vascular membrane. This cavity, with its vascular wall, is analogous to an enlarged cancellus or Haversian canal of Bone ; and, as we shall pre- sently see, it is formed in a similar manner. Upon the walls of the cavity, all the tubuli open; and they radiate from this towards the surface of the upper part of the tooth, as shown in the accompanying figure. The central cavity is continued as a canal through each fang or root; and the dentinal tubes in like manner radiate from this, towards the surface of the fang.—In the teeth of many of the lower animals, however, we find a network of canals extending through the substance of the tooth, instead of a single cavity; and these canals are 13 194 STRUCTURE AND DEVELOPMENT OF DENTINE. frequently continuous with the Haversian canals of the subjacent bone, so that the analogy between the two is complete. From each canal the dentinal tubuli radiate, just in the manner of the canaliculi of bone (§ 279); and thus we may regard a tooth of this kind as repeating, in each of the parts surrounding one of these canals, the structure of the human tooth. 313. The process by which the cellular mass, or pulp, of the dental papilla, becomes converted into the dentine of the perfect tooth, is thus described by Prof. Owen, from his investigations into the his- tory of the Shark's dentition.—The pulp becomes vascular, by the extension of the capillary network into its substance ; the vessels are also accompanied by fine branches of nerves. The cells arrange themselves in lines, radiating from the centre to the circumference of the pulp ; and they become somewhat elongated in that direction. A series of changes takes place in the nuclei of the cells, consisting chiefly in their elongation and subdivision; so that they form a series of parallel lines within each cell. The subdivided and elongated nuclei become attached, by their extremities, to the corresponding nuclei of the cells in advance, and the attached extremities form con- tinuous lines; so that in each row or file of cells, extending from the inner part to the circumference of the pulp, there are several dark lines, apparently continuous, which are formed by rows of granules (or perhaps incipient cells) thus derived from the once single and rounded nuclei of the parent-cells. During the same time, the walls of the adjacent cells come into closer proximity, to the exclusion of the gelatinous matter, that originally intervened between them ; and they secrete calcareous matter, derived from the blood, into their own cavities. The cells thus become completely filled with that material (probably combined with gelatin, as in bone), excepting in the part occupied by the rows of granules, which are thus left unconsolidated; and which, when the granules disappear at a subsequent period, remain as the dentinal tubes. This consolidation first takes place on the exterior of the pulp ; and the calcifying process gradually extends itself inwards, causing the blood-vessels to retreat, as it were, towards the centre, where an unconsolidated portion usually remains. 314. Thus the substance of the outer portion of the pulp is actually converted into dentine, and does not form it by a process of excretion, as was formerly supposed. In general, the coalescence of the original cells is so complete, that their boundaries altogether disappear, and the substance that intervenes between the tubuli seems quite homo- geneous.; but distinct traces of the original division into cells may often be met with, in the dentine of Man (Fig. 50), as well as in that of other animals; which satisfactorily confirms what has been just stated, as to the mode of its formation. Although in the most charac- teristic form of Dentine, no blood-vessels exist, yet there are certain species, both among Mammals, Reptiles, and Fishes, in which the Dentine is traversed by cylindrical prolongations of the central cavity, conveying blood-vessels into its substance ; and the presence of these STRUCTURE AND DEVELOPMENT OF DENTINE. 195 medullary canals thus gives to the Dentine a vascular character; and thus increases its resemblance to bone.—The central portion of the pulp is sometimes converted into a substance still more nearly resem- bling bone, having its stellate lacunae as well as its vascular canals. This change is normal or regular in certain animals, as in the extinct Iguanodon and Icthosaurus, and in the Cachalot or Sperm-whale ; and the ossified pulp bears a close resemblance to the bones of the respect- ive animals, although it is not formed in continuity with them. A similar change occurs in the Human tooth;—sometimes, it would appear, rapidly, as the result of disease ; but in general more slowly, increasing gradually with the advance of age. 315. It is not easy to ascertain the amount of nutritive change that takes place in the substance of Dentine, when it is once fully formed. When young animals are fed with colouring-matter, it is found to tinge their teeth, as well as their bones ; and if the tooth be in process of rapid formation at the time of the experiment, the progressive cal- cification of the pulp, from without inwards, is marked by a series of concentric lines. Even in the adult, some tinge will result from a prolonged use of this substance ; and it has been noticed that the teeth of persons, who have long suffered from Jaundice, sometimes acquire a tinge of bile. These facts show that, even after the com- plete consolidation of the Dentine, it is still pervious to fluids: and that in this manner it may draw into itself, from the vascular lining of the pulp-cavity, a substance capable of repairing its structure, is proved by the circumstance, that a new layer of hard matter is occa- sionally thrown out upon a surface which has been laid bare by caries. 316. In those simple teeth which consist solely of Dentine, the mode of production already described,—that of the consolidation of a papilla upon the mucous membrane of the mouth,—is all which is requisite. When the formation of the tooth itself is complete, it may remain attached only to the mucous membrane, which is the case in the Shark, or it may grow downwards, by the addition of new dental structure at its base, until it comes in contact with the bone of the jaw. Where it is only attached to the mucous membrane, as in the Shark, it is very liable to be torn away ; but a new tooth, formed from a distinct papilla, is ready to replace it; and this process is continually repeated, the development of new papillae being apparently unlimited. On the other hand, where the root of the tooth comes in contact with the jaw, it may completely coalesce with it, which is the case in many Fishes, the Haversian canals of the bone being continued as medullary canals into the dentine ; or it may send long spreading roots into the bone, which are united to it at their extremities. In the classes of Fishes and Reptiles (with scarcely any exceptions) the teeth are by no means permanent, as among Mammalia; but new teeth are con- tinually succeeding the old ones. The mode in which these teeth originate, by small buds from the capsules of the preceding, will be understood when the capsular development of all the higher forms of the dental apparatus has been described. 196 STRUCTURE OF ENAMEL. Fig. 52. 317. It is obvious that there is no provision, in the simple calcifi- cation of the dental papilla, for any variations or density, other than those which may result from the different degrees of hardness in the substance of the dentine itself. Now in the teeth of Man and most other Mammals, and in those of many Reptiles and some Fishes, we find two other substances, one of them harder, and the other softer, than Dentine; the former is termed Enamel; and the latter Cementum or Crusta petrosa. For the development of these, a peculiar modifi- cation of the apparatus is requisite. 318. The Enamel is composed of long prismatic cells, exactly resembling those of the prismatic shell-substance formerly described, but on a far more minute scale ; the diameter of the cells not being more, in Man, than l-5600th of an inch. The length of the prisms corresponds with the thickness of the layer of enamel; and the two surfaces of this layer present the ends of the prisms, which are usually more or less regularly hexagonal. The quantity of animal matter in the tooth of the adult is extremely minute,—not above two parts in 100 ; and it is only at a very early age, that the true character of the animal structure can be distinctly seen. The course of the pris- matic cells is more or less wavy; and they are marked by numerous transverse striae, resembling those of the prismatic shell-sub- stance, and probably originating in the same cause,—the coalescence of a line of shorter cells, to form the lengthened prism. No trace of tubuli or of blood-vessels is to be found in the completely formed Enamel of higher animals ; but in the teeth of certain Fishes, it is penetrated by calcigerous tubes, which enter its substance from the exterior, and ramify and subdivide like those of the dentine. Of the 98 parts of mineral matter in the enamel, 88£ consist (according to Berzelius) of phosphate of lime, 8 of car- bonate of lime, and 1£ of phosphate of magnesia. In density and resisting power, the Enamel far surpasses any other organ- ized tissue, and approaches some of the hardest of mineral substances. In Man, and in Carnivorous animals, it covers the crown of the tooth only, with a simple cap or superficial layer of tolerably uniform thickness (Fig. 52, 1), which follows the surface of dentine in all its inequalities; and its component prisms are directed at right angles to that surface, their inner extremities resting in slight but regular depressions on the exte- rior of the dentine. In the teeth of many Herbivorous animals, how- ever, the Enamel forms (with the Cementum) a series of vertical plates, which dip down (as it were) into the substance of the dentine Vertical section of human mo- lar tooth;—1, enamel j 2, cemen- tum or crusta petrosa; 3, dentine or ivory ; 4. osseous excrescence, arising from hypertrophy of ce- mentum; 5, cavity; 6, osseous cells at outer part of dentine. CEMENTUM.—FORMATION OF DENTAL CAPSULE. 197 and present their edges alternately with it, at the grinding surface of the tooth ; and there is in such teeth no continuous layer of dentine over the crown. The purpose of this arrangement is evidently to provide, by the unequal wear of these three substances,—of which the Enamel is the hardest and the Cementum the softest,—for the constant maintenance of a rough surface, adapted to triturate the tough vegetable substances on which these animals feed.—The Enamel is the least constant of the Dental tissues. It is more fre- quently absent than present in the teeth of the class of Fishes ; it is wanting in the entire order of Ophidia (Serpents) among existing Reptiles; and it forms no part of the teeth of the Edentata (Sloths, &c.) and Cetacea (Whales) amongst Mammals. 319. The Cementum, or Crusta Petrosa, has the characters of true bone ; possessing its distinctive stellate lacunae and radiating canali- culi. Where it exists in small amount, we do not find it traversed by medullary canals ; but, like Dentine, it is occasionally furnished with them, and thus resembles Bone in every particular. These medullary canals enter its substance from the exterior of the tooth ; and conse- quently pass towards those, which radiate from the central cavity towards the surface of the dentine, where it possesses a similar vas- cularity,—as was remarkably the case in the teeth of the extinct Megatherium.—In the Human tooth, however, the Cementum has no such vascularity. It forms a thin layer, which envelops the root of the tooth, commencing near the termination of the capping of Enamel (Fig. 52, 2). This layer is very subject to have its thickness increased, especially at the extremity of the fangs, by hypertrophy, resulting ftom inflammation; and sometimes large exostoses are thus formed (Fig. 52, 4), which very much increase the difficulty of extracting the tooth. When the tooth is first developed, the Cementum envelops its crown, as well as its body and root; but the layer is very thin where it covers the Enamel, and being soft, it is soon worn away by use. In the teeth of many Herbivorous Mammals, it dips down with the Enamel to form the vertical plates of the interior of the tooth ; and in the teeth of the Edentata as well as of many Reptiles and Fishes, it forms a thick continuous envelop over the whole of the surface, until worn away at the crown. 320. The development of these additional structures is provided for by the enclosure of the primitive papilla, from which the Dentine is formed, within a Capsule, which, at one period, completely covers it in: between the inner surface of the capsule, and the outer surface of the dentinal papilla, a sort of epithelium is developed, by the cal- cification of which, the Enamel is formed; and the Cementum is generated by the conversion of the capsule itself into a bony sub- stance.—The processes by which this capsular investment is pro- duced, and the tooth completed and evolved, will now be briefly described, as they occur in the Human fcetus. 321. The dental papillae begin to make their appearance, at about the seventh week of embryonic life, upon the mucous membrane 198 DEVELOPMENT OF HUMAN TEETH. covering the bottom of a deep narrow groove (Fig. 53, a), that runs along the edge of the jaw (Fig. 53, 6); and during the tenth week, processes from the sides of this " primitive dental groove," particu- Fig. 53. Successive stages of the development of the deciduous or temporary teeth, and of the origin of the sacs of the permanent set. larly the external one, begin to approach one another, so as to divide it, by their meeting, into a series of open follicles, at the bottom of which the papillae may still be seen. At the thirteenth week, all the follicles being completed, the papillae, which were at first round blunt masses of cells, begin to assume forms more characteristic of the teeth which are to be developed from them; and by their rapid growth, they protrude from the mouths of the follicles (Fig. 53, c). At the same time, the edges of the follicles are lengthened into little valve- like processes, or opercula, which are destined to meet and form covers to the follicles (Fig. 53, d). There are two of these opercula in the Incisive follicles, three for the Canines, and four or five for the Molars. And by the fourteenth week, the two lips of the dental groove meet over the mouths of the follicles, so as completely to enclose each papilla in a distinct capsule (Fig. 53, e). At this period, before the calcification of the primitive pulps commences, a provision is made for the production of the second or permanent molars ; whose capsules originate in buds or offsets from the upper part of the cap- sules of the temporary or milk-teeth (Fig. 53, f). These offsets are in the condition of open follicles, communicating with the cavity of the primitive tooth ; but they are gradually closed in, and detached altogether from the capsules of the milk-teeth (Fig. 53, g, h, i). 322. Soon after the closure of the follicles of the Milk-teeth, the conversion of the cells of the original papilla into Dentine commences, according to the method already described (§ 313). Whilst this is going on, the follicles increase in size, so that a considerable space is left between their inner walls and the surface of the dental papillae ; which space is filled up with a gelatinous granular matter, the Enamel- pulp* The portion of this which is converted into enamel, however, is Very slnall; being only a thin layer, which is left on the inner sur- face oT the capsule after the remainder has disappeared. The interior of the dental sac, at the time when the conversion-process has reached the base of the dentinal pulp, is in the villous and vascular condition DEVELOPMENT AND RENEWAL OF TEETH. 199 of a Mucous membrane,—which indeed it really is, having been, as we have seen, once continuous with the lining of the mouth ; and the layer of prismatic cells which covers its free surface, and by the cal- cification of which the enamel is produced, may be regarded as an epithelium.—The completion of the Enamel, and the ossification of the capsule so as to form the Cementum, take place at a subsequent period. 323. We have thus seen that the history of the first development of the human teeth may be divided into three stages, the papillary, the follicular, and the saccular. The papillary corresponds precisely with the complete mode of dental development in the Shark and other Fish,—as already mentioned. The follicular, which commences with the enclosure of the papilla in open follicles, and terminates when the papillae are completely hidden by the closure of the mouths of those follicles, has also its permanent representation in the develop- ment of the teeth of many Reptiles and Fishes ; the primitive papillae of which, though enclosed in follicles, are never covered in at the summit, and thus free themselves from their envelops by simply growing upwards through their open mouths. But in Man, in all other animals which agree with him in going on to the saccular stage, there must also be an eruptive stage, which consists in the bursting- forth of the tooth from the enclosing capsule ; the summit of the tooth being carried against the lid of the sac, by the growth of its root (Fig. 53, h). By the continuance of the same growth, the teeth are caused to penetrate the gum, and are gradually raised above its surface (Fig. 53, i). 324. All the permanent teeth, which are destined to replace the temporary set, originate, as already stated, in buds or offsets from the capsules of the latter. But behind the last temporary molars, which are replaced by the permanent bicuspids, three permanent molars are to be developed, on each side of either jaw. The first of these is formed on precisely the same plan with the milk-teeth; but is not completed until a later period. The capsule of the second is formed at a later period from that of the first, by a process of budding ex- actly analogous to that by which the other permanent capsules are formed from the corresponding temporary ; and at a still later period, the capsule of the third permanent molar is formed as a bud from that of the second. The evolution of this molar does not usually take place until the system has acquired its full development; and the process of budding then ceases in Man,—being limited to a single act of reproduction in the case of the ordinary Milk-teeth, and to a double one in that of the first permanent Molar. In many animals of the lower classes, however, the process goes on through the whole of life without any limit; the newly-formed teeth, however, usually replacing those of the previous set, and not being developed at their sides like the second and third permanent molars of Man. 325. By a process of this kind, the continual renewal of the Teeth takes place in those Reptiles and Fishes, whose dentition goes on to 200 DEVELOPMENT AND SUCCESSION OF TEETH. the saccular stage; in those at which it stops at the papillary, the successive teeth are formed from new and independent papillae. The only exception to the rule, that no Reptiles or Fishes have permanent teeth, is found in the curious Dicynodon; an extinct Reptile which had two large tusks growing from persistent pulps, like those of the Elephant, the front teeth of the Rodentia, and the grinders of the Edentata. In such teeth, the base of the pulp remains unconverted, and a new development of cells is continually taking place in that situation ; these new cells are in their turn converted into dentine, in continuity with that previously formed ; and thus the tooth or tusk is continually lengthening at its base, in a degree which compensates for its usual wear at its summit. If anything should prevent that wear, —as when the opposite tooth has been broken off,—there is an abso- lute increase in the length of the tooth, from the continued growth at its base; which may become a source of great inconvenience to the animal. There is nothing, in the human subject, at all analogous to this mode of development from persistent pulps; the process being checked by the closure of the root around the base of the pulp, which obstructs the supply of blood it receives. The analogy between the continued succession of teeth in the lower Vertebrata, by the gem- miparous reproduction of their capsules, and the development of the capsules of the permanent teeth of Man from those of the temporary set, is made further evident by the fact, that a third set occasionally makes its appearance in persons advanced in life; the development of which would not be intelligible, if we could not refer it to the continuance of the same process in the other capsules, as that which regularly takes place to a limited extent in the permanent molars of Man, and which goes on without limit through the whole lives of the lower Vertebrata. 326. The following table shows the usual periods at which the different teeth of the two sets first show themselves above the gum. It must be borne in mind, however, that these periods are subject to very great variation; and that the average alone can therefore be expressed. Temporary or Deciduous Teeth. Months Central Incisors 7 Lateral Incisors . 8—10 Anterior Molars . 12—13 Canines . 14—20 Posterior Molars . . 18—36 Permanent Teeth. Years. First Molar Central Incisors Lateral Incisors First Bicuspid Second Bicuspid Canines Second Molars Third Molars 6* to 7 — 8 — 9 — 10 —11 —12\ 12^—14 16 —30 7 8 9 10 12 327. We have seen that the Teeth are formed, in the first instance, upon the surface of the Mucous membrane of the mouth; and con- STRUCTURE AND DEVELOPMENT OF HAIR. 201 sequently they really form a part of the external or dermo-skeleton, and not of the internal or osseous skeleton. They correspond, there- fore, with the external skeletons of the Invertebrata; and thus the analogy which has been pointed out, between the enamel of teeth and the prismatic cellular substance of the shells of Mollusca, and between the dentine and the shells of the higher Crustacea, holds good also in regard to the situation of these structures. Although the teeth are the only ossified portions of the dermo-skeleton in Man, we find the body partially or completely enclosed in an armour of bony scales or plates, in certain Mammalia, Reptiles, and Fishes; and in some species of the last-named class, which have now ceased to exist, the scales seem to have had the texture of Enamel. 328. In connection with the teeth, the structure and development of the Hair may be described ; this substance being generated very much in the same manner as dentine,—by the conversion of a pulp enclosed in a follicle; though the product of the transformation is different. The Hair-follicle is formed by the inversion of the Skin, as the Tooth-follicle is by an inversion of the Mucous membrane; and it is lined by a continuation of the epidermis. From the bottom of the follicle, a sort of papilla rises up, formed of cells; the exterior of this, which is the densest part, is known as the bulb ; whilst the softer interior is termed the pulp. The follicle itself is extremely vascular; and even the bulb is reddened by a minute injection; though no distinct vessels can be traced into it.—It has been imag- ined until recently, that the Hair, like the other extra-vascular tissues, is a mere product of secretion ; its material, which is chiefly horny matter of the same composition with that of the epidermis and its other appendages (§t227), being elaborated from the surface of the pulp. This, however, proves to be a very erroneous account of it; as is shown by the results of recent microscopic inquiries into its structure. Although the Hairs of different animals vary considerably in the appearances they present, we may generally distinguish in them two elementary parts ;—a cortical or investing substance, of a fibrous horny texture; and a medullary or pith-like substance, occupying the interior. In some instances, however, there is scarcely any me- dullary substance to be traced; whilst in other cases (as in the hair of the Musk-Deer) the entire hair seems made up of it. 329. The fullest development of both substances is to be found in the spiny hairs of the Hedgehog, and in the quills of the Porcupine, which are but hairs on a magnified scale. The cortical substance forms a dense horny tube, to which the firmness of the structure seems chiefly due ; whilst the medullary substance is composed of an aggre- gation of very large cells, which seem not to possess any fluid con- tents in the part of the hair that is completely formed. In the hair of the Mouse and other small Rodents, we see the horny tube crossed at intervals by partitions, which are sometimes complete, sometimes only partial; these are the walls of the single or double line of cells, of which the medullary substance is made up. In the Human hair, 202 STRUCTURE AND DEVELOPMENT OF HAIR. the chief part is composed of a tube of a horny substance, correspond- ing with the cortical sheath of the hairs of other animals; this is fibrous in its texture, as may be shown by crushing the hair, after it has been softened by maceration in dilute acid ; and the outlines of the fibres are indicated by very delicate longitudinal striae, which may be traced through its whole thickness. This fibrous structure sometimes makes up the whole thickness of the hair; but there is usually a central medulla, composed of colourless cells, with which pigment-cells are mingled. The hair is invested by a series of very minute scales, resembling those of the epidermis, but much smaller; these are arranged in rows, like tiles upon a roof, and their edges form delicate lines upon the surface of the hair, which are sometimes transverse, sometimes oblique, sometimes apparently spiral. The colouring matter of the hair appears to be related to Haematosine; it is bleached by Chlorine ; and its hue seems dependent in part upon the presence of iron, which is found in larger proportion in dark than in light hair. 330. The fibres of the cortical substance are probably cells, which have become elongated by the process formerly described, and which have at the same time secreted horny matter into their interior. This change is continually going on in the bulb of the hair, at the base of the part previously completed ; and by the progressive formation of new cells in the bulb, a constant growth of the cortical substance is provided for. The mode in which the medullary substance is gene- rated, does not seem very clear ; but it probably consists of the con- tents of the cells of the pulp, in which a continuous growth goes on, at the same rate with that of the bulb. Thus the Hair is constantly undergoing elongation by the addition of new substance at its base; precisely in the same manner as the teeth of certain Mammals grow from persistent pulps. The part once formed usually undergoes no subsequent alteration; but there is evidence that it may be affected by changes at its base, the effect of which is propagated along its whole extent. Thus it is well known that cases are not unfrequent, in which, under the influence of strong mental emotion, the whole of the hair has been turned to gray, or even to a silvery white, in the course of a single night;—a change which can scarcely be accounted for in any other way, than by supposing that a fluid, capable of che- mically affecting the colour, is secreted at the base of the hair, and transmitted by imbibition through the medullary substance to the opposite extremity. The knowledge of the organized structure of hair, enables us better to understand some of the effects of disease ; and especially of that peculiar affection termed Plica Polonica. The hair of individuals suffering from it is disposed to split into fibres, often at a considerable distance from the roots, and to exude a glu- tinous substance; and these two causes unite in occasioning that peculiar matting of the hair, which has given origin to the name of the disease. In the hair thus affected, there is evidently a power of transmitting fluid absorbed at the roots; and it is said that even OF CELLS COALESCED INTO TUBES. 203 blood exudes from the stumps, when the hairs are cut off close to the skin. 6. Of Cells coalesced into Tubes, with Secondary Deposit. 331. Most of the tissues which have been hitherto described, differ in no essential particulars from those of Plants; the chief departure from the forms presented by the latter, being in the Fibrous tissues, which, as already observed, are introduced for the sake of facilitating the movements of the several parts of the structure, one upon the other. The various cellular tissues find their exact representatives in those of the Vegetable fabric; and the denser parts of the Animal, such as Bone, Cartilage, &c, are represented by the solid substances formed by the Plant in the heart-wood of the stem, the stone of fruits, &c,—these substances acquiring their density in precisely the same manner with the Osseous tissues, by the secreting action of their own cells, which draw a solidifying material from the general circulating fluid. But we now come to two tissues of the highest importance in the Animal fabric ; the presence of which is, indeed, its distinguishing characteristic. These are the Muscular, and the Nervous tissues. The former is the one, by which all the sensible movements of the body are effected ; and the latter serves as the instrument, by which sensations are received, and by which the will excites the muscles to action,—besides serving as the medium for other operations, in which motion is produced, without the intervention of either sensation or will. These tissues, with the apparatus of bones and joints on which the muscles act, constitute the purely animal portion of the fabric; and if a being could be constructed, in which they should be capable of continued activity without any other assistance, it would be in all essential particulars an Animal. But, as we shall presently see, the plans on which these tissues are formed, in fact the very conditions of their existence and activity, are such, that they require constant nutrition and re-formation; so that the Animal cannot exist, without an apparatus for preparing, circulating, and maintaining in constant purity, a fluid, by which nutrient operations may be effected, and which shall also be the means of carrying off the products of the waste consequent upon the action of those tissues. This apparatus consti- tutes the Vegetative portion of the frame ; the elementary parts con- cerned in which have been already noticed. 332. When we examine an ordinary Muscle with the naked eye, we observe that it is made up of a number of fasciculi or bundles of fibres ; which are arranged side by side with great regularity, in the direction in which the muscle is to act; and which are united by areolar tissue. These fasciculi may be separated into smaller parts, which appear like simple fibres; but when these are examined by the microscope, they are found to be themselves fasciculi composed of minuter fibres bound together by delicate filaments of areolar tissue. By carefully separating these, we may obtain the ultimate Muscular 204 STRUCTURE OF MUSCULAR FIBRE. Fibre. Fig. 54 Fasciculus striated Muscular Fibre, showing at a the transverse strias, and at b, the longitudinal striae, more distinctly. This fibre exists under two forms, the striated and the non- striated; the former makes up the whole substance of those muscles, over which the will has control, or which are usually called into operation through the nerves; whilst the latter exists in the muscles which the will cannot influence, and which are excited to contraction by stimuli that act directly upon them. The muscles of the former class minister to the ani- mal functions; those of the latter to the functions of organic or vegetative life. The appearance presented by the striated fibres of voluntary muscles, is shown in Fig. 54 ; that of the non-striated fibres of the mus- cles of organic life, in Fig. 55. 333. When the fibre of voluntary muscle is more closely examined, it is seen to consist of a delicate tubular sheath, quite distinct on the one hand from Fig. 55. i the areolar tissue which binds the fibres into fasciculi, and equally distinct from the internal substance of the fibre. This cannot always be brought into view, on account of its transparency; it becomes most evident, when (as oc- casionally happens) the contents of the fibre are separated transversely by the drawing apart of its extremities, without the rupture of the sheath; but it may also be sometimes seen rising up in wrinkles upon the surface of the fibre, when the latter is in a state of contraction. This membranous tube, which has been termed the Myolemma, has nothing to do with the production of the striae; these being due, as will be presently shown, to the peculiar arrangement of its contents. It is not per- forated either by nerves or capillary vessels ; and forms, in fact, a complete barrier be- tween the real elements of Muscular struc- ture, and the surrounding parts. That it has no share in the contraction of the fibre, is evident from the fact just mentioned, in regard to its wrinkled aspect wThen the fibre is shortened. 334. Although Muscular fibres are com- monly described as cylindrical in form, yet they are in reality rather polygonal, their sides being flattened against those of the adjoining fibres. In some instances, the angles are sharp and decided ; in others they are rounded off, so as to leave spaces between the con- tiguous fibres, for the passage of vessels. In Insects, the fibres often present the form of flattened bands, on which the transverse striae are very beautifully marked. The size of the fibres is subject to great U i:« Non-striated Muscular Fibre; at 6, in its natural state; at a, show- ing the nuclei after the action of acetic acid. STRIATED MUSCULAR FIBRE. 205 variation, not merely in different classes of animals, but in different species, in different sexes of the same species, and even in different parts of the same muscle. Thus Mr. Bowman estimates the average diameter of the fibres in the Human male at l-352d of an inch; the largest being l-192d, and the smallest l-507th. In the female, he found the average to be l-454th of an inch; whilst the largest was l-384th, and the smallest l-615th. The average size of the Mus- cular fibre is greater among Reptiles and Fishes, than in other Verte- brata ; but on the other hand the extremes are much wider. Thus its dimensions vary in the Frog from l-100th to l-1000th of an inch ; and in the Skate from l-65th to l-300th. 335. When the striated Muscular Fibre is examined still more closely, it is found to contain an assemblage of very minute elements, which appear to be flattened disk-like cells, of very uniform size. These primitive particles are adherent to each other both by their flat surfaces, and by their edges. The former adhesion is usually the most powerful; and causes the substance of the fibre, when it is broken up, to present itself in the form of delicate fibrillae, each of which is composed of a single row of the primitive particles (Fig. 56). On the other hand, the lateral ad- hesion is sometimes the stronger; and causes the fibre to break across into disks, each of which is composed of a layer of the primitive particles (Fig. 57). That the fibre is a solid collection of these elementary parts, and not hollow in the centre, as some have supposed, is shown by making a thin transverse section of a fasciculus (Fig. 58); by which also the polygonal form of the fibre is made apparent. Fig. 56. Striated Muscular fibre separating into fibrillae, from a preparation by Mr. Lealand. Fig. 57. Fig. 58. An ultimate fibre, in which the transverse splitting into disks, in the direction of the Btriaiion of the ultimate fibrils, is seen. Transverse section of ultimate fibres of the biceps. In this figure the poly- gonal form of the fibres is seen, and their composition of ultimate fibrils. 206 STRIATED MUSCULAR FIBRE. Fig. 59. 336. When the fibrillae are separately examined, under a high mag- nifying power, they are seen to present a cylindrical or slightly-beaded form, and to be made up of a linear aggregation of distinct cells. We observe the same alternation of light and dark spaces, as when the fibrillae are united into fibres or into small bundles; but it maybe distinctly seen, that each light space is divided by a transverse line; and that there is a pellucid border at the sides of the dark spaces, as well as between their contiguous extremities (Fig. 59). This pellucid border seems to be the cell-wall; the dark space enclosed by it (which is usually bright in the centre) being the cavity of the cell, which is filled with a highly-refracting substance. When the fibril is in a state of relaxation, as seen at a, the diameter of the cells is greatest in the longitudinal direction : but when it is contracted, the fibril increases in diameter as it diminishes in length ; so that the transverse dia- meter of each cell becomes equal to the longitudinal diameter, as seen at b; or even exceeds it. Thus the act of Muscular contraction seems to consist in a change of form in the cells of the ultimate fibrillae, consequent upon an attraction between the walls of their two extremities; and it is interesting to observe, how very closely it thus corresponds with the contrac- tion of certain Vegetable tissues, of which the com- ponent cells (§ 345) appear to produce a movement, when they are irritated, by means of a similar change of form. The essential difference, therefore, between the muscular tissue of Animals, and the contractile tissues of Plants, consists in the subjection of the former to nervous influence (§ 353). The diameter of the ultimate fibrillae will of course be subject to variations, in accordance with their contracted or re- laxed condition ; but seems to be otherwise tolerably uniform in different animals, being for the most part about l-10,000th of an inch. It has been observed, however, as high as l-5000th of an inch, and as low as l-20,000th, even when not put upon the stretch. The average distance of the striae, too, is nearly uniform in different animals ; though considerable variations present themselves in every individual, and in different parts of the same muscle. Thus the maximum distance varies in different animals from 1-I5,000th to l-20,000th of an inch ; the minimum from l-7500th to l-4500th of an inch ; while the mean does not depart widely in any instance from l-10,000th. 337. The Muscular fibre of Organic Life is very different from that which has been now described. It consists of a series of filaments, which do not present transverse markings; but which are tubular like the preceding, their contents having a granular consistence, without m m n Q A B 1 1 E i § i i ■ i Structure of the ultimate fibrillae of striated muscu- lar fibre :—o, a fi- bril in a state of ordinary relaxa- tion ; b, a fibril in a state of partial contraction. NON-STRIATED MUSCULAR FIBRE. 207 a. A muscular fibre of organic life from the uri- nary bladder, magnified 600 times, linear measure. Two of the nuclei are seen. b. A muscular fibre of organic life, from the sto- mach, magnified 600 times. The diameter of ihis and of the preceding fibre, mid- way between the nuclei, was l-4750th of an inch. any definite arrangement of the particles into disks or fibrillae. Their size is usually much less than that of the striated muscular fibre; but owing to the extreme varia- tion in the degree of flattening which they undergo, it is difficult to make even an average estimate of their dimensions. Those of the alimentary canal of Man are stated by Dr. Baly to measure from about the l-2500th to the l-5600th part of an inch in diameter. They generally present nodosities or enlargements at frequent intervals (Fig. 60) ; the character of which will be presently apparent. These fibres are, like those of the other muscles, arranged in a parallel manner into bands or fasci- culi ; but these fasciculi are generally interwoven into a network, without having any fixed points of attachment. It is of this kind of structure, that the proper muscular coat of the oesophagus, of the stomach and intestinal canal, and of the bladder, is composed; it makes up, also, the substance of the pregnant uterus; and it is found in no incon- siderable amount in the trachea and bronchial tubes. The fibres of the uterus somewhat differ in their aspect from those of other parts; being much broader at their centre, and tapering off towards their extremities. In the Heart, a mixture of the striated and non-striated fibres is found ; a modification of the latter form of tissue exists in the middle coat of the arteries, especially in the smaller branches; and fibres of the same kind are interwoven with the other fibrous tissues in the substance of the skin, and especially in the dartos, giving it a contractility which is mani- fested under the influence of cold or of mental emotions, and thus producing that general roughness and rigidity of the surface, which are known as cutis anserina, and throwing the scrotum into wrinkles. 338. From the study of the early development of Muscular Fibre, it appears that the Myolemma, or external transparent tube, is the part first formed ; this being distinctly visible, long before any traces of fibrillae can be observed in it. This tube takes origin, like the straight ducts of Plants, in cells laid end to end ; the cavities of which coalesce, by the disappearance of the partitions, at a subse- quent period. The nuclei of these original cells may be distinctly seen, for some time after the appearance of the transverse striae, which indicate the formation of the fibrillae in their interior; and they pro- ject considerably from the sides of the fibres. In the fully-formed muscle of animal life, however, they are not perceptible, except when the fibre is treated with weak acid; the effect of which is to render the nuclei more opaque, whilst the surrounding structure becomes more transparent. They are usually numerous in proportion to the size of the fibre. There is every probability that these nuclei con- tinue to act, like the " germinal spots" of the glandular follicles, as 208 DEVELOPMENT AND GROWTH OF MUSCULAR FIBRE. Fig 61. Mass of ultimate fibres from the pectoralis major of the human foetus, at nine months. These fibres have been immersed in a solution of tartaric acid, and their " numerous corpuscles, turned in various directions, some present- ing nucleoli," are shown. centres of nutrition; from which the minute cells that compose the fibrillae are developed as they are required. The diameter of the Muscular Fibre of the fcetus is not above one-third of that which it possesses in the adult; and as the size of the ultimate par- ticles is the same in both cases, their number must be greatly multiplied during the growth of the structure. But we shall find reason to believe, that the decay of these particles is constantly taking place, with a rapidity proportional to the functional activity of the Muscle; and their generation, which occurs as continually, when the nutrient operations proceed in their regular course, is proba- bly accomplished by a development from these centres, at the expense of the blood with which the Muscle is copiously sup- plied. 339. From the preceding history it appears, that there is no dif- ference, at an early stage of development, between the striated and the non-striated forms of muscular fibre. Both are simple tubes, containing a granular matter in which no definite arrangement can be traced, and presenting enlargements occasioned by the presence of the nuclei. But whilst the striated fibre goes on in its development, until the fibrillae, with their alternation of light and dark spaces, are fully produced, the non-striated fibre retains throughout life its ori- ginal embryonic condition. 340. We have seen that the Muscular tissue, properly so called, is as extra-vascular as cartilage or dentine; for its fibres are not penetrated by vessels ; and the nutriment required for the growth of its contained matter is drawn by absorption through the myolemma. But the substance of Muscle is extremely vascular; the capillary vessels being dis- tributed in nearly parallel lines, in the minute interspaces betwreen the fibres; so that it is probable that there is no fibre, which is not in close relation with a capillary. Hence there is every pro- vision for the active nutrition of this tissue; the arterial circulation bringing the materials for its growth and renova- tion ; whilst the venous conveys away the products of the waste or disintegration, which is consequent upon its active exercise. The supply of blood is not merely requisite for the nutrition of the muscular tissue; but it also affords a condition which is requisite for its action. This condition is oxygen. It is not enough that blood should circulate through the muscles; for that blood, to exercise any beneficial influence, must be arterialized. Fig. G2. Capillary network of Muscle. NUTRITION AND COMPOSITION OF MUSCLE. 209 Consequently the muscles of warm-blooded animals soon lose their contractile power, after the supply of arterial blood has been sus- pended, either by the cessation of the circulation, or by the want of aeration of the blood; but those of cold-blooded animals preserve their properties for a much longer period, in accordance with the general principle formerly stated,—that, the lowrer the usual amount of vital energy, the longer is its persistence, after the withdrawal of the conditions on wThich it is dependent. 341. The Muscles of Animal Life are, of all the tissues except the Skin, the most copiously supplied with Nerves. These, like the blood- vessels, lie on the outside of the Myolemma of each fibre ; and their influence must consequently be exerted through it. The arrangement of these nerves is shown in the succeeding figure. Their ultimate fibres or tubes cannot be said to terminate anywhere in the muscular substance; for after issuing from the trunks, they form a series of loops, which either return to the same trunk, or join an adjacent one. The occasional appearance of a termination to a nervous fibril is caused by its dipping-down between the muscular fibres, to pass towards another stratum.—The non-striated muscles, however, are very sparingly supplied with nerves; and these are derived,—for the most part, if not entirely,—from the Sympathetic system, rather than from the Cerebro-spinal. 342. Every Muscular Fibre, of the striated kind at least, is at- tached at its extremities to fibrous tissue; through the medium of which it exerts its contractile power on the bone or other substance, which it is destined to move. The muscular fibre usually ends ab- ruptly by a perfect disk ; and the myolemma seems to terminate there. The tendinous fibres are attached to the whole surface of the disk; Fig. 63. Portion of muscle, showing the arrangement of the motor nerves supplying it. and probably become continuous with the myolemma. Thus the whole muscle is penetrated by minute fasciculi of tendinous fibres; and these collect at its extremities into a tendon. Sometimes the 14 210 CONTRACTILITY OF VEGETABLE TISSUES. muscular fibres are attached obliquely to the tendon, which forms a broad band that does not subdivide; this is seen in the legs of Insects and Crustacea, in which the muscular fibres have what is called a penniform arrangement, being inserted into the tendon, on either side, like the laminae of a feather into its stem.—The forms which different muscles present, have reference purely to the mechanical purposes, which they have respectively to accomplish. The elements are the same in all, both as regards structure and properties. 343. Notwithstanding the energy of growth in Muscular tissue, it is doubtful if it is ever regenerated, wrhen there has been actual loss of substance. Wounds of muscles are united by Areolar Tissue, which gradually becomes condensed; but its fibres never acquire any degree of contractility. 344. It is probable that the pure Muscular Fibre is identical in Chemical composition,—or nearly so,—with the Fibrin of the blood. It is, however, impossible to separate it completely from the areolar tissue, nerves, blood-vessels, fatty matter, &c, which enter into the substance of the muscle ; so that it cannot be precisely analyzed. In ordinary muscle, the solid matter forms about 23 parts in 100 ; the remainder consisting of water.—The solid matter contains about 7^ per cent, of fixed salts. 345. We now come to investigate the remarkable property, which is the distinguishing characteristic of Muscular tissue;—that of con- tracting on the application of a stimulus. Some approaches to this property are manifested by certain Vegetable structures. Thus, if the small enlargement at the base of the footstalk of the leaf of the Sensitive Plant, be touched ever so slightly, the leaf will be imme- diately drawn down by the contraction of the tissue of the part irritated. If the leaf itself be touched, the same effect results, but apparently through a different channel; the tissue of the leaf contracts where it is touched, and forces some of its fluid along the vessels of the footstalk into the upper side of the little excrescence at its base, by the distension of which the leaf is forced down. In the Dioncea muscipula, or Venus's Fly-trap, there is a similar transmission of the eflect of the stimulus from one part to another; for the two lobes of the leaf, which form the trap, are made to close together, when an insect settles upon either one of three spines which project from the surface of each lobe, or when the points of these spines are touched with any hard body. Many other instances of Vegetable movement might be brought together. Some of them are obviously produced by an enlargement or contraction of the cells, occasioned by variations in the amount of fluid they contain ; and these variations depend upon the hygrometric state of the atmosphere. With these we have nothing to do. But there are many, in which (as in the case of the Sensitive^ plant first mentioned)a stimulus applied to a part occasions the imme- diate contraction of its cells, and a consequent motion in the same part. And there are also several, in which the contraction produces motion in a distant part, as in the Dionaea; but this propagation INHERENT CONTRACTILITY OF MUSCULAR FIBRE. 211 appears to be of a simply mechanical character; being accomplished through the medium of fluid, which is forced from one part by its own contraction, and caused to distend another. 346. From these examples, however, it is evident that the property of contractility is not entirely restricted to the Animal kingdom; and we shall find that the simplest form under which it manifests itself in the Animal body, bears a close relationship with that which is dis- played in Plants. The non-striated fibre of the alimentary canal, which is subservient to the functions of Vegetative life alone, is called into action much more readily by a stimulus directly applied to itself, than it is in any other mode. Such is not the case, however, with the striated fibre, of which the muscles of Animal life are composed; this being much more readily called into action by a peculiar stimulus conveyed through the nerves supplying those muscles, than in any other more directly applied to them. 347. The Contractility of Muscular Fibre shows itself under two forms. Its most obvious and striking manifestations are those that occur in the voluntary muscles and in the heart; which, when in action, exhibit powerful contractions alternating with relaxations. The property which is concerned in these is distinguished as Irritability. On the other hand, we find that these same muscles exhibit a tend- ency to a moderate and permanent contraction, which is not shown by them when they are dead, and which cannot, therefore, be the result of elasticity or of any simple physical property; this endow- ment, which seems to exist in the greatest amount in certain forms of the non-striated fibre, is called Tonicity. 348. That the irritability of Muscles is a property inherent in them, and in this respect analogous to the peculiar vital endowments of any other forms of tissue, cannot be any longer a matter of doubt;—though many Physiologists have sought to show, that it is in some way derived from the nerves. Not only may an entire Muscle be made to contract, by the application of a proper stimulus, long after the division of the nervous trunks supplying it; but even a single fibre, completely isolated from all its nervous connections, may be seen to contract under the Microscope. Moreover, in the non-striated mus- cular fibre, it is often difficult to excite contractions through the nerves at all, when a stimulus directly applied to itself will immediately pro- duce sensible and vigorous movements. The energy of the contractile power depends in great part upon the state of nutrition of the muscle; and this again is influenced by the degree in which it is exercised. Now as the Muscles of Animal Life are all excited to action, in the usual state of things, through the medium of their nerves, it follows that if the nerves be paralyzed, the muscles will be seldom or never called into use. When disused, they will receive very little nourish- ment; the disintegrating changes will not be counterbalanced by reparative processes; and in consequence, the muscular structure will be gradually so far impaired, as to lose its peculiar properties,—and will even, in time, almost totally disappear. Yet even after the 212 INHERENT CONTRACTILITY OF MUSCULAR FIBRE. almost complete departure of muscular contractility, through the metamorphosis of the structure consequent upon disuse, it may be again recovered, if the muscles be called into exercise; but the recovery of the power is very slow, and proceeds pari passu with the improvement in the nutrition of the part, being more tedious in pro- portion to the length of the previous disuse. 349. That the Irritability of Muscular fibre belongs to itself, and is not derived in any way from the nerves, is further shown in the following manner. If a set of muscles (as those of the leg of a Rabbit or Frog) be repeatedly thrown into action by galvanism, until the stimulus will no longer occasion their contraction, their irritability is then said to be exhausted; by rest, however, it is recovered,—the nutritive processes making good the loss previously suffered. Now it has been shown by Dr. J. Reid, that this recovery may take place, even after the division of all the nerves supplying the limb; provided that the nutrition of the part be not interfered with. It has been further shown by the same excellent Physiologist, that, if the nerves of a limb be divided, the loss or retention of the contractility of the muscles entirely depends upon the degree of exercise to which they are subjected, and consequently upon the nutrition they receive. The muscles of the hind-leg of a Rabbit, whose sciatic nerve had been divided, were found to lose their contractility almost completely in the course of seven weeks. They were much smaller, paler, and softer, than the corresponding muscles of the opposite leg; and they scarcely weighed more than half as much as the latter. Now when the nerves of both hind-legs of a Frog were cut, and the muscles of one of the limbs thus paralyzed were daily exercised by a weak gal- vanic battery, whilst those of the other were allowed to remain at rest, it was found after the lapse of two months that the muscles of the exercised limb retained their original size and firmness, and con- tracted vigorously, whilst those of the other had shrunk to one-half their former size. Though the latter still retained their contractility, there could be no doubt that they would soon lose it, in consequence of the change already far advanced in their physical structure ; this change not being as rapid in cold-blooded animals, as in Birds and Mammals. 350. By these and other facts, then, it may be regarded as com- pletely proved, that the Irritability of Muscles is a vital endowment, belonging to them in virtue of their peculiar structure ;—that, so lon« as this structure is maintained in its normal condition by the nutritive processes, so long is the property capable of being manifested ;—but that any cause which interferes with the nutrition of a muscle, impairs or destroys its irritability. No cause is so effectual in doing this, as complete disuse; and no means is so sure to produce complete disuse of a muscle, as the division of its nerve, since its being called into exercise in any other way is very improbable; hence the section of the nerve is almost certain to produce, in time, the loss of the con- tractility of the muscle. But if a means be devised, by which the EFFECTS OF STIMULI ON MUSCULAR FIBRE. 213 muscle may still be called into action in any other way,—as in Dr. Reid's experiment just quoted,—its irritability is retained, because its regular nutrition is continued. 351. We have now to inquire, then, into the circumstances under which this peculiar endowment acts; or the means by which it may be called into operation, the mode in which the contraction takes place, and the conditions which are necessary for its performance.— All Muscular Fibre, which has not lost its contractility, may be made to contract by a stimulus applied directly to itself; and this stimulus may be of different kinds. The simplest is the contact of a solid sub- stance; thus we may excite muscular contractions by simply touching the fibre,—just as we cause contraction in the tissue of the Dionaea or Sensitive Plant. Most substances of strong chemical action, such as acids and alkalies, will call forth the contractility of muscular fibre, when applied to it; and the same result is produced by heat, cold, and electricity,—the last-named agent being the most powerful of all. The effect of the application of any of these stimuli varies con- siderably, according to the kind of Muscle on which it is exerted. If we irritate a portion of a muscle composed of striated fibre (any one of the voluntary muscles for example), the fasciculus of fibres which is touched will immediately contract, and that one only; and the contracted fasciculus will soon relax, without communicating its movement to any other. 352. If we irritate a portion of non-striated fibre, however, as that of the Alimentary canal, the fasciculus which is stimulated will con- tract less suddenly, but ultimately to a greater amount; its relaxation will be less speedy; and before it takes place, other fasciculi in the neighbourhood begin to contract; their contraction propagates itself to others; and so on. In this manner, successive contractions and relaxations may be produced through a considerable part of the canal, by a single prick with a scalpel; a sort of wave of contraction being transmitted in the direction of its length, and being followed by relaxa- tion. Again, in the Muscular structure of the Bladder and Uterus, powerful contractions are excited by irritation, and these produce a great degree of shortening ; but they do not alternate in the healthy state with any rapid and decided elongation; whilst, on the other hand, an irritation applied to one spot causes more extensive contrac- tions than are seen to occur as its immediate consequence in the pre- ceding cases. In the Heart, the muscular structure of a large part of the organ is thrown into rapid and energetic contraction, by a stimu- lus applied at any one point; and this contraction is speedily followed by relaxation. And in the fibrous tissue of the middle coat of the Arteries, the contraction takes place rather after the manner of that of the bladder and uterus, and a prolonged application of the stimulus is often necessary to produce the effect; but when the contraction commences, it produces a considerable degree of shortening, which takes place mother fasciculi than those directly irritated, and does not speedily give way to relaxation. 214 ACT OF MUSCULAR CONTRACTION. 353. On the other hand, when the stimuli which excite muscular contraction are applied to the nerve, which supplies a voluntary mus- cle composed of striated fibre, they produce a simultaneous contraction in the whole muscle; the effect of the stimulus being at once exerted upon every part of it. In the ordinary action of such muscles, the nervous system is always the channel through which they are called into play, whether to carry into effect the determinations of the mind (§ 391), or to perform some office necessary to the continuance of life, such as the movements concerned in Respiration (§ 394). The nerves of the striated fibre are all derived at once from the brain or spinal cord.—The ordinary actions of the non-striated fibre, on the contrary, are executed in respondence to stimuli applied directly to themselves. It is so difficult to excite contractions in it through the medium of its nerves, that many Physiologists have denied the possi- bility of doing so ; and the nerves lose their power of conveying the influence of stimuli very soon after death, although the contractility of the muscles may remain for a considerable time. The nerves of the non-striated fibre are chiefly those belonging to the Sympathetic system; but, as will be shown hereafter (Chap. XII.), those which excite motion are probably derived in reality from the Cerebro-spinal system, through the communicating branches which unite the two. 354. When a Muscle is thrown into contraction, its bulk does not appear to be at all affected. Its extremities approach, so that it is shortened in the direction of its fibres; but its diameter enlarges in the same proportion. It was formerly supposed that the ultimate fibres, in the act of contraction, threw themselves into zigzag folds; but this is now well ascertained not to be the case. The fibre, like the entire muscle, preserves its straight direction in shortening, and increases in diameter. The fibrillae themselves, as already mentioned (§ 336) exhibit an evident change, in regard to the distances of their successive light and dark portions; and the fibre, which is made up of these, exhibits, in its contracted state, a very close approximation of the transverse striae; to such an extent that they become two, three, or even four times as numerous in a given length, as they are in a similar length of a non-contracted fibre. According to Mr. Bowman's observations, the contraction usually commences at the extremities of a fibre; but it may occur also at one or more intermediate points. The first appearance of contraction is a dark spot, caused by the ap- proximation of the striae; and this gradually extends itself, so as to involve a greater or less proportion of the length of the fibre. The approximation of the solid portions forces out the fluid, which was previously contained amongst the fibrillae ; and this is seen to lie in bullae or blebs beneath the myolemma, which is drawn up into wrinkles. 355. The successive stages of the act of contraction can only be thus observed, when it takes place very slowly, as in the rigor mortis or slow contraction after death, the phenomena of which will be pre- sently noticed. But the resulting change in muscular fibres, which ACT AND CONDITIONS OF MUSCULAR CONTRACTION. 215 have been made to contract by galvanism or any other stimulus, is essentially the same. This may be best seen in transparent Entozoa, Crustacea, and others among, the lower Articulated Animals, whilst alive. Again, in persons who have died from Tetanus, a considerable number of the fibres are found to have been ruptured by the violent spasmodic action ; the contractile force, called into action by the pow- erful stimulation of the nerves, having overcome the tenacity of the fibre: and in such cases, the same approximation of the transverse striae, and proportional increase in the diameter of the fibre, are to be observed. 356. It appears that, even when considerable force of contraction is being exerted, the whole fibre is seldom or never in contraction at once; but that a continual interchange is taking place amongst its different parts,—some of them passing from the contracted to the relaxed state, as shown by the separation of the transverse striae,— whilst others are taking up the duty, and passing from the relaxed to the contracted condition, as shown by the approximation of the striae. But it is not only among the different parts of the individual fibres, that this interchange seems to take place. There is good reason to believe that, when a muscle is kept in a contracted state, by an effort of the will, for any length of time, only a part of its fibres are in con- traction at any one time; but that a constant interchange of condition takes place amongst them, some contracting whilst others are relaxing, so that the entire muscle remains contracted, whilst the state of every individual fibre may have undergone a succession of alterations. When the ear is applied to a muscle in vigorous action, an exceed- ingly rapid faint silvery vibration is heard, which seems to be attri- butable to this constant movement in its substance. 357. Thus it appears that the prolongation of the contraction of a muscle, through any length of time, is not opposed to the fact that, in the individual fibres, relaxation speedily follows contraction; but is only a peculiar manifestation of it. The ordinary movements of the Heart exhibit a different manifestation ; its fibres contracting simulta- neously, and relaxing together, instead of alternating amongst them- selves like those of a voluntary muscle.—The occasional zigzag arrangement of the fibres, which has been supposed to be their con- tracted state, is really dependent upon the approximation of their extremities, in consequence of the contraction of some neighbouring fibres, whilst their own condition is that of relaxation, it may be artificially produced by bringing together the two extremities of a fasciculus, after the irritability of the fibre has ceased; so that the flexure at determinate points must be owing simply to the physical arrangement of the parts,—perhaps to the passage of nerves or ves- sels in a transverse direction. 358. We have now to consider the conditions, which are requisite for the manifestation of Muscular Irritability. It has been already pointed out, how close is the dependence of the property upon the due nutrition of the tissue; but the property cannot be long exercised ex- 216 DEPENDENCE OF MUSCULAR CONTRACTION UPON OXYGEN. cept under another condition, which is consequently of almost equal importance,—the circulation of arterial blood through the substance of the muscle. The length of time during which the contractility remains, after the circulation has ceased, has been shown by Dr. M. Hall to vary inversely to the activity of the respiration of the animal. Thus in cold-blooded animals, the standard of whose respiration is low, the contractility remains for many hours after death, even in the voluntary muscles; and the muscles of organic life retain it with great tenacity. Thus the heart of a Frog will go on pulsating for many hours after its removal from the body ; and the heart of a Stur- geon, which had been inflated with air and hung up to dry, has been seen to continue beating, until the auricle had become absolutely so dry, as to rustle during its movements. An exceedingly feeble Gal- vanic current is sufficient to excite the muscles of these animals to contraction ; so that Matteuci, in his experiments upon Animal Elec- tricity, has been accustomed to use the prepared hind-leg of a Frog as the best indicator of the passage of an electric current. Among warm-blooded animals, the same rule holds good, in regard to the inverse proportion of the duration of irritability, and the amount of respiration; for the muscles of Birds lose this property at an earlier period after the cessation of the circulation, than do those of Mammals. From experiments on the bodies of executed criminals, who were pre- viously in good health, Nysten ascertained that, in the human subject, the contractility of the several muscular structures, as tested by Galva- nism, departs in the following time and order :—the left ventricle of the heart first; the intestinal canal at the end of 45 or 55 minutes; the urinary bladder nearly at the same time; the right ventricle after the lapse of an hour; the oesophagus at the expiration of an hour and a half; the iris a quarter of an hour later; and lastly, the ventricles of the heart, especially the right, which in one instance contracted 16£ hours after death. 359. That the circulation of arterial or oxygenated blood through the muscles, is the essential condition of the continuance of their irrita- bility, appears from this,—that after the general death of the system, and even after the removal of the brain and spinal cord, the muscles will preserve their irritability, and the action of the heart itself will continue for a long time, provided that the circulation be kept up through the lungs by artificial respiration, on the principles hereafter to be explained. (§ 688.) But if, whilst the general circulation continues, the circulation through a particular muscular part be inter- rupted, that organ will lose its contractility earlier than usual. Thus it has been shown by Mr. Erichsen, that, if the coronary arteries (supplying the substance of the heart) be tied in a dog or a rabbit, after the animal has been pithed, and the circulation is being main- tained by artificial respiration, the pulsation of the heart will only go on for about 23 minutes after the ligature has been applied, or about 33 minutes after the death of the animal; instead of continuing for 90 minutes, which it will do under other circumstances. Further, if blood DISINTEGRATION OF MUSCLE BY USE. 217 charged with carbonic acid, instead of with oxygen, circulate through the muscles, their irritability is speedily impaired, and is even de- stroyed. This is best seen, when animals are killed by being caused to breathe an atmosphere highly charged with carbonic acid ; the irri- tability of their muscles departing as soon as they are dead. In fact, the destruction of the irritability of the heart, by the circulation of venous blood through its substance, is one of the immediate causes of death. A similar effect is produced by the respiration of other gases, which are either poisonous in themselves, or which prevent the inter- change of carbonic acid and oxygen, which ought to take place in the lungs. On the other hand, when animals have been made to respire oxygen, and their blood has been consequently highly arterialized, the contractility of their muscles is retained for a longer time than usual. 360. Hence we may conclude the presence of oxygen in the blood to be one of the conditions of muscular contraction; although it is much less essential in the case of cold-blooded, than in that of warm- blooded animals. It is interesting to remark, that the muscles of hy- bernating warm-blooded Mammals are reduced for a time to the level of those of cold-blooded animals; their contractility being retained almost as long as that of the latter; thus confirming the general prin- ciple already stated as to the relation between the amount of respira- tion, and the duration of the irritability. 361. The Muscles, as we have seen, are largely supplied with blood; and the flow of blood into them increases with the use that is made of them. The demand for nutrition is obviously augmented, in proportion to the activity of the exercise of the Muscular system; for the slightest observation suffices to show, that a much smaller am ount of nourishment is sufficient to sustain the body in its normal condition, when the Muscular system is not actively exercised, than when it is in energetic operation. The quantity which is ample for an individual leading an inactive life, is far too little for the same person in the full exercise of his muscular powers. Again, there is evidence derived from observation of the relative amount of the solid matters excreted from the body under different circumstances, that a waste or disintegra- tion of the muscular tissue takes place, whenever it is actively em- ployed ; and this in a degree strictly proportional to the amount of force which it is called upon to exercise. In fact, it would appear that this waste is a necessary consequence of the exercise of the muscle ;—every act of contraction involving the death and decomposition of a certain amount of tissue. And as the presence of oxygen is always necessary for the decomposition of organic substances, so do we find that the penetration of the muscular tissue by oxygenated blood is essential to the manifestation of its contractile power. 362. Every act of contraction, then, may be said to involve the death of a certain amount of muscular tissue; and the products of decomposition which consist of the elements of muscular fibre united with the oxygen of the arterial blood, are carried off by the 218 DEPENDENCE OF IRRITABILITY UPON NUTRITION. venous current. On the other hand, the muscular substance is re- paired by an act of nutrition, at the expense of the fibrin supplied to it by the circulating fluid. There are certain muscles, as the heart, and the muscles of respiration, whose action is necessarily constant; and their reparation must take place as unceasingly as their waste. In these muscles no sense of fatigue is ever experienced. But in the muscles which are usually put in action by the will, this is not the case. Any prolonged exertion of them induces fatigue; and this fatigue is an evidence of their impaired condition, and of the necessity of rest to impart to them a renewal of vigour. The rest of muscles is essen- tial to the recovery of their powers; and this recovery is due to the nutritive operations, which then take place unchecked, and which repair the losses previously sustained. The permanently increased flow of blood to a muscle, which takes place when it is continually being called into vigorous action, is thus on the one hand occasioned by the demand for oxygenated blood created by its use, whilst on the other hand it tends to increase the power of the muscle by an aug- mentation of its nutrition. Hence it is, that the more a muscle is exercised, the more vigorous and more bulky does it become. This is equally the case whether the exercise of the muscle be voluntary or not. We see examples of it in the arms of the smith and in the legs of the opera dancer; and we have a still more striking manifesta- tion of it in those cases, in which an obstruction to the exit of urine through the urethra, has called for increased efforts on the part of the bladder, the continuance of which gives rise to an extraordinary aug- mentation in the thickness of its muscular coat. 363. Thus we see that the property of Irritability is a vital endow- ment peculiar to muscular tissue, and dependent for its existence upon due nutrition of that tissue; that it may be called into exercise by cer- tain stimuli, applied either to the muscle itself, or to the nerve sup- plying it, provided that the muscle be also permeated with oxygen ; that it may be exhausted by repeated stimulation, but is then recov- ered by rest, provided that there be no obstacle to the nutrition of the muscle; that the nutrition of the muscle is impaired by continued repose, and that its irritability diminishes in the same proportion ; that the nutrition is increased by frequent use, and that the power of the muscle then augments in like degree ; and finally that the departure of muscular power, which ensues upon the general death of the system, is dependent in part upon the cessation of the supply of oxygen, and in part upon changes in the composition of the muscle itself, which are no longer compensated by the functions that keep it in its normal condition during life.—The rapidity of these changes is the greatest in warm-blooded animals, in which also the muscular irritability is most dependent upon the presence of oxygen in the muscular sub- stance ; consequently the irritability departs after death much more speedily in these than in cold-blooded animals. 364. We have now to consider the other form of Contractility which produces a constant tendency to contraction in the Muscular TONICITY OF MUSCLES.—RIGOR MORTIS. 219 fibre, but which is so far different from simple Elasticity, that it abates after death, before decomposition has taken place. This Tonicity manifests itself in the retraction which takes place in the ends of a living muscle, when it is divided; the retraction being permanent, and greater than that of a dead muscle. It also shows itself in the permanent flexure of joints, when, by paralysis of the extensors, the tonic contraction of the flexors is not antagonized. In the healthy state, it would seem as if the tonicity of the several groups of muscles was so adjusted, as to be in mutual counterpoise ; but the balance is destroyed when, in consequence of paralysis, or of impaired nutrition from other causes, the tonicity of one set is weakened. This is the case, for example, in the lead-palsy; in which the extensors of the forearm and hand lose their power, so that the tonic contraction of the flexors keeps the fingers constantly bent upon the palm. It would seem, however, that the tonicity of the flexors is usually greater than that of the extensors; as the former predominate, when all are equally withdrawn from the control of the nervous system, in profound sleep. 365. The Tonicity is much greater, relatively to the amount of irritability, in the non-striated, than in the striated fibre; and it is particularly remarkable in the fibrous coat of the arteries, in which it is difficult to procure any decided indication of irritability by the ap- plication of stimuli. It is by this tonicity of the walls of the arteries, that they are kept in a state of constant moderate contraction upon their contents; and that, when they are emptied, they contract until the tube is nearly obliterated. If its amount be too great (as some- times happens) the artery approaches the condition of a rigid tube ; which, as will be shown hereafter, is unfavourable to the regularity of the flow of blood through it, though the rate is increased. On the other hand, if it be unduly diminished, the circulation is retarded, by the tendency of the arterial walls to yield too much to the pulse- wave. 366. This property is very greatly affected by temperature; being diminished by wTarmth, and increased by cold. Thus when an artery is exposed to the air for some time, the lowering of its temperature occasions its contraction to such an extent, that its tube may be almost obliterated. And in the operation of crimping fish, immersion of the body in cold water, after the muscles have been divided, increases the tonic contraction of the muscles, and thus improves the firmness to their substance, wThich it is the object of this operation to produce. 367. The Rigor Mortis, or death-stiffening of the muscles, is pro- bably to be regarded as a manifestation of this property, occurring after all the Irritability of the muscles has departed, but before any putrefactive change has commenced. This phenomenon is rarely absent; although it may be so slight, and may last for so short a time, as to escape observation.' The period which elapses before its com- mencement is as variable as its duration; and both seem to be de- pendent upon the vital condition of the system at the time of death. 220 RIGOR MORTIS. When it has been weakened or depressed by previous disease, the irritability of the muscles speedily departs ; and the stiffening comes on early, and lasts but a short time. Thus, after death from Typhus, the limbs have been sometimes known to stiffen within 15 or 20 minutes. On the other hand, when the general vigour of the system has not been previously impaired, and death has resulted from some sudden cause, the irritability of the muscles is of longer duration, and their stiffening is consequently deferred. The commencement of the rigidity usually takes place within seven hours after death; but twenty or even thirty hours may elapse before it shows itself. Its general duration is from twenty-four to thirty-six hours ; but it may pass off much more rapidly, or it may be prolonged through several days. It affects all the muscles composed of the striated fibre with nearly the same intensity; except that the flexors usually contract more strongly than the extensors (as in sleep), the fingers being closed upon the palm, the hand bending on the forearm, and the lower jaw being drawn firmly against the upper. And it even mani- fests itself in muscles that have been thrown out of use by paralysis, provided that their nutrition has not been seriously impaired. 368. This tonic contraction, however, is most remarkably mani- fested in the non-striated fibre ; and especially in the heart and blood- vessels. As soon as the muscular walls of the several cavities lose their irritability, they begin to contract forcibly upon their contents, and thus become stiff and firm, although they were previously flaccid. In this manner, the ventricles of the heart, which are the first parts to lose their irritability, become rigid and contracted within an hour or two after death; and usually remain in that state for ten or twelve hours, sometimes for twenty-four or thirty-six, then again becoming relaxed and flaccid. This rigid contracted state of the heart, in which the walls are thickened and the cavities diminished, was formerly supposed to be a result of disease, and was termed concentric hyper- trophy; but it is now known to be the natural condition of the organ, at the period when the rigor mortis occurs in it.—The contraction of the arterial tubes is so great, as to produce for the time a great dimi- nution in their calibre; and this, doubtless, contributes to the passage of the blood from the arterial into the venous system, which almost invariably takes place within a few hours after death. The arteries then enlarge again, and become quite flaccid, their tubes being emptied of the previous contents; and it was from this circumstance, that the ancient physiologists were led to imagine that the arteries are not destined to carry blood, but air. 369. As soon as the Rigor Mortis departs, the muscles pass into a state of decomposition; in fact, it is by the commencement of de- composition, that the cessation of this vital property is occasioned. Thus we may regard the Rigor Mortis as the last act of the Muscu- lar Contractility; and in this respect it corresponds with the coagula- tion of the blood, which also is the closing act of its life, when it is drawn from the living body, or has ceased to circulate (§ 184). FORCE DEVELOPED IN MUSCULAR CONTRACTION. 221 There are, indeed, many remarkable points of correspondence be- tween the two phenomena; which have induced some physiologists to believe, that rigor mortis is in fact nothing else than the coagula- tion of the blood in the muscles. It has been shown by Mr. Bowman, however, that the stiffening of the muscles after death is due to the permanent contraction of their component fibres, and that the coagu- lation of the blood can have nothing to do with it. Nevertheless, this contraction may be considered as being, for the muscular fibre, very much the same kind of phenomenon as the coagulation of the fluid fibrin of the blood,—especially resembling the subsequent con- traction of the clot, which takes place gradually, within a few hours after its separation. The causes which prevent the coagulation of the blood after death (§ 187), usually prevent also this last manifes- tation of the tenacity of the muscles; their vitality being completely destroyed, like that of the blood, by sudden and powerful shocks operating on the nervous system, or by the complete exhaustion con- sequent upon violent and long-continued exertion, as when animals are run to death. And again, the tonicity of muscles survives the freezing process; manifesting itself by contraction and rigidity, in a muscle that has been frozen immediately after death, and is subse- quently thawed; just as the peculiar properties of the fibrin of the blood cause its coagulation upon being thawed, if it have been frozen immediately upon being drawn from the vessels. 370. The power by which the elements of Muscular fibre are caused to approach one another in the exercise of their Contractility, differs from any other with which we are acquainted. Its complete dependence upon the life of the tissue is remarkably shown by the fact (ascertained by Valentin), that, after the cessation of the irritabi- lity, the muscles tear with a far less weight, than they were previously able to draw, when excited by galvanism; so that their contractile force is much greater than that, which the simple cohesiveness of the tissue can sustain. Moreover, it has been shown by the experiments of Schwann, that the contractile force is greatest, when the muscle is most extended; so that, with the same stimulus, it can overcome a greater resistance by its contraction, when it has been previously stretched to its full length, than it can when it has been already in part shortened by the exercise of its contractile force. The power diminishes progressively with the further shortening of the muscle ; until at last no further contraction can be produced by any stimulus, the extreme limit having been reached. Hence it seems as if the contractile force of Muscles differs completely from other forms of Attraction, as those of Gravitation, Electricity, &c; since it is the universal law of their operation, that the force increases, in proportion to the decrease between the squares of the distances between the attracting bodies; whilst, in the case of muscle the force decreases, in proportion as the distance between the attracting particles decreases. But it is to be remembered that the law of attraction just quoted sup- poses the particles to be quite free to approach one another; and this 222 NERVOUS SYSTEM;—ITS GENERAL STRUCTURE. they obviously are not in the contraction of a Muscle, since the ap- proach cannot take place without a change of place between the solid and fluid elements (§ 354). Hence it is difficult, if not impossible, to discover the law, which shall truly express the nature of the at- traction between the ultimate particles of Muscle at different dis- tances ; but the law discovered by Schwann expresses the force actually developed, at the different states of muscular contraction. 371. It has been ascertained by the researches of MM. Becquerel and Breschet, that the temperature of a muscle rises, when it is thrown into energetic contraction. The increase is ordinarily but about 1° Fahr.; but it may amount to twice as much, if the muscle be kept in action for some time, as in the exercise of sawing. Two causes may be assigned for this increase. It may depend upon the chemical changes which take place in the Muscles, as a necessary condition of the production of its force (§ 361); or it maybe the result of the fric- tion taking place between different parts, during the constant inter- change of their actions (§ 356). Perhaps both these causes concur in producing the effect. 372. The Nervous System, taken as a whole, is the instrument of all those operations, which peculiarly distinguish the Animal from the Plant; and it serves many additional purposes, connected with the Organic or Vegetative functions, which the peculiar arrangements of the Animal body involve. Wherever a distinct Nervous System can be made out (which has not yet been found possible in the lowest Animals), it consists of two very different forms of structure ; the presence of both of which, therefore, is essential to our idea of it as a whole. We observe, in the first place, that it is formed of trunks, which are distributed to the different parts of the body, especially to the muscles and to the sensory surfaces ; and of ganglia, which some- times appear merely as knots or enlargements on these trunks, but which, in other cases, have rather the character of central masses, from which the trunks proceed. Now it is easily established by experi- ment, that the active powers of the nervous system reside in the ganglia; and that the trunks serve merely as conductors of the influence which is to be propagated towards or from them. For if a trunk be divided in any part of its course, all the parts to which the portion thus cut off from the ganglion is distributed, are completely paralyzed ; that is, no impression made upon them is felt as a sensation, and no motion can be excited in them by any act of the mind. Or if the substance of the ganglion be destroyed, all the parts, which are exclusively supplied by nervous trunks proceeding from it, are in like manner paralyzed. But if, when a trunk is divided, the portion still con- nected with the ganglion be pinched, or otherwise irritated, sensations are felt, which are referred to the points supplied by the separated portion of the trunk; which shows that the part remaining in con- nection with the ganglion is still capable of conveying impressions and that the ganglion itself receives these impressions and makes them felt as sensations. On the other hand, if the separated portion STRUCTURE OF NERVOUS FIBRES. 223 of the trunk be irritated, motions are excited in the muscles which it supplies; showing that it is still capable of conveying the motor influence, though cut off from the usual source of that influence. 373. When we minutely examine the trunks of the nerves, we find that they are composed, in the first place, of a Neurilemma or nerve- sheath, consisting of white fibrous tissue ; the office of which is evi- dently that of protecting the nerve-tubes, and of isolating them from the surrounding structures, at the same time that it allows blood- vessels to pass into the interior of the trunk. From the interior of the neurilemma, thin layers of areolar tissue pass into the midst of the enclosed bundle of nervous fibres ; separating it into numerous smaller fasciculi, which are thus bound together, and supplied with blood-vessels. The capillaries are distributed very much on the same plan as those of Muscular tissue (Fig. 62); the network being com- posed of straight vessels, which run along the course of the nerve, between the nerve-tubes, and which are connected at intervals by transverse vessels.—When the neurilemma has been removed, and the trunk has been separated into its component fasciculi, we may still further subdivide the fasciculi themselves, by careful dissection, until we arrive at the ultimate nervous fibre, which is the essential element of the structure. Two forms of this fibre exist in the nerves of higher animals, bearing a considerable analogy to the two forms of the Mus- cular fibre ; one appearing to be the special instrument of the animal functions; and the other, which seems to be less perfectly formed, having a connection (the nature of which is not yet well understood) with the organic. These require a separate description. 374. The Nervous fibre, in its most complete form, is distinctly tubular. It is composed externally of a very delicate transparent mem- brane, which is apparently quite homogeneous; this is obviously analogous to the myolemma of the Muscular fibre, and serves, like it, to isolate the contained substance most completely from surrounding structures. This membranous tube is not penetrated by blood-vessels, nor does it branch or anastomose wTith others; and there is reason to believe it to be continuous from the origin to the termination of the nervous trunk. Within the tube is a hollow cylinder of a material, known as the White substance of Schwann, which differs in composi- tion and refracting power from the matter that occupies the centre of the tube, and of which the outer and inner boundaries are marked out by two distinct lines. And the centre or axis of the tube is occu- pied by a transparent substance, which is termed the axis-cylinder. There is reason to believe that this last is the essential component of the nervous fibre ; and that the hollow cylinder which surrounds it, serves, like the external investment, chiefly for its complete isolation. The whole of the matter contained in the tubular sheath is extremely soft; yielding to very slight pressure. The tubular sheath itself varies in density in different parts; being stronger in the nervous trunks, than in the substance of the brain and spinal cord. In the former, it is not difficult to show, that the regular form of the nerve-tube is a 224 STRUCTURE OF NERVOUS FIBRES AND TRUNKS. perfect cylinder ; though a little disturbance will cause an alteration in this,—a small excess of pressure in one part forcing the contents of the tube towards another, where they are more free to distend it, and thus producing a swelling. The greater delicacy of the tubular sheath in the latter causes this result to take place with yet more readiness; so that a very little manipulation, exercised upon the fibres of the brain and spinal cord, or on those of special sense, occasions them to assume a varicose or beaded appearance, which, when first observed by Ehrenberg, was thought to be characteristic of them. When the fibres of these parts, however, are examined without any such preparation, they are found to be as cylindrical as the others.— The diameter of the tubuli is usually between l-2000th and l-4000th of an inch. Sometimes, however, it is as much as l-1500th ; and occasionally as little as l-14000th. They are larger in the nerve- trunks than in the brain ; and they diminish in the latter as they ap- proach the cortical substance. The fibres of the nerves of special sense are smaller than the average, in every part of their course. 375. The organic nervous fibres (termed gelatinous by Henle) are chiefly found in the Sympathetic system, and may be regarded as its distinctive element; but, as we shall see hereafter (Chap. XII.), they are mixed up with the preceding in the ordinary nervous trunks. These fibres cannot be shown to consist of the same variety of parts as the preceding ; no tubular envelop can be distinguished; and the white substance of Schwann seems wanting. They are flattened, soft, and homogeneous in their appearance, bearing a considerable resem- blance to the unstriped Muscular fibres; and, like them, they contain numerous cell-nuclei, which are arranged with tolerable regularity. These nuclei are brought into view by acetic acid, which dissolves the rest of the fibre, leaving them unchanged. The organic fibres are usually of smaller size than the tubular, their diameter averaging between the l-6000th and the l-4000th of an inch ; and they some- times show a disposition to split into very delicate fibrillae. Being of a yellowish-gray colour, they have been sometimes distinguished as the gray fibres. 376. Both classes of fibres appear to run continuously, from one extremity of the nervous cord to the other, without anything like union or anastomosis; each ultimate fibre probably having its distinct office, which it cannot share with another. The fasciculi, or bundles of fibres, however, occasionally intermix and exchange fibres with each other; and this interchange may take place among either the fasciculi of the same trunk, or among those of different trunks. Its object is evidently to diffuse among the different branches the endow- ments of a particular set of fibres. Thus we shall hereafter see that, in all the Spinal Nerves of Vertebrata, one set of roots ministers to sensation, and another to motion; the sensory fibres are principally distributed to the skin, and the motor fibres to the muscles ; but every branch contains both sensory and motor fibres, which are brought together by the interlacement of those connected with both sets of COMMUNICATIONS BETWEEN NERVOUS TRUNKS. 225 roots. In the head, we have some nervous trunks which have sen- sory roots alone; and others which have motor roots only ; these in like manner acquire each other's functions in some degree by an interchange of filaments,—the sensory trunk receiving motor fibres, and the motor trunk receiving sensory fibres. An interchange of this kind, upon a very extensive scale, takes place between the Cerebro-spinal system, whose ganglionic centres are the brain and spinal cord, and the sympathetic system, whose centres consist of a number of scattered ganglia. The former sends a large number of tubular fibres into the latter, by the twigs of communication near the origins of the Spinal nerves, as well as by their connecting branches; whilst the latter sends a smaller number of gray or organic fibres into the former. 377. Sometimes we find the fasciculi of several distinct trunks united into an extensive plexus; the sole object of which appears to be, to give a more advantageous distribution to fibres, which all possess corresponding endowments. Thus the brachial plexus mixes together the fibres arising by five pairs of roots, on either side, from the spinal cord; and sends off five principal trunks to supply the arm. Now if each of these trunks had arisen by itself, from a distinct seg- ment of the spinal cord, so that the parts on which it is distributed had only a single connection with the nervous centres, they would have been much more liable to paralysis than they are. By means of the plexus, every part is supplied with fibres arising from each of the five segments of the spinal cord; and the functions of the whole must, therefore, be suspended, before complete paralysis of any part could occur, from a cause which operates above the plexus. This may be experimentally shown on the Frog, whose crural plexus is formed by the interlacement of the component fasciculi of three trunks on each side; for section of the roots of one of these produces little effect on the general movements of the limb; and even when two are divided, there is no paralysis of any of its actions, all being weakened in nearly an equal degree. It is possible that by the plexiform arrangement, a consentaneousness of action is in some degree favoured, where several distinct motions are to be combined in one movement; something of the same kind is to be met with in numerous instances, among the lower animals, in which the same purpose has to be attained. 378. The second primary element of the Nervous System, without which the fibrous portion would seem to be totally inoperative, is composed of nucleated cells, containing a finely granular substance, and lying somewhat loosely in the midst of a minute plexus of blood- vessels. Their normal form may be regarded as globular (hence they have been termed nerve or ganglion-globules); but this is liable to alteration from the compression they suffer, so that they may be- come oval or polygonal. The most remarkable change of form, however, which they undergo, is by an extension into one or more long processes, giving them a caudate or a stellate aspect. These 15 226 VESICULAR NERVOUS SUBSTANCE. Fig. 64. processes, according to Messrs. Todd and Bowman, are composed of a finely-granular substance, resembling that of the interior of the vesicle, with which they seem to be dis- tinctly continuous. They are very liable to break off near the vesicle; but if traced to a distance, they are found to divide and subdivide, and at last to give off some extremely fine transparent fibres, which seem to interlace with those of other stellate cells, and which may perhaps (though this is at present only a surmise) become continuous with the axis-cylin- ders of the nerve-tubes. The size of the vesicles is liable to great variation; the globular ones are usually between l-300th and l-1250th of an inch in diameter.—Besides the finely-granular substance just mentioned, these cells usually contain Capillary Network of Nervous Centres. Primitive fibres and ganglionic globules of human brain, after Purkinje. a, ganglionic globules lying amongst varicose nerve-tubes, and blood-vessels, in substance of optic thalamus; a, globule more enlarged; 6, small vascular trunk, b, b, globules with variously-formed peduncles, from dark portion of crus cerebri. 350 Diam. a collection of pigment-granules, which give them a reddish or yel- lowish-brown colour. This, however, is frequently absent, especially among the lower animals. 379. The vesicles just described are aggregated together in masses of variable size ; and are in some degree held together by the plexus of blood-vessels, in the midst of which they lie. They are sometimes imbedded in a soft granular substance, which adheres closely to their exterior and to their processes; this is the case in the outer part of the cortical substance of the human brain. In other instances, each cell is enclosed in a distinct envelop, composed of smaller cells closely adherent to each other and to the contained cell; such an STRUCTURE OF NERVOUS GANGLIA. 227 arrangement is common in the smaller ganglia, and in the inner por- tion of the cortical substance of the brain.—The substance, which is made up of these peculiar cells, of the plexus of the blood-vessels in which they lie, and of the granular matter that is disposed amongst them, is altogether commonly known as the cineritious or gray sub- stance ; being distinguished by its colour, in Man and the higher ani- mals at least, from the white substance (composed of nerve-tubes) of which the trunks of the nerves, as well as a large part of the brain and spinal cord, are made up. But this distinction is by no means constant; for the gray colour, which is partly due to the pigment- granules of the cells, and partly to the redness of the blood in the vessels, is wanting in the Invertebrata generally, and is not charac- teristically seen in the classes of Fishes and Reptiles. Moreover, when the ganglionic substance exists in small amount, even in Man, its colour is not sufficiently intense to serve to distinguish it; and, as we have already seen, there are nerve fibres which possess a grayish hue. The real distinction evidently lies in the form of the ultimate structure, which is fibrous in the one case, and cellular or vesicular in the other; and these terms will be henceforth used to characterize the two kinds of Nervous tissue, which have been now described. 380. A ganglion, then, essentially consists of a collection of nerve- vesicles or ganglion-globules, interspersed among the nerve-fibres; and it is in the presence of the former, that it differs from a plexus, which it frequently resembles in the arrangement of the latter. When a nerve enters a ganglion, its component fibres separate and pass through the ganglion in different directions, so as to be variously distributed among the branches which pass out of it. In their Dorsal o-nnglion of Sympathetic nerve of Mouse;— a, b, cords of connection with adjacent sympa- thetic ganglia; c, e, c, c, branches to the viscera and spinal nerve; d, ganglionic globules or cells; e, nervous fibres crossing the ganglion. course, they come in contact with the vesicular matter, which occu- pies the interior of the ganglion: and it appears from Mr. Newport's 228 CONNECTION OF FIBROUS AND VESICULAR STRUCTURES. observations, that they then become softer, and that their diameter increases.—The only exception to the general fact, that the vesicular matter occupies the centre of the ganglia, occurs in the brain of Vertebrata, in which it is chiefly disposed on the exterior, forming the cortical envelop. The reason for this variation is probably to be found in the very large amount of this substance, which the brain of the higher Vertebrata contains; and in the necessity of the free access of blood-vessels to it, which is provided for by a great extension of its surface beneath the investing vascular membrane (pia mater), more readily than it could be in any other mode. 381. But the vesicular matter is not found in the central masses only of the Nervous System; for it presents itself also at those parts of the surface or periphery, which are peculiarly destined to receive the impressions that are to be conveyed to the central organs. Thus on the expansion of the optic nerve which forms the retina, there is a distinct layer of ganglionic corpuscles or nerve-cells, with a minute plexus of vessels; possessing all the essential characters of the vesi- cular substance of the brain. Something of the same kind has been seen in connection with the corresponding expansions of the olfactive and auditory nerves ; and it is probable that the same elements exist in the papillce of the tongue and skin, to which the nerves of taste and touch are distributed. In these papillae, the nervous fibres seem to form loops, which are accompanied by similar loops of blood-ves- sels (Figs. 67 and 68). Hence we may state it as a general fact, Fig. 67. Fig. 68. Distribution of the tactile nerves at the surface of Capillary network at margin of lips. the lip; as seen in a thin perpendicular section of the skin. that, wherever a change is to be originated, we find vesicular matter with capillary blood-vessels; whilst for the conduction of such a change to distant parts, the fibrous structure is alone required. 382. The connection between the fibrous and vesicular portions of the Nervous system, has not yet been clearly traced. It is quite certain that, as already remarked, many of the nerve fibres which enter a ganglion, come into contact with its cells, passing over or amongst them, and then issuing from it again. And this seems to be the case also with many of the fibres which enter the vesicular matter CHEMICAL COMPOSITION OF NERVOUS SUBSTANCE. 229 of the Spinal cord, and the cortical substance of the brain. Some observations recently made by Dr. Lonsdale on the structure of the nervous system of foetuses, in which the brain and spinal cord were wranting, present a remarkable confirmation of this view. The nervous cords were for the most part developed; and at their origins (so called), or central extremities, they were found to hang as loose threads in the cavities of the cranium and spine. On examining these threads, it was found that the nerve-tubes of which they con- sisted formed distinct loops, each of which was composed of a fibre that entered the cavity and then returned from it. These loops were imbedded in granular matter, resembling that interposed between the vesicles in the cortical substance of the brain, and perhaps to be regarded as vesicular substance in an early stage of its formation. All that is known of the laws regulating the formation of irregular productions like these, leads us to the belief, that we may rightly consider this arrangement of the nerve-tubes as one which exists in the nervous centres when they are fully developed. On the other hand, the appearances observed by Messrs. Todd and Bowman appear to indicate, that some of the fibres originate directly in the subdivisions of the filamentous prolongations of the nerve-cells; this, however, must still be regarded as an unsettled question. 383. We have now to speak briefly of the Chemical Composition of the Nervous matter;—a consideration which will be presently shown to be of much importance. As formerly remarked (§7), the vital activity of a tissue is usually greater, as the proportion of its solid to its fluid contents is less; and this rule holds good most strik- ingly in regard to the Nervous substance, the vital activity of which is far greater than that of any other tissue, and the solid matter of which usually constitutes no more than a fourth, and occasionally does not exceed an eighth, of its entire weight. The proportion of water is greatest in infancy and least in middle life; and it has been observed to be under the average in idiots. Of the solid matter of the brain, about a third consists of fibrin or albumen; which is pro- bably the material of the membrane of the tubuli, as well as of the tissue that connects them.—It is chiefly with the Fatty matter, which constitutes about a third of the solid substance, that the attention of Chemists has been occupied. This is stated by M. Fremy (one of the most recent analysts) to contain, besides the ordinary fatty mat- ters, and Cholesterine or biliary fat, two peculiar fatty acids, termed the Cerebric and the Oleo-phosphoric. Cerebric acid, when purified, is white, and presents itself in crystaline grains. It contains a small proportion of Phosphorus; and differs from the ordinary fatty matter in containing Nitrogen, as also in containing twice their proportion of oxygen. Oleo-phosphoric acid is separated from the former by its solubility in ether; it is of a viscid consistence; but when boiled for a long time in water or alcohol, it gradually loses its viscidity, and resolves itself into a pure oil, which is elaine, while phosphoric acid remains in the liquor. The proportion of phosphorus in the brain is 230 WASTE AND RENEWAL OF NERVOUS SUBSTANCE. considerable; being from 8 to 18 parts in 1000 of the whole mass, or from l-20th to l-30th of the whole solid matter. It seems to be un- usually deficient in the brain of idiots.—The remaining third and sometimes more, is composed of a substance termed Osmazome (which seems to be a proteine-compound in a state of decomposi- tion), together with saline matter.—No satisfactory examination has yet been made into the comparative composition of the vesicular and fibrous substances ; but according to Lassaigne, the former contains much more water than the latter, and little colourless fat, but nearly 4 per cent, of red fat, which does not exist in the other. 384. Various circumstances lead to the belief, that the Nervous tissue, during the whole period of active life, is continually undergoing changes in its substance, by decay and renewal. We know that, after death, it is one of the first of all the animal tissues to exhibit signs of decomposition ; and there is no reason to suppose, that this tendency is absent during life. Hence for the simple maintenance of its normal character, a considerable amount of nutritive change must be required. But many circumstances further lead to the conclusion, that, like all other tissues actively concerned in the vital operations, Nervous matter is subject to a waste or disintegration, which bears an exact proportion to the activity of its operations;—or, in other words, that every act of the Nervous system involves the death and decay of a certain amount of Nervous matter, the replacement of which will be requisite in order to maintain the system in a state fit for action. We shall hereafter see, that there are certain parts of the Nervous system, particularly those which put in action the respiratory muscles, wThich are in a state of unceasing, though moderate, activity; and in these, the constant nutrition is sufficient to repair the effects of the constant decay. But those parts, which operate in a more power- ful and energetic manner, and which therefore waste more rapidly when in action, need a season of rest for their reparation. Thus a sense of fatigue is experienced, when the mind has been long acting through its instrument—the brain ; indicating the necessity for rest and reparation. And when sleep, or cessation of the cerebral func- tions, comes on, the process of nutrition takes place with unchecked energy, counterbalances the results of the previous waste, and pre- pares the organ for a renewal of its activity. In the healthy state of the body, when the exertion of the nervous system by day does not exceed that, which the repose of the night may compensate, it is maintained in a condition which fits it for constant moderate exercise ; but unusual demands upon its powers,—whether by the long-con- tinued and severe exercise of the intellect, by excitement of the emo- tions, or by the combination of both in that state of anxiety which the circumstances of man's condition too frequently induce,—produce an unusual waste, which requires, for the complete restoration of its powers, a prolonged repose. 385. There can be no doubt that (from causes which are not known) the amount of sleep required by different persons, for the WASTE AND RENEWAL OF NERVOUS SUBSTANCE. 231 maintenance of a healthy condition of the nervous system, varies considerably; some being able to dispense with it, to a degree which would be exceedingly injurious to others of no greater mental ac- tivity. Where a prolonged exertion of the mind has been made, and the natural tendency to sleep has been habitually resisted, by a strong effort of the will, injurious results are sure to follow. The bodily health breaks down, and too frequently the mind itself is permanently enfeebled. It is obvious that the nutrition of the Nervous system becomes completely deranged ; and that the tissue is no longer formed, in the manner requisite for the discharge of its healthy func- tions. 386. As the amount of Muscular tissue that has undergone disin- tegration, is represented (other things being equal) by the quantity of urea in the urine, so do we find that an unusual waste of the nervous matter is indicated by an increase in the amount of phosphatic depo- sits. No others of the soft tissues contain any large proportion of phosphorus; and the marked increase in these deposits, which has been continually observed to accompany long-continued wear of mind, whether by intellectual exertion, or by anxiety, can scarcely be set down to any other cause. The most satisfactory proof is to be found in cases, in which there is a periodical demand upon the mental powers; as, for example, among clergymen, in the preparation for, and discharge of, their Sunday duties. This is found to be almost invariably followed by the appearance of a large quantity of the phos- phates in the urine. And in cases in which constant and severe intellectual exertion has impaired the nutrition of the brain, and has consequently weakened the mental power, it is found that any pre- mature attempt to renew the activity of its exercise, causes the re-appearance of the excessive phosphatic discharge, which indicates an undue waste of nervous matter. 387. As the disintegration of the Nervous System is thus propor- tional to its exercise, so must its reparation make a corresponding demand upon the nutritive processes. And accordingly we find, that it is very copiously supplied with blood-vessels; and that the amount of food appropriated to its maintenance in an active condition, is very considerable. This we know from the fact, that persons of active minds, but sedentary bodily habits, commonly require nearly as much food as those in whom the waste of the Muscular system is greater, and that of the Nervous system less, in virtue of their bodily activity and the less energetic operation of their minds. 388. The first development of the nerve-fibres appears to take place, like that of Muscular fibre, by the coalescence of a number of primary cells into a continuous tube; the granular fatty matter within being the product of a subsequent secreting-action. The nuclei of the original cells may be frequently seen in the nerve-tubes at a later period, lying between their membranous walls and the substance deposited in their interior. It is probable that the nerve-tubes undergo little change, from the period of their first production to 232 REGENERATION OF NERVOUS SUBSTANCE. that of their final decay; their function, as will be presently shown, being of a much more passive character, than that of the vesicular substance. On the other hand, the vesicular matter appears to be in a state of continual change, as is the case with all cells whose func- tions are active. The appearances observed by Henle in the cortical substance of the brain lead to the belief, that there is as continual a succession of nerve-cells as there is of epidermic cells; their de- velopment commencing at the surface, where they are most copiously supplied with blood-vessels from the investing membrane, and pro- ceeding as they are carried towards the inner layers, where they come into more immediate relation with the fibrous portion of the nerve-structure. This change of place is probably due to the con- tinual death and decay of the mature cells, where they are connected with the fibres; and the constant production of new generations at the external surface,—thus carrying the previously-formed cells in- wards, in precisely the same manner that the epidermic cells are progressively carried outwards. 389. The regeneration of Nervous tissue that has been destroyed, takes place very readily in continuity with that which is left sound. This may be more easily proved by the return of the sensory and motor endowments of the part, whose nerves have been separated, than by microscopic examination of the reunited trunks themselves, which is not always satisfactory. All our knowledge of the functions of the nervous system leads to the belief, that perfect continuity of the nerve-tubes is requisite for the conduction of an impression of any kind,—whether this be destined to produce motion or sensation; and various facts, well known to Surgeons, prove that such restora- tion may be complete. In the various operations which are prac- tised for the restoration of lost parts, a portion of tissue removed from one spot is grafted, as it were, upon another; its original at- tachments are more or less completely severed,—frequently entirely destroyed,—and new ones are formed. Now in such a part, as long as its original connections exist, and the new ones are not completely formed, the sensation is referred to the spot from which it was taken; thus when a new nose is made, by partly detaching and bringing down a piece of skin from the forehead, the patient at first feels, when anything touches the tip of his nose, as the contact were really with his forehead. After time has been given, however, for the establishment of new connections with the parts, into whose neigh- bourhood it has been brought, the old connections of the grafted portion are completely severed; and an interval then ensues, during which it frequently loses all sensibility; but after a time its power of feeling is restored, and the sensations received through it are referred to the right spot.—A more familiar case is the regeneration of Skin, containing sensory nerves, which takes place in the well-managed healing of wounds involving loss of substance. Here there must obviously be, not merely a prolongation of the nerve-tubes from the subjacent and surrounding trunks, but also a formation of new sen- FUNCTIONAL CONNECTION OF BRAIN AND NERVES. 233 sory papillae.—A still more striking example of the regeneration of Nervous tissue, however, is to be found in those cases (of which there are now several on record), in wThich portions of the extremi- ties, that have been completely severed by accident, have been made to adhere to the stump, and have in time completely recovered their connection with the Nervous as with the other systems,—as indi- cated by the restoration of their sensory and motor endowments.— Of the degree in which the vesicular substance of the Nervous sys- tem may be regenerated, we have no certain knowledge; but there can be little doubt, from the activity of its usual nutritive changes, that a complete reproduction may be effected in cases of loss of sub- stance, where it can commence from a neighbouring mass of the same tissue. 390. We have now to inquire into the conditions under which the peculiar properties of the Nervous System are manifested in an active form; and it will first be desirable to explain, somewhat more in detail, the nature of the different operations to which it is sub- servient. These operations present themselves, in their most com- plex form, in Man and the higher animals; but they may often be most satisfactorily studied in the lower. In the first place, when an impression is made upon any part of the surface of the body by mechanical contact, by heat, electricity, or any other similar agent, —or upon the organs of special sense (the eye and ear, the nose and tongue), by light or sound, by odorous or sapid bodies,—these im- pressions, in the healthy and wakeful state of the Nervous system are felt as sensations; that is, the mind is rendered conscious of them. Now there can be no doubt that the mind is immediately influenced, not by the impression in the remote organ, but by a cer- tain change in the condition of the brain, excited or aroused by that which has originated elsewhere. For if the communication with the brain be cut off, no impression on the distant parts of the nervous system is felt, notwithstanding that the mind remains perfectly capa- ble of receiving it. The mind, then, is only rendered conscious of external objects by the influence which they exert upon the brain, or upon a certain part of it, which, being the peculiar seat of sensation, is called the sensorium. Hence we recognize, in the process by which the mind is rendered conscious of external objects, three dis- tinct stages ;—first, the reception of the impression at the extremities of the sensory nerve; second, the conduction of the impression, along the trunk of the nerve, to the sensorium; third, the change excited by it in the sensorium itself, through which sensation is produced. Here, then, the change in the condition of the nervous system commences at the circumference, and is transmitted to the centre; and the fibres which are concerned in this transmission are termed sensory. 391. On the other hand, when an emotion, an instinctive impulse, or an act of the will, operates through the brain to produce a muscular contraction, the first change is in the condition of the brain itself or of 234 ACTION OF NERVES INDEPENDENTLY OF THE BRAIN. a certain part of it. The influence of this change is transmitted by the motor nerves to the muscles, among which they are distributed ; and the desired movement is the result. Here, too, we have three stages ; first, the origination of the change by the act of the mind upon the brain ; second, the conduction of that change along the motor nerves ; and third, the stimulation of the muscles to contraction. But the operation here commences at the centre; and the effects of the change in the brain are transmitted to the circumference, by a set of nervous fibres which are termed motor. The complete distinctness of these two classes of fibres was first established by Sir C. Bell. It is best seen in the nerves of the head, of which some are purely sen- sory, and others purely motor; but it may also be clearly proved to exist at the roots of the spinal nerves (although their trunks possess mixed endowments), the posterior being sensory, whilst the anterior are motor. 392. But although sensations can only be felt through the brain, and voluntary motions can only be produced by an action of the mind through the same organ, yet there are many changes in the animal body, in which the nervous system is concerned, which yet do not involve the operation of the brain, being produced without our con- sciousness being necessarily excited, and without any act of the will, or even in opposition to its efforts. Of these actions, the spinal cord of Vertebrata, and its prolongation within the cranium, are the chief instruments ; in the Invertebrate animals, they are performed by various ganglia, which are usually disposed in the neighbourhood of the organs to which they minister. If the spinal cord of a Frog be divided in its back, above the crural plexus, so as entirely to cut off the nerves of the lower extremities from connection with the brain, the animal loses all voluntary control over these limbs, and no sign of pain is produced by any injury done to them. But they are not thereby rendered motionless ; for various stimuli applied to the limbs themselves will cause movements in them. Thus if the skin of the foot be pinched, or if a flame be applied to it, the leg will be violently retracted. Or, if the cloaca be irritated by a probe, the feet will endeavour to push away the instrument. We have no reason hence to believe, that the animal feels the irritation, or intends to execute these movements, in order to escape from it; for motions of a similar kind are exhibited by men, who have suffered injury of the lower part of the spinal cord, and who are utterly unconscious, either of the irritation, or of the action their limbs perform. 393. We are not to suppose, however, that the stimulus acts at once upon the muscles, without the nervous system being concerned at all; throwing them into contraction by its direct influence. For it is quite certain that, unless the nervous trunks remain continuous with the spinal cord, and unless the part of the spinal cord with which they are connected remains sound (although cut off from connection with the parts above, and with the brain), no action will be the result. If the trunks be divided, or either of the roots by which they are con- REFLEX ACTION.—DISTINCT NERVOUS FIBRES. 235 nected with the spinal cord be severed, or the lower portion of the spinal cord itself be injured, no stimulation will cause the muscular movements just described. A very good example of the necessity of the completeness of the nervous trunks, which convey impressions to and from the central organ, is found in the movements of the iris, for the contraction and dilatation of the pupil. Here, the stimulus of light upon the retina gives rise to a change in the condition of the optic nerve ; which, being transmitted to a certain portion of the encephalon with which that nerve is connected, excites there a motor impulse ; and this impulse is conveyed through a distinct nerve (a branch of the third pair) to the iris, occasioning contraction of the pupil. Every one knows that this adjustment of the size of the pupil to the amount of light, is effected without any exertion of the will on his own part, and even without any consciousness that it is taking place. It is performed, too, during profound sleep ; when the influ- ence of light upon the retina excites no consciousness of its presence, —when no sensation, therefore, is produced by it. 394. The class of actions thus performed, is termed reflex; and we see that every such action involves the following series of changes. In the first place, an impression is made upon the extremity of a nerve, by some external agent; just as when sensation is to be pro- duced. Secondly, this impression is transmitted by a nervous trunk to the spinal cord in Vertebrata, or to some ganglionic mass which answers to it in the Invertebrata. But instead of being communi- cated by its means to the mind, and becoming a sensation, it imme- diately and necessarily executes a motor impulse ; which is reflected back as it were to certain muscles, and by their contraction, gives rise to a movement. We shall hereafter see, that nearly all those move- ments in the animal body, which are immediately connected with the maintenance of the organic functions, such as those of respiration, deglutition (or swallowing), the expulsion of the feces, urine and fcetus, &c.—are performed in this manner. 395. Now there is no reason to believe, that the mode in which impressions are conducted by the nervous trunks, whether towards or from the nervous centres, is in any way different from that which takes place, when sensations are to be produced, or voluntary motions executed. The endowments of the trunks appear to be the same in both instances ; but those of the centres are different. We shall here- after see, that the very same trunks contain fibres, originating at the same part of the surface, of which some go to the brain, and others to the spinal cord; impressions on the former, therefore, will produce sensations, whilst similar impressions on the latter will give rise to no sensations, but will excite a motor influence in immediate respondence to their call. Again, the motor fibres which pass forth from the spinal cord, and which convey the reflex influence created by its vesicular substance, are bound up in the same trunk with others, which proceed from the brain, and which convey the influence of the will, communi- cated through its gray or vesicular matter. Thus we have at least 236 NATURE OF THE NERVOUS POWER. two sets of fibres conveying impressions inwards or centripetally; one of them being sensory (as already explained § 390) in virtue of its connection with the brain ; whilst the other is excitor, or destined to excite reflex movements, through the spinal cord. These, taken collectively, may be termed afferent or centripetal fibres. On the other hand, there are at least two sets of fibres conveying motor im- pulses to the muscles; one of them communicating the influence of the mind, operating through the brain; whilst the other merely trans- mits the reflex power of the spinal cord. These in conjunction may be called efferent or centrifugal fibres. The following diagram may assist the Student in comprehending the relations of the elementary parts of the Nervous System. 396. Of the mode by which the effects of changes in one part of the Nervous System, are thus instantaneously transmitted to another, nothing whatever is known. There is evidently a strong analogy between this phenomenon, and the instantaneous transmission of the Fig. 69. I Diagram of the origins and terminations of the dif- ferent groups of nervous fibres;—a, a, vesicular sub- stance of the spinal cord; 6, b, b, vesicular substance of the brain ; e, vesicular substance at the commence- ment of afferent nerve, which consists of, el, the ce- rebral division, or sensory nerve, passing on to the brain, and si, the spinal division, or excitor nerve, which terminates in the vesicular substance of the spinal cord; on the other side we have the efferent or motor nerve proceeding to the muscle d, likewise con- sistingof two divisions,—c2, the cerebral portion, pro- ceeding from the brain, and conveying the influence of the will or of instinct; and s2, the spinal division, conveying the reflex power of the spinal cord. Electric power along good conductors; and there is this further analogy between the Nervous and Electric agencies, that the latter will produce many of the effects of the former. Thus a very feeble galvanic current transmitted along a motor nerve, serves to excite contractions in the muscles supplied by it; and in like manner, a galvanic current transmitted along any of the sensory nerves, gives rise to a sensation of the kind to which the nerve ministers. Moreover, we shall hereafter see, that certain animals are capable of generating Electric power in a very remarkable manner (Chap. X.); and that the nervous system is in some way essentially concerned in this opera- tion. But on the other hand, it is quite certain that the influence transmitted along the nerves of the living body is not ordinary electricity; for all attempts to procure manifestations of electric changes in the state of nerves, that are acting most energetically on muscles, have completely failed ; and a nerve remains capable of conveying the influence of electricity, when it has been rendered unable to transmit the influence of the brain,—as by tying a ligature CONDITIONS OF NERVOUS ACTION. 237 round it, or by tightly compressing it between the forceps, which gives no interruption to the one agency, whilst it completely checks the other.—Notwithstanding, then, the strong analogy which exists between these two powers, we are not warranted in regarding them as identical; although it is very possible that the Nervous power may be in time shown (as Magnetism has been proved) to be a peculiar modification of ordinary Electricity, acting under circumstances in many respects dissimilar, and therefore appearing to possess distinct properties. 397. It is more desirable, however, that we should understand the conditions under which the phenomena of the Nervous system take place, than that we should spend much time in discussing the identity of its peculiar powers with any others in Nature. The conducting power of the nervous fibres appears to remain with little decrease for some time after death, especially in cold-blooded animals ; for we can, by pinching, pricking, or otherwise stimulating the motor trunks, give rise to contractions in the muscles supplied by them, exactly as during life. This power is much lessened by the influence of narcotics; so that if a nervous trunk be soaked in a solution of opium, belladonna, or other powerful narcotic, it ceases to be able to convey the effects of stimuli to the muscles, some time before the muscles themselves lose their contractile power. On the other hand, it seems to be exalted by various irritating influences; so that, when the nervous trunk has been treated with strychnia, or w-hen it has been subjected to undue excitement in other ways, a very slight change is magnified (as it were) during its transmission, and produces effects of unusual intensity. 398. Now although the conducting power of the fibrous structure will continue for a time, after the circulation through it has ceased, the peculiar endowments of the vesicular substance, by which it ori- ginates the changes which the former transmits, are only manifested, when blood is moving through its capillaries. Thus if the circula- tion through the brain cease but for a moment, total insensibility, and loss of the power of voluntary motion, immediately supervene. The brain is supplied with blood through four arteries,—the two internal carotids, and the two vertebrals ; and by the communication of these with each other through the circle of Willis, the circulation will still be kept up, if only one of them should convey blood into the cavity of the cranium. Hence it is necessary that the flow of blood should be checked through all of them, in order that the functions of the brain should be suspended; and the suspension is then complete and instantaneous. The best method of effecting this was devised by Sir Astley Cooper. He tied both the carotid arteries in a dog; which, for the reason just mentioned, did not produce any decided influence on the functions of the brain, the circulation being kept up through the vertebrals. But upon compressing the latter, so as to suspend the flow of blood through them, immediate insensibility, and loss of vo- luntary power, were the result. When the compression was taken 238 DEPENDENCE OF NERVOUS POWER ON SUPPLY OF BLOOD. off, the animal immediately returned to its usual state ; and again became suddenly insensible, when the pressure was renewed. Al- though the functions of the brain were thus suspended, those of the spinal cord were not; as was shown by .the occurrence of convulsive movements. But in the state called Syncope, or fainting, the suspen- sion of the circulation, by a failure in the heart's action, causes an entire loss of power in both these centres; and a complete cessation of muscular movement is the result. This condition may come on instantaneously, under the influence of powerful mental emotion, or of some other cause, which act primarily in suspending the heart's action, and consequently in checking the circulation; the insensi- bility, and loss of muscular power, are secondary results, depending upon the suspension of the powers of the nervous centres, consequent upon the cessation of the flow of blood through them. 399. The due activity of the vesicular nervous matter is not only dependent upon a sufficient supply of blood, but it requires that this blood should be in a state of extreme purity; for there is no tissue in the body, whose functions are so readily deranged, by any departure from the regular standard in the circulating fluid,—whether this con- sist in the alteration of the proportions of its normal ingredients, or in the introduction of other substances which have no proper place in it. One of the most fertile sources of disturbance in the action of the brain, consists in the retention of substances within the blood, which ought to be excreted from it. We shall hereafter see, that three of the largest and most important organs in the body,—the lungs, the liver, and the kidneys,—have it for their special office, to separate from the circulating fluid the products of the decomposition, which is continually taking place in the body ; and thereby to main- tain its purity, and its fitness for its important functions. Now. if these, from any cause, even partially fail in their office, speedy dis- turbance of the functions of the nervous centres is the result. Thus if the lungs do not purify the venous blood of its impregnation of carbonic acid, or restore to it the proper proportion of oxygen, the functions of the brain are seriously affected. The sensations become indistinct, the will loses its control over the muscles, giddiness and faintness come on, and at last complete insensibility supervenes. Corresponding symptoms occur, though to a less serious degree, when the excretion of carbonic acid is but slightly impeded. Thus when a number of persons are shut up in an ill-ventilated apartment, for a sufficient length of time to raise the proportion of carbonic acid in the air to 1 or 2 per cent, the continued purification of their blood by respiration is but insufficiently performed, for reasons which will be stated hereafter (Chap. VIII.); and the carbonic acid accumulates in their blood in a sufficient degree, to produce headache and obtuse- ness of the mental powers.—Similar results take place, as will be shown hereafter, from the retention of the substances, which ought to be drawn off by the liver and kidneys; these, when they accumulate in even a trifling degree, produce torpor of the functions of the brain; EFFECTS OF STIMULANTS UPON NERVOUS POWER. 239 and, when their proportion increases, complete cessation of its pow- ers is the result, their action being precisely that of narcotic poisons. Various substances introduced into the blood may exert similar influ- ences ; depressing the activity of the vesicular substance of the nerv- ous centres, and consequently producing torpidity, not merely in regard to the reception of impressions, and the performance of volun- tary motions, but also in the mental operations generally. 400. On the other hand, various conditions of the blood, espe- cially those depending on the presence of certain external agents, produce an undue energy in the functions of the nervous centres; which energy, however, is almost invariably accompanied by irregu- larity, or want of balance among the different actions. Of this we have a familiar example in the operation of alcohol. Its first effect, when taken in moderate quantity, is usually to produce a simple increase in the activity of the cerebral functions. A further dose, however, occasions not merely an increase, but an irregularity; de- stroying that power of self-control, which is so important a means of balancing the different tendencies in the healthy condition of the mind. And a still larger dose has the effect of a narcotic poison; producing diminution or suspension of activity in all the functions of the brain. In some persons, this is the mode in which the alcohol acts from the first,—its stimulating effects being altogether wanting. —A similar activity is usually produced by the respiration of the nitreous oxide; which seems to increase all the powers of the mind, save that of self-control, which it diminishes; the individual, while under its influence, being the slave of his impulses, which act on his muscular system with astonishing energy. Very analogous to this, is the incipient stage of mania, which is simply an undue eneigy of the cerebral functions, at first in some degree under the control of the will, but afterwards increasing to an extent that ren- ders the individual completely powerless over himself; and showing itself in the intensity of the sensations produced by external objects, in the vividness of the trains of thought, (which, being entirely uncontrolled, succeed each other with apparent irregularity, though probably according to the laws of association and suggestion,) and in the violence of the muscular actions. Such a state may continue for some time, without the intervention of sleep; but the subsequent exhaustion of nervous power is proportioned to the duration of the excitement; and frequent attacks of mania almost invariably subside at last into imbecility. 401. In these cases of undue excitement, there is obviously an increase in the supply of blood to the head, as indicated by the suf- fusion of the face, the injection of the conjunctiva, the throbbing at the temples, the pulsation of the carotids; and we find that measures which diminish the activity of the circulation through the brain are those most effectual in subduing the excitement. But it does not at all follow, that this undue action of the brain should be connected with an excess in the whole amount of nutritive material, and should 240 DEPENDENCE OF NERVOUS POWER ON SUPPLY OF BLOOD. require general depletion for its treatment. In fact, a very similar class of symptoms may present itself under two conditions of an entirely opposite kind,—inflammation, accompanied with an increase in the proportion of fibrin in the blood, and requiring treatment of a lowering kind,—and irritation, depending on a state of blood in which there is a deficiency of solid materials, and requiring a strengthening and even a stimulating regimen. The skill of the practitioner is often put to the test, in the due discrimination between these states. 402. The preceding examples mark the influence of various causes upon the actions of the vesicular matter of the brain; others might be adduced to show that the vesicular substance of the spinal cord is also liable to have its powers depressed or excited; but these will be best adverted to hereafter, when the distinct functions of that organ are under consideration (Chap. XII.). We may simply notice, that the stimulating effect of Strychnia is peculiarly and most re- markably exerted upon the vesicular substance of the spinal cord; and that a corresponding state, in which violent convulsive actions are excited by the most trifling causes, sometimes presents itself as a peculiar form of disease, named Tetanus, which may be either idio- pathic, depending probably upon a disordered condition of the blood, or traumatic, consequent upon the irritation of a wound. 403. But, as formerly remarked, it is not in the Nervous centres only that changes originate. Whenever an impression is made upon the surface of the body, or upon the organs of special sense, which, being conducted to the nervous centres, either excites a sensation in the brain (§ 390), or a reflex action through the spinal cord (§ 392), the reception and propagation of such impression at the extremities of the sensory nerve requires a set of conditions of the same kind with those which we have seen to exist in the nervous centres. In fact, if we regard the course of the motor nerves as commencing in the nervous centres and terminating in the muscles, we may with equal justice consider that of the sensory nerves as originating in their peripheral extremities, and terminating in the sensorium. And, as already stated (§ 381), precisely the same kind of vesicular structure exists in some (probably in all) of the peripheral expansions of the sensory nerves as makes up the gray substance of the brain and spinal cord. Now it is easily shown, that the circulation of blood through these parts is just as necessary for the original reception of the impressions, as is the circulation through the brain to their re- ception as sensations, and to the origination of motor impulses by an act of the will. We find that anything which retards the circula- tion through a part supplied by sensory nerves, diminishes its sensi- bility ; and that if the flow of blood be completely stagnated, entire insensibility is the result. A familiar example of this is seen in the effects of prolonged cold, which, by diminishing, and then entirely checking, the flow of blood through the skin, produces first numb- ness, and then complete insensibility of the part. This result, how- DEPENDENCE OF NERVOUS POWER ON SUPPLY OF BLOOD. 241 ever, may be partly due to the direct influence of the cold upon the nerve-vesicles themselves, depressing their peculiar vital powers (§ 97). The same effect is produced, however, when the supply of blood is checked in any other way; as, for example, by pressure on the artery, or by obstruction in its interior. Thus, when the main artery of a limb is tied, numbness of the extremities is immediately perceived; and this continues until the circulation is re-established by the collateral branches, when the usual amount of sensibility is restored. Again, in the gangrene which depends upon obstruction of the arterial trunks by a fibrinous clot in their interior, diminution of sensibility, consequent upon the insufficient circulation, is one of the first symptoms. 404. On the other hand, increased circulation of blood through a part produces exaltation of its sensibility ; that is, the ordinary im- pressions produce changes of unusual energy in its sensory nerves. This is particularly evident in the increased sensibility of the genital organs of animals during the period of heat; and in those of Man, when in a state of venereal excitement. Moderate warmth, friction, exercise, and other causes which increase the circulation through a part, also augment its sensibility ; and this augmentation is one of the most constant indications of that state of determination of blood, or active congestion, which usually precedes inflammation, and which exists in the parts surrounding the centre of inflammatory action. But it must be borne in mind, here as elsewhere (§ 401), that such exaltation of function in a limited part, is quite consistent with gene- ral debility ; and in fact we may often observe, that the tendency to such local affections is particularly great, when the blood is in a very poor condition. (See Chap. V.) 405. To sum up, then, we may compare the vesicular substance, wherever it exists, to a galvanic combination : the former being capa- ble of generating nervous influence, and transmitting it along the fibrous structure, to the part on which it is to operate; in the same manner as the latter generates electric power, and transmits it along the conducting wires, to the point at which it is to effect a decompo- sition or any other change. In one of the most perfect forms of the galvanic battery (that invented by Mr. Smee), although the metals remain inserted in the acid solution, and are consequently always ready for action, no electricity is generated until the circuit is com- plete ; and the waste of the zinc produced by its solution in the acid, is therefore exactly proportional to the electric effects to which it gives rise. The condition of the nervous system, in the healthy and waking state, bears a close analogy to this; for it is in a state constantly ready for action, but waits to be excited ; and its waste is proportional to the activity of its function. The vesicular matter, diffused over the surface of the body, is inactive, until an impression is made upon it by some external agent; but a change then takes place in its con- dition f of which we know no more, than that the presence of arterial 16 242 CONNECTION OF NERVOUS SYSTEM WITH MIND. blood and a certain amount of warmth are necessary for it), which is transmitted to the central organs by the sensory trunks. It would appear that the excitement of this change has a tendency to increase the afflux of blood to the part; thus when a lozenge or some similar substance is allowed to lie for a time in contact with the tongue, or with the side of the mouth, a roughness is produced, which is due to the erection of the sensory papillae, by the distension of their blood- vessels. On the other hand, the change in the vesicular matter of the central organs, by which motion is produced in the distant mus- cles, may be excited either by the stimulus conveyjedhv the afferent nerves (as in reflex action, § 392), or by an act oflM^jiLi^This act may be voluntary, originating in the will; or it may be instmcrrre or emotional, resulting from certain states of mind excited by sensations, and altogether independent of the will. Of the mode in which the mind thus acts upon the nervous system, we know nothing whatever, and probably never shall be informed in our present state of being. But it is sufficient for us to be aware of the physiological fact of the peculiar connection between the mind and the brain ; a connection so intimate, as to enable the mind to receive through the body a know- ledge of the condition of the Universe around it, and to impress on the body the results of its own determinations; and of such a nature, that the regularity of the working of the mind itself is dependent upon the complete organization of the brain in the first instance, upon the constant supply of pure and well-elaborated blood, and upon all those influences which favour the due performance of the nutritive opera- tions in general. BOOK II. ^FECIAL PHYSIOLOGY. CHAPTER IV. OF FOOD, AND THE DIGESTIVE PROCESS. 1. Sources of the Demand for Aliment. 406. The dependence of all Organized beings upon food or aliment, must be evident from the facts stated in the preceding portion of this Treatise. In the first place, the germ requires a large and constant supply of materials, with which it may develop itself into the perfect being, by the properties with which it is endowed. In all but the lowest tribes of Plants, we find the materials required for the earliest stages of the process prepared and set apart by the parent. Thus in the seed, the germ itself forms but a small proportion of the whole substance, the principal mass being composed of starchy matter, which is laid up there for its nutrition; and the act of germination consists in the appropriation of that nutriment by the germ, and the consequent development of the latter, up to the point at which it becomes inde- pendent of such assistance, and is able for itself to procure, from the soil and atmosphere that surround it, the materials for its continued growth. So in the egg of the Animal, the principal mass is composed of Albumen and oily matter; the germ itself being, at the time the egg is first deposited, a mere point invisible to the naked eye ; these materials serve as the food or aliment of the germ, which gradually draws them to itself, and converts them into the materials of its own structure, and at the end of a certain period the young animal comes forth from the egg ready to obtain for itself the food which is neces- sary for its continued increase in size. 407. In many instances among the lower animals, the form in which the young animal emerges from the egg is very different from that which it is subsequently to assume; and the latter is only attained by a process of metamorphosis. This change has been longest known, and most fully studied, in the case of Insects and Frogs; which 244 SOURCES OF THE DEMAND FOR ALIMENT. were formerly thought to constitute an exception to all general rules in this respect,—the Insect coming forth from the egg in the state of a Worm, and the Frog in the condition of a Fish. But it is now known that changes of form, as complete as these, occur in a large propor- tion of the lower tribes of Animals; so that the absence of them is the exception. The fact seems to be, that the supply of nutriment laid up within the egg, among the lower classes, is by no means sufficient to carry on the embryo to the form it is subsequently to attain; and its development is so arranged that it may come into the world in a condition which adapts it to obtain its own nutriment, and thus to acquire for itself the materials of its further development. Thus the Insect, in its larva or Caterpillar state, is essentially a fcetus in regard to its grade of development; but it is a fcetus capable of acquiring its own food. In this condition it attains its full growth as regards size, though its form remains the same ; but it then, in passing into the Chrysalis state, reassumes (as it were) the condition of an embryo within the egg,—the development of various new parts takes place, at the expense of the nutriment stored up in its tissues,—and it comes forth as the perfect insect. In many of the lower tribes, the animal quits the egg at a still earlier period in comparison ; thus it has been lately shown by M. Milne Edwards, that some of the long marine worms consist only of a single segment, forming a kind of head, when they leave the egg ; and that the other segments, to the number, it may be, of several hundred, are gradually developed from this, by a process that resembles the budding of Plants. 408. Up to the period, then, when the full dimensions of the body have been attained, and the complete evolution of all its organs has taken place, a due supply of food is necessary for these purposes. In the Plant nearly the whole of the alimentary materials taken into the system, are thus appropriated ; the extension of its structure going on almost indefinitely, and the waste occasioned by decay being compara- tively small. Thus the carbon, which is given out by the respiratory process in the form of carbonic acid, bears but a small proportion to that, which is introduced by the decomposition of that same gas under the influence of light (§ 81). And the fall of the leaves, which takes place once a year or more frequently, and which gives back a large quantity of the matter that has undergone the organizing process does not occur, until by their means a considerable addition has been made to the solid and permanent substance of the tree. 409. This is not the case, however, with the Animal. Its period of increase is limited. The full size of the body is usually attained and all the organs acquire their complete evolution at a comparatively early period. The continued supply of food is not then requisite for the extension of the structure, but simply for its maintenance ; and the source of the demand lies in the constant waste, to which, during its period of activity, it is subjected. We have seen that every action of the Nervous and Muscular systems involves the death and decay 0f a certain amount of the living tissue,—as indicated by the appearance of SOURCES OF DEMAND FOR FOOD. 245 the products of that decay in the Excretions; and a large part of the demand for food will be consequently occasioned by the necessity for making good the loss thus sustained. Hence we find that the demand for food bears a close relation to the activity of the animal functions; so that a diet, which would be superfluous and injurious to an indi- vidual of inert habits, is suitable and beneficial to one who is leading a life of continual exertion; and this difference manifests itself in the requirements of the same individual who makes a change in his habits, —the indolent man acquiring an appetite by vigorous exertion, and the active man losing his disposition to hearty feeding by any cause that keeps him from his accustomed exercise. We see precisely the same contrast between Animals of different tribes, whose natural instincts lead, them to different modes of life. The Birds of most active flight, and the Mammals which are required to put forth the greatest efforts to obtain their food, need the largest and most constant supplies of nutriment; but even the least active of these classes stand in remarkable contrast with the inert Reptiles, whose slow and feeble movements are attended with so little waste, that they can sustain life for weeks and even months, with little or no diminution of their usual activity, without a fresh supply of food. 410. The waste and decay just adverted to, however, do not affect the muscular and nervous tissues alone; for all the operations of nutrition involve it to a certain extent. It has been already shown that the acts of absorption, assimilation, respiration, secretion, and reproduction,—all those, in fact, by which the material for the nutri- tion of the nervous and muscular tissues is first prepared, and sub- sequently maintained in the requisite purity,—are effected in the Animal, as in the Plant, by the agency of cells, wThich are continually dying and requiring renewal. In most Vegetables, the death of the parts concerned in these functions takes place simultaneously, as soon as they have performed them; the whole crop of leaves ceasing at once to perform its proper actions, and dropping off;—to be replaced by another, at an interval that solely depends upon the temperature under which the tree is living (§ 99). In the evergreen, however, the process bears a close resemblance to that which we observe in the Animal; for the leaves die one by one, and not simultaneously; and are constantly undergoing replacement, so that the vigour of the system and the activity of its nutritive processes never suffer a com- plete suspension. 411. In the Animal body, the different classes of cells, to which allusion has been made, are in like manner constantly undergoing death and renewal; and this with a rapidity proportioned to the energy of their functions. Hence a supply of food is as requisite to furnish the materials of their growth, as it is in Plants to furnish the materials of the growth of the leaves. A large part of these materials are subsequently used for other purposes in the economy; thus, as the leaves prepare the sap which is to nourish the woody stem, and to form new shoots, so do the absorbing and assimilating cells prepare 246 SOURCES OF DEMAND FOR FOOD. and furnish the fluid elements of the blood, which are to repair the waste of nerve and muscle, bone and cartilage, &c. But still a con- siderable amount is expended in the simple nutrition of these organs themselves, whose duration is transient, and whose solid parts are cast off as of no further use. Thus the skin and all the mucous sur- faces are continually forming and throwing off epidermic and epithelial cells, whose formation requires a regular supply of nutriment; and only a part of this nutriment (that which occupies the cavity of the cells) consists of matter, that is destined to serve some other purpose in the system, or that has already answered it; the remainder (that of which the solid walls are composed) being furnished by the nutritive mate- rials of the blood, and being henceforth altogether lost to it.—Thus every act of Nutrition involves a waste or decay of Organized tissue. 412. We may observe a marked difference, however, between the amount of aliment required, and the amount of waste occasioned, by the simple exercise of the nutritive or vegetative functions in the building-up and maintenance of the animal body, and that which results from the exercise of the animal functions. The former are carried on, with scarcely any intermixture of the latter, during fcetal life. The aliment, in a state of preparation, is introduced into the fcetal vessels; and is conveyed by them into the various parts of the structure, which are developed at its expense. The amount of waste is then very trifling, as we may judge by the small amount of excre- tory matter, the product of the action of the liver and kidneys, which has accumulated at the time of birth ; although these organs have attained a sufficient development to act with energy when called upon to do so. But as soon as the movements of the body begin to take place with activity, the waste increases greatly; and we even observe this immediately after birth, when a large part of the time is still passed in sleep, but when the actions of respiration involve a constant employment of muscular power.—In the state of profound sleep, at subsequent periods of life, the vegetative functions are performed, with no other exercise of the animal powers, than is requisite to sus- tain them ; and we observe that the waste, and the demand for food, are then diminished to a very low point. This is well seen in many animals, which lead'a life of great activity during the warmer parts of the year, but which pass the winter in a state of profound sleep, without, however, any considerable reduction of temperature ; the demand for food, instead of being frequent, is only felt at long inter- vals, and the excretions are much reduced in amount. And those animals which become completely inert, either by the influence of cold, or by the drying-up of their tissues, do not suffer from the pro- longed deprivation of food; because not only are their animal func- tions suspended, but their nutritive operations also are in complete abeyance; and the continual decomposition of their tissues, which would otherwise be taking place, is checked by the cold or desicca- tion ; so that the whole series of changes which goes on in their active condition is completely at a stand. SOURCES OF DEMAND FOR FOOD. 247 413. But there is another most important cause of demand for food, amongst the higher Animals, which does not exist either amongst the lower Animals, or in the Vegetable kingdom. We have seen (Chap. II.) that Mammals and Birds, and to a certain extent Insects also, are able to sustain the heat of their bodies at a fixed standard, and thus to be independent of variations in external temperature. This they are enabled to do, as will be explained hereafter, by a process strictly analogous to ordinary combustion; the carbon and hydrogen directly supplied by their food, or after having been employed for a time in the composition of the living tissues and then set free, being made to unite with oxygen introduced by the respiratory process, and thus giving off the same heat as if the same materials were burned in a furnace. And it has been further shown, that the immediate cause of death in a warm-blooded animal, from which food has been entirely withheld, is the inability any longer to sustain the temperature, which is requisite for the performance of its vital operations (§ 117). Hence we see the necessity for a constant supply of aliment, in the case of warm-blooded animals, for this purpose alone; and the de- mand will be chiefly regulated by the external temperature. When the heat is rapidly carried off from the surface, by the chilling influ- ence of the surrounding air, a much greater amount of carbon and hydrogen must be consumed within the body, to maintain its proper heat, than when the air is nearly as warm as the body itself; so that a diet which is appropriate to the former circumstances, is superfluous and injurious in the latter; and the food which is amply sufficient in a wrarm climate, is utterly destitute of power to enable it to resist the influence of severe cold. This is a fact continually experienced; both in the ordinary recurrence of changes of temperature in our own climate ; and, still more remarkably, when the same individual is subjected to the extremes of heat and cold, in successively visiting the tropical and frigid zones. 414. Thus we find that in the Animal body, aliment is ordinarily- required for four different purposes. First, for the original construc- tion or building-up of the organism. Second, to supply the loss occa- sioned by its continual decay, even when in a state of repose. Third, to compensate for the waste occasioned by the active exercise of the nervous and muscular systems. And Fourth, to supply the materials for the heat-producing process, by which the temperature of the body is kept up.—The amount required for these several purposes will vary according to the conditions of the body, as regards exercise or repose, and external heat or cold. It is also subject to great varia- tion with difference of age. During the period of growth, it might be anticipated that a larger supply of food would be required, than when the full stature has been attained; but a very small daily addi- tion would suffice in the case of a child or youth, to produce the entire increase of a whole year. Yet every one knows that the child requires much more food than the adult, in proportion to his compa- rative bulk. This results from the much more rapid change in the 248 SOURCES OF DEMAND FOR FOOD. constituents of his body; which is evident from the large proportional amount of his excretions, from the quickness with which the effects of illness or of deficiency of food manifest themselves in the diminu- tion of the bulk and firmness of the body, from the short duration of life when food is altogether withheld, and from the readiness with which losses of substance by disease or injury are repaired, when the nutritive processes are restored to their full activity. The converse of all this holds good in the aged person. The excretions diminish in amount, the want of food may be sustained for a longer period, losses of substance are but slowly repaired, and everything indicates that the interstitial changes are performed with comparative slowness; and, accordingly, the demand for food is far less in proportion to the bulk of the body than it is in the adult, and may be even absolutely less than in the child of a fourth of its weight. 415. The demand for food is increased by any cause, which creates an unusual drain or waste in the system. Thus an extensive suppu- rating action can be sustained only by a large supply of highly-nutri- tious food. The mother, who has to furnish the daily supply of milk which constitutes the sole support of her offspring, needs an unusual sustenance for this purpose. And there are states of the system, in which the solid tissues seem to possess an unusual tendency to decom- position, and in which an increased supply of aliment is therefore required. This is the case, for example, in diabetes; one of the first symptoms of which disease is the craving appetite, that seems as if it would never be satisfied. And there can be no doubt that, putting aside all the other circumstances that have been alluded to, there is much difference amongst individuals, in regard to the rapidity of the changes which their organism undergoes, and the amount of food required for its maintenance. 416. The influence of the supply of food upon the size of the indi- vidual, is very evident in the Vegetable kingdom; and it is most strikingly manifested, when a plant naturally growing in a poor dry soil is transferred to a damp rich one, or when we contrast two or more individuals of the same species, growing in localities of opposite characters. Thus, says Mr. Ward, " I have gathered, on the chalky borders of a wood in Kent, perfect specimens in full flower of Ery- thrcea Centaurium (Common Centaury), not more than half an inch in height; consisting of one or two pairs of most minute leaves, with one solitary flower: these were growing on the bare chalk. By trac- ing the plant towards and in the wood, I found it gradually increasing in size, until its full development was attained in the open parts of the wood, where it became a glorious plant four or five feet in eleva- tion, and covered with hundreds of flowers." On the other hand, by starvation, naturally or artificially induced, Plants may be dwarfed, or reduced in stature: thus the Dahlia has been diminished from six feet to two ; the Spruce Fur from a lofty tree to a pigmy bush ; and many of the trees of plains become more and more dwarfish as they ascend mountains, till at length they exist as mere underwood. Part INFLUENCE OF VARIATIONS IN SUPPLY OF FOOD. 249 of this effect, however, is doubtless to be attributed to diminished temperature; which, as already remarked, concurs with deficiency of food in producing inferiority of size. 417. Variations in the supply of food would not appear to be effectual in producing a corresponding variety of size in the Animal kingdom : this is not, however, because Animals are in any degree less dependent than Plants upon a due supply of food ; but because such a limitation of the supply, as would dwarf a Plant to any con- siderable extent, would be fatal to the life of an Animal. On the other hand, an excess of food, which (under favourable circum- stances) would produce great increase in the size of the Plant, would have no corresponding influence on the Animal; for its size appears to be restrained within much narrower limits,—its period of growth being restricted to the early part of its life, and the dimensions proper to the species being rarely exceeded in any great degree. Even in the case of giant individuals, it does not appear that the excess of size is produced by an over-supply of food; but that the larger sup- ply of food taken in is called for by the unusual wants of the system, —those wants being the result of an extraordinary activity in the pro- cesses of growth, and being traceable rather to the properties inherent in the system, than to any external agencies. Thus we not unfre- quently hear of children, who have attained an extraordinary size at the age of a few years; and this excess of size is usually accompanied with other marks of precocious development. We shall hereafter see, that a provision exists in the Digestive apparatus, which absolutely prevents the reduction and preparation of the food, in any amount greatly surpassing that which the wants of the system demand (§ 474); and it is probably to this cause, in part, that we are to attribute the small degree of influence exerted by an excess of food, in producing an increased development of the Animal frame. 418. The influence of a diminished supply of food, in producing a marked inferiority in the size of Animals, is most effectually exerted during those early periods of growth, in which the condition of the system is most purely Vegetative. Thus it is well known to Ento- mologists, that, whilst it is rare to find Insects departing widely from the average size on the side of excess, dwarf-individuals, possessing only half the usual dimensions, or even less, are not uncommon ; and there can be little doubt that these have suffered from a diminished supply of nutriment during their larva state. This variation is most apt to present itself in the very large species of Beetles, which pass several years in the larva state ; and such dwarf-specimens have even been ranked as sub-species. Abstinence has been observed to pro- duce the effect, upon some Caterpillars, of diminishing the number of moults and accelerating the transformation; in such cases, the Chry- salis is more delicate, and the size of the perfect Insect much below the average. 419. One of the most remarkable examples known, of the effect of food in modifying the development of Animals, is to be found in the 250 INFLUENCE OF VARIATIONS IN SUPPLY OF FOOD. economy of the Hive-Bee. In every community, the majority of Individuals consists of neuters; which may be regarded as females, having the organs of the female sex undeveloped ; and which, whilst incapable of reproduction, perform all the labours of the hive. The office of continuing the race is restricted to the queen; who is the only perfect female in the community. If by any accident the queen be destroyed, or if she be purposely removed for the sake of experi- ment, the bees choose two or three from amongst the neuter eggs, which have been deposited in their appropriate cells; and these they cause to be developed into perfect queens. The first operation is to change the cells in which they lie into royal cells; these differ con- siderably from the ordinary ones in form, and are of much larger dimensions. This is accomplished by breaking down the walls of the surrounding cells, removing the eggs or grubs they may contain, and rebuilding the central upon an enlarged scale, and upon the same plan as the royal cells in which the queens are ordinarily reared. When the eggs are hatched, the maggot is supplied with food of a very different nature from the farina or bee-bread (composed of a mixture of pollen and honey) which has been stored up for the nourish- ment of the workers: this food being of a jelly-like consistence and pungent, stimulating character. After the usual transformations, the grub becomes a perfect queen ; differing from the neuter bee, into which it would otherwise have been changed, not only in the develop- ment of the reproductive system, but in the general form of the body, the proportionate shortness of the wings, the shape' of the tongue, jaws, and sting, the absence of the hollows on the thighs in which the pollen is carried, and the loss of the power of secreting wax. 420. That insufficiency of wholesome food, continued through suc- cessive generations, may produce a marked effect, not merely upon the stature, but upon the form and condition of the body, even in the Human race, appears from many cases, in which such influence has operated on an extensive scale. Thus there are parts of Ireland inhabited by a population descended from those who were treated by the English as rebels two centuries since, and who were driven into mountainous tracts, bordering the sea, where they have been since exposed to the two great brutalizers of the human race, hunger and ignorance. The present race is distinguished physically from the kindred race of Meath and other neighbouring districts, where the same causes have not been in operation, by their low stature (not exceeding five feet two inches), their pot-bellies and bow-legs; whilst their open projecting mouths, with prominent teeth and exposed gums, their advancing cheek-bones and depressed noses, bear bar- barism in their very front. " These spectres of a people that once were well-grown, able-bodied, and comely, stalk abroad into the daylight of civilization, the animal apparitions of Irish ugliness and Irish want."—The whole aboriginal population of New Holland pre- sents a similar aspect; and apparently from the operation of the same causes. EFFECTS OF EXCESS OF FOOD. 251 421. When a larger quantity of food is habitually consumed than the wants of the system require, it is not converted into solid flesh; but it is got rid of by the various processes of excretion. The in- creased production of Muscular fibre depends, as we have already seen (§ 362), upon nothing so much as the exercise of the muscle. It cannot take place, unless the blood supply it with the materials : but no degree of richness of the blood can alone produce it. Consequently, the accumulation of nutritive matter in the blood is so far from being a condition of health, that it powerfully tends to produce disease,— either of an inflammatory character, if the fibrin predominate,—or of the hemorrhagic character, if the red corpuscles predominate. This state is most apt to present itself in those who live well and take little exercise ; and the remedy for it is either to diminish the diet, or to increase the amount of exercise, so as to bring the two into harmony. 422. The continued over-supply of food has several injurious effects; it disorders the digestive processes, by stimulating them to undue activity, and lays the foundation for a complete derangement of them ; it gives a predisposition to the various diseases of repletion, as already noticed; and it throws upon the excreting organs much more than their proper amount of labour, besides tending to produce a depraved condition of the matters to be drawn off by them, which renders the proper act of excretion still more difficult. When this is the case, various disorders arise, caused by the retention, within the circulating current, of substances which are very noxious to the general system, and which become most fertile sources of disease. What are com- monly regarded as diseases of the biliary and urinary organs, are really, in a large proportion of cases, nothing else than disordered actions of these organs, occasioned by the irregular mode in which the products of decomposition are formed within the blood, and de- pendent upon some error in diet, either as regards quantity or quality. Thus the " lithic acid diathesis," in which there is an undue propor- tion of that substance in the urine, and of which Gout is a particular manifestation, is due, not to disorder of the kidney, but to an undue production of lithic acid in the blood ; so long as the excreting action of the kidney is sufficient to prevent its accumulation in the blood, so long the general health is but little affected ; but whenever that action receives a check, various constitutional symptoms indicate that the system is disturbed by the presence of this product of decomposition. And though our remedies may be rightly directed, in part, to facili- tating its escape through the kidneys, yet the radical cure is to be sought only in the regulation of the diet, and in the prevention of the first production of the substance in question.—Similar remarks might probably be applied to disorders of the Liver; but we are, from various causes, far less perfectly acquainted with their character than we are with those of the Kidney. 423. There is only one tissue, the increase of which is directly produced by an over-supply of food. This is the Adipose or fatty. It is formed almost entirely at the expense of the non-azotized con- 252 INFLUENCE OF FOOD UPON PRODUCTION OF FAT. stituents of the food; the walls of the cells, into which the fatty matter is secreted, being the only part of this tissue that is derived from the proteine-compounds of the blood. The production of the adipose tissue is most directly favoured by the presence of a large amount of fatty matter in the food ; but it may also take place, as will be presently shown, by the conversion of starchy and saccharine substances into fatty compounds. It cannot occur, unless there be in the food a larger proportion of substances that can be thus appro- priated, than is sufficient to maintain the heat of the system by the respiratory process. Consequently, whatever increases the demand for heat, is unfavourable to the deposition of fat; and vice versd. The fattening of animals is now brought to a regular system ; and expe- rience has shown that rest and a warm temperature, with food con- taining a large amount of oily matter, are most conducive to the accu- mulation. Rest acts by keeping the respiration at a low standard ; for it will hereafter be shown (Chap. VIII.), that a much larger pro- portion of carbonic acid is thrown off when the body is in active movement, than when it is in repose. External warmth has the same effect; the demand upon the calorifying power being diminished, and more of the combustible material being left, to be stored up as fat. 424. The deposition of fat affords a supply of combustible matter, against the time when it may be needed ; and it is consequently found, that the duration of life in warm-blooded animals, when they are com- pletely deprived of food, is in great degree proportional to the amount of fat they have previously accumulated. There is no sufficient reason to believe that fatty matter can be converted, within the ani- mal body, into a proteine-compound, which can serve for the nutri- tion of the muscular and other tissues. But the greatest and most constant waste, when an animal is undergoing starvation, is that which is occasioned by the heat-producing process ; this, so long as the sup- ply lasts, is kept up by the store of fat, which is gradually consumed ; and when it is completely exhausted, the temperature falls, hour by hour, until life can no longer be sustained, (§ 117). The use of this store of fat, in supplying any temporary deficiency in the food, be- comes evident from such experiments ; for when it has been com- pletely exhausted, the withholding of a single meal proves fatal, from the want of power to sustain the calorifying process. We find that animals, which are likely to suffer from deficiency of food in the winter, or which spend that period in a state of quiescence, have a tendency .to accumulate a store of fat in the autumn ; which tendency seems principally to depend upon the nature of their food. We observe it chiefly in those Birds and Mammals which live upon seeds and grains; and these, when ripe, contain a large quantity of oily matter, which thus becomes a valuable store against the time of need. There are many birds, such as the beccafico so much esteemed in Italy, which are described, if killed at this season, as being " lumps of fat." 425. It is well known to breeders of cattle, that some varieties or breeds have a much greater tendency to the production of Adipose VALUE OF DIFFERENT MATERIALS OF FOOD. 253 tissue, than others placed under the same circumstances; and the former are therefore selected to undergo the fattening process. Cor- responding differences may be met with among different individuals of the Human race ; some persons having a remarkable tendency to the production of fat, under circumstances which do not seem by any means favourable to it, whilst others appear as much indisposed to this deposit. The latter condition we notice particularly in that temperament which is commonly termed the " bilious;" and it is important to bear in mind that, where such an indisposition exists, any superfluity of fatty matter in the food taken into the system, must be excreted again through the liver, instead of being retained and stored up in the body. It is very desirable, therefore, that such persons should abstain from any. excess of this kind ; since an habitual call upon the liver, to relieve the system of a superfluity of fatty matter, is certain to produce a dis- ordered state of that organ; and in order to prevent it, the diet should be altered so as to include less of fatty matter, or the amount of exer- cise should be increased, so that it may be burned off by the addi- tional respiration which then takes place. 426. We see, then, that the amount of food which can be properly appropriated by the system varies considerably in different individuals, and in the same individual under different circumstances. Conse- quently it is impossible to give any general rule, which shall apply to every one alike. The average quantity required by adult men, leading an active life, and exposed to the ordinary vicissitudes of temperature in our own climate, seems to be from 30 to 36 ounces of dry aliment. But a healthy condition may be kept up on scarcely more than half this allowance, if the muscular powers are but little exerted, and the surrounding temperature be high; provided that it consist of sub- stances of a nutritious kind, united in proper proportions. 427. The value of different substances as aliment, depends in. the first place upon the quantity of solid matter they contain; being of course the greater, as the solids form the larger proportion of the entire weight. Many esculent vegetables contain so large a quantity of water, that the nutriment they afford is very slight in proportion to their bulk.—Next it depends upon the proportion of digestible matter which the solid parts include; for it is not every substance containing the requisite ingredients, that is capable of being reduced to a state which enables it to be absorbed. Thus woody fibre is composed of the same elements as starch-gum; but it passes out of the intestinal canal unchanged, and therefore affords no nutriment. In the same manner, the horny tissues of animals, though nearly allied to proteine in their composition, are completely destitute of nutritive properties to Man and the higher animals, because not capable of being reduced by their digestive process; though certain insects appear capable of living exclusively upon them. 428. But when the watery and indigestible parts of the food are put out of consideration, and our attention is directed only to the soluble solids, we find a most important difference in the chemical 254 VALUE OF DIFFERENT MATERIALS OF FOOD. composition of different substances, which renders them more or less appropriate to the different purposes which have to be answered in the nutrition of the body. It has been already pointed out, that Vege- tables possess the power of combining the elements furnished by the inorganic world into two classes of compounds,—the ternary, consist- ing of oxygen, hydrogen, and carbon,—and the quaternary, which consists of these elements, with the addition of azote or nitrogen. These two classes are hence termed the non-azotized and the azotized. 429. Now the azotized compounds which are formed by Plants, are essentially the same with those which are furnished by the flesh and by the albuminous fluids of Animals, as already shown (§ 169); and these are required for the reparation of the waste of the muscular tissue and for the general nutrition of the body. Consequently, unless the food contain a sufficient proportion of these substances, the body must be insufficiently nourished, and the strength must diminish, even though other elements of the food be in superabundance. The other azotized compounds existing in the animal body may be elaborated by the transformation of these proteine-compounds ; so that when they are duly supplied, the system cannot become enfeebled for want of support.—But there is another azotized compound, Gelatin, that is furnished by Animals, to which nothing analogous exists in Plants; and this, although it cannot sustain life by itself, is a valuable adjunct to the proteine compounds. For as the gelatinous tissues suffer waste in common with the others, it is evident that if the gelatin be sup- plied already prepared, it may be at once applied to their nutrition; and thus the proportion of proteine, which they would otherwise re- quire, is not demanded, and the labour of transformation is also saved. Further, there is this great advantage in combining a proportion of gelatin with the food,—especially when the digestive powers are feeble,—that being already in a state of perfect solution, it is taken up at once by the simple act of physical absorption or endormose, instead of requiring the selective absorption, which involves an act of cell- formation (§ 494). But there is no evidence that gelatin can ever be transformed into a proteine-compound, and can thus be applied to the nutrition of the muscular and other fibrous tissues; and .the pre- sumption, derived from the results of various experiments, is very strong the other way. 430. The quantity of azotized substances furnished by Plants is usually small in proportion to that of the non-azotized ; being consider- able only in the Corn-grains, and in the seeds of Leguminous plants, which the universal experience of ages has demonstrated to be the most nutritious of Vegetable substances. The non-azotized compounds exist under various forms; of which the principal are starch, sugar, and oil. The two former may be regarded as belonging to one class; because we know that starch and the substances allied to it may be converted into sugar by simple chemical processes, and that this trans- formation takes place readily both in the Vegetable and Animal econo- my. On the other hand, the oily matters contained in vegetable and VALUE OF DIFFERENT MATERIALS OF FOOD. 255 animal food, are usually ranked as a distinct group of alimentary sub- stances ; and it has been maintained that, under no circumstances, has the Animal the power of elaborating fatty matter from starchy or sac- charine compounds. But this is now known to be an unfounded limi- tation ; since the transformation of a saccharine into a fatty compound takes place in the case of bees, which form wax when fed upon pure sugar; and it has been recently shown that it may take place in the laboratory of the Chemist, butyric acid (the fatty acid of rancid butter) being one of the products of the fermentation of sugar taking place under peculiar circumstances. 431. The great use of these substances in the Animal economy, is to support the respiratory process, and thus maintain the temperature of the body. We have seen that, in the compounds of the Saccharine group (in which Starch is included) the amount of oxygen is no more than sufficient to form water with the hydrogen of the substance (§ 12); so that the carbon is free to combine with the oxygen taken in by the lungs, and thus becomes a source of calorifying power. Again, in the oily matters taken in as food, the proportion of oxygen is far smaller; so that they contain a large quantity of surplus hydrogen, as well as of carbon, ready to be burned off in the system, and thus to supply the heat required. This is obviously the ordinary destina- tion of the alimentary matters belonging to these classes; and the greatest economy in the choice of diet is therefore exercised, when it is composed of azotized substances in sufficient amount to repair the waste of the system, and of non-azotized compounds which include free carbon and hydrogen in sufficient quantity to develop (with the aid of other processes) the requisite amount of heat by combination with oxygen. But if there be a deficiency in either of these kinds of aliment, the body must suffer. Should the supply of duly-prepared azotized matter be less than is required to repair the waste of the albuminous and gelatinous tissues, then these diminish in bulk and in vital power, though the heat of the body may be kept up to its proper standard. But if the non-azotized matter would be supplied in sufficient amount, or in a form in which it cannot be appropriated, the heat of the body cannot be sustained in any other way than by drawing upon the store of fat previously laid up. 432. Various circumstances lead to the belief, that the saccharine compounds are thus carried off by the respiratory process, within a short time after they have been introduced into the system. They have not been detected in the chyle drawn from the lacteal absorbents; but there seems reason to believe that, in consequence of their ready solubility, they are directly taken up by the blood (§ 493), and that they are so rapidly burned off there, as to escape notice in that fluid. But it has been lately shown by Dr. Buchanan, that, if the blood be examined within a short time after a meal consisting in part of fari- naceous and saccharine substances, a very appreciable quantity of saccharine matter is found in it. This soon disappears, however; being eliminated or separated from the blood by the action of the 256 VALUE OF DIFFERENT MATERIALS OF FOOD. lungs. In fact it is very probable, that a large proportion of the mat- ter thus taken in never enters the general circulation at all; as the blood of the mesenteric veins proceeds to the lungs, after passing through the liver, before it is transmitted to the systemic arteries, and may there lose its saccharine matter, as fast as this is taken in from the stomach. After a meal containing the ordinary admixture of sac- charine, oily, and albuminous compounds, it is probable that the sac- charine are first received into the blood, and are the first to be elimi- nated from it; and that, by the time they have been all consumed, the oily matter, introduced through the more circuitous channel of the lacteal system, is ready to answer the same purpose. If these are exhausted before a fresh supply of food is taken in, cold as well as hunger is experienced; and the body is in this condition peculiarly liable to suffer from any depressing causes, such as a low external temperature, poisonous miasmata, &c; hence the prudence of avoid- ing exposure to such influences upon an empty stomach. 433. We can thus in part account for the fact, which universal experience has established, that in warm-blooded animals, a mixture of azotized and non-azotized substances is the diet most conducive to the welfare of the body ; and that, in all but the purely carnivorous tribes, the diet provided by Nature consists not only of albuminous, gelatinous, and oily substances, such as are furnished by the flesh and fat of animals, but also of saccharine or farinaceous matter. This is the diet to which Man is evidently best adapted ; and it is remarkable how completely accordant is his use of the ordinary materials of food, with the principles now established by chemical and physiological research, in regard to the wants of his bodily system, and the best mode of supplying them. Thus, good wheaten bread contains, more nearly than any other substance in ordinary use, that proportion of azotized and non-azotized matter, which is adapted to repair the waste of the system, and to supply the necessary amount of combus- tible material, under the ordinary conditions of civilized life in tem- perate climates; and we find that the health and strength can be more perfectly sustained upon that substance, than upon any other taken alone. The addition of a moderate quantity of butter increases its heat-producing powers; and this is especially useful when the tem- perature is low,—under which condition, there is usually an increased disposition to the employment of fatty matters as articles of food. On the other hand, if the body be subject to violent exertion, advantage is gained by increasing the proportion of the proteine-compounds, by the addition of animal flesh; and, under any circumstances, there is an economy in the use of gelatin, in the form of soup, which dimin- ishes the demand for other azotized matter. The use of animal flesh, however, as a principal article of diet, except when the indi- vidual is leading the incessantly-active life of a carnivorous animal is very far from being economical, and is positively injurious to the welfare of the body. 434. On the other hand, in rice, potatoes, casava-meal, and simi- VALUE OF DIFFERENT MATERIALS OF FOOD. 257 lar substances, the farinaceous or saccharine components form so very large a proportion of the whole mass, and the proteine-compounds are present in so very small an amount, that they are insufficient to sup- port the bodily vigour when taken alone, unless a larger quantity be ingested, so as to supply the requisite proportion of azotized matter. But when these substances form part of a mixed diet, the other ingre- dient of which consists of animal flesh, a much smaller quantity of them suffices; and the same kind of combination is then formed, as exists in the single article of bread. Those in whose diet the farina- ceous elements predominate largely, and the azotized compounds exist in the smallest amount compatible with the maintenance of the bodily vigour, are exempt from many diseases incident to those who live more highly; thus among the potato-eating Irish, and the oatmeal- feeding Scotch, gout is a disease never heard of; whilst among the richer classes of the same countries, there is no peculiar exemption from it. 435. The oily constituents of food are most abundant in the diet of the inhabitants of frigid zones, who feed upon whales, seals, and other animals loaded with fat, and who devour this fat with avidity, as if instinctively guided to its use. It is by the enormous quantity of this substance taken in by them, that they are enabled to pass a large part of the year in a temperature below that of our coldest win- ter, spending a great portion of their time in the open air ; as well as to sustain the extreme of cold, to which they are occasionally sub- jected. And in consequence of its being more slowly introduced into the system than most other substances, a larger quantity may be taken in at one time, without palling the appetite; whilst its bland and non-irritating character favours its being retained until it is all absorbed. In this manner, the Esquimaux and Greenlanders are enabled to take in 20 or 30 pounds of blubber at a meal; and, when thus supplied, to pass several days without food.—On the other hand, among the inhabitants of warm climates there is comparatively little disposition to the use of oily matter as food; and the quantity of it contained in most articles of their diet is comparatively small. 436. In the Milk, which is the sole nutriment of young Mammalia during the period immediately succeeding their birth, we find an ad- mixture of albuminous, saccharine, and oleaginous substances; which indicates the intention of the Creator, that all these should be era- ployed as components of the ordinary diet. The Casein or cheesy matter is a proteine-compound; the Butyrine of butter is but a slight modification of its ordinary fats; and its sugar differs from that in common use, only by its larger proportion of wTater. The relative amount of these ingredients in the milk of different animals is subject, as we shall hereafter see, to considerable variation; but they con- stantly exist, at least in the milk of the Herbivorous Mammalia, and of those which, like Man, subsist upon a mixed diet. But it has been recently asserted, that the milk of the purely Carnivorous animals is 17 258 IMPORTANCE OF VARIETY IN MATERIALS OF FOOD. destitute of Sugar, consisting, like their food, of proteine-compounds and fatty matter only. 437. No fact in Dietetics is better established than the impossibi- lity of long sustaining health, or even life, upon any single alimentary principle. Neither pure albumen nor fibrin, gelatin nor gum, sugar nor starch, oil nor fat, taken alone for any length of time, can serve for the due nutrition of the body. This is partly due, so far as the non- azotized compounds are concerned, to their incapability of supplying the waste of the albuminous tissues. This reason does not apply, however, to the proteine-compounds; which can serve not only for the reparation of the body,'but can also afford the carbon and hydrogen requisite for the sustenance of its temperature. The real cause is to be found in the fact, that the continued use of single alimentary sub- stances excites such a feeling of disgust, that the animals experi- mented on seem at last to prefer starvation, rather than the ingestion of them. Consequently it is quite impossible to ascertain, by such experiments, the nutritive power of the different alimentary princi- ples ; no animal being capable of sustaining life upon less than two of them at least. The same disgust is experienced by Man, when too long confined to any article of diet, which is very simple in its composition ; and a craving for change is then experienced, which the strongest will is scarcely able to resist. Thus, in the treatment of Diabetes, a disease in which there is an undue tendency to the production of sugar in the system, it is very important to abstain completely from the introduction of saccharine or farinaceous matters in the food; but the craving for vegetable food, which is experienced when the diet has long consisted of meat alone, is such as to make perseverance in the latter very difficult; and a means has been latterly devised of supplying this want without injury, by the use of bread from which the starchy portion has been removed, the gluten or azo- tized matter alone being eaten.* 438. The organic compounds, which have been enumerated as supplying the various wants of the system, would be totally useless without the admixture of certain inorganic substances, which also form a constituent part of the bodily frame, and which are constantly being voided by the excretions, especially in the Urine. These sub- stances have various uses in the system. Thus common Salt, or the Chloride of Sodium, appears to afford, by its decomposition, the mu- riatic acid which is concerned in the digestive process, and the soda • As an illustration of the advantage of this treatment, even in unpromising cases, the Author may cite an instance which has come under his own observation. The patient was a man 72 years of age; the disease had lasted at least a year, and was decidedly on the increase; considerable loss of flesh and of muscular vigour had taken place; and the quantity of sugar in the urine was such as to make it quite sweet to the taste. By the careful restriction of his diet to animal flesh and gluten-bread, this individual has kept the disease in complete check for more than 15 months; he has gained flesh, and improved in strength; and his urine is no longer sweet. Having two or three times ventured upon a return to his ordinary diet, his old symptoms have immediately manifested themselves, warning him of the necessity of perseverance in the strict regimen prescribed for him. NECESSARY MATERIALS OF ANIMAL FOOD. 259 which is an important constituent of the bile. Its presence in the serum of the blood, also, and in the various animal fluids which are derived from this, probably aids in preventing the decomposition of the organic constituents of these fluids,—Phosphorus has been sup- posed, until recently, to be chiefly requisite as one of the materials of the nervous tissue (§ 383); and also, when acidified by oxygen, to unite with lime in forming the bone-earth by which bone is consoli- dated. But there is reason to believe, from the results of late inqui- ries, that the acid and alkaline phosphate of lime and soda are very important constituents of the various fluid secretions, and have a large share in their respective actions. It has even been maintained that the acid phosphate of lime is the essential ingredient in the gastric juice, by which the first solutions of the food are effected.—Sulphur exists in small quantities in several animal tissues ; but its part appears to be by no means so important as that performed by Phosphorus.—Lime is required for the consolidation of the bones, and for the production of the shells and other hard parts that form the skeletons of the Inverte- brata ; and also as the base of the acid phosphate, which has been just referred to as an important constituent of the animal fluids.— Lastly, Iron is an essential constituent of Heematosine; and is conse- quently required for the production of the red corpuscles of the blood in Vertebrated animals. 439. These substances are contained, more or less abundantly, in most of the articles generally used as food ; and where they are defi- cient, the animal suffers in consequence, if they be not supplied in any other way.—Thus, common Salt exists, in no inconsiderable amount, in the flesh and fluids of animals, in the milk, and in the substance of the egg; it is not so abundant, however, in Plants ; and the deficiency is usually supplied to herbivorous animals in some other way. Thus, salt is purposely mingled with the food of domesticated animals ; and in most parts of the world inhabited by wild cattle, there are spots wThere it exists in the soil, and to which they resort to obtain it. Such are the " buffalo-licks" of North America.—Phosphorus exists also, in combination with proteine-compounds, in all animal substances com- posed of these ; and in the state of phosphate, combined with lime, magnesia, and soda, it exists largely in many vegetable substances ordi- narily used as food. The phosphate of lime is particularly abundant in the seeds of the grasses ; and it also exists largely, in combination with casein, in Milk.—Sulphur is also derived alike from vegetable and animal substances. It exists, in union with proteine-compounds, in flesh, eggs, and milk ; also in several vegetable substances ; and, in the form of sulphate of lime, in most of the river and spring water that we drink. 440. Lime is one of the most universally diffused of all mineral bodies; for there are very few Animal or Vegetable substances in which it does not exist. The principal forms in which it is an element of Animal nutrition, are the carbonate and phosphate. Both these are found in the ashes of the grasses, and of other plants used as food; 260 MINERAL SUBSTANCES REQUIRED BY ANIMALS. the phosphate of lime being particularly abundant (as already men- tioned) in the corn-grains. The production of these cannot take place, to their fullest extent, unless the soil previously contain phos- phate of lime in a state in which the plant can receive it; and it is now understood, that the diminished fertility of many lands is due, in great part, to the exhaustion of the soil as regards this ingredient. The restoration of the alkaline and earthy phosphates to the soil, in the form of manure, is the obvious means of preserving its fertility; but so long as a very large proportion of the excrements of animals (the materials of which are originally derived from the earth, through the vegetables it supplies), is allowed to run to waste, so long will it be necessary that the requisite amount of phosphate of lime should be drawn from foreign sources. 441. The phosphate of lime, as already mentioned, seems to per- form important offices of a chemical nature in the animal economy, besides being the chief solidifying ingredient of bones and teeth ; but the carbonate would seem principally destined to mechanical uses only; and we find it predominating, or existing as the sole mineral ingredient, in those non-vascular tissues of the Invertebrated animals, which give support and protection to their soft parts (§ 289). The degree of development of these tissues depends in great part upon the supply of carbonate of lime which the animals receive. Thus, the Mollusca which inhabit the sea, find in its waters the proportion of that substance which they require; but those which dwell in streams and fresh water lakes, that contain but a small quantity of lime, form very thin shells ; whilst the very same species inhabiting lakes, which, from peculiar local causes, contain a large impregnation of calcareous matter, form shells of remarkable thickness.—The Crustacea, which periodically throw off their calcareous envelop (§ 297), are enabled to renew it with rapidity by a very curious provision. There is laid up in the walls of their stomachs a considerable supply of calcareous matter, in the form of little concretions, which are commonly known as " crabs' eyes." When the shell is thrown off, this matter is taken up by the circulating current, and is thrown out from the surface, mingled with the animal matter of which the shell is composed. This hardens in a day or two, and the new covering is complete. The concretions in the stomach are then found to have disappeared ; but they are gradually replaced, before the supply of lime they contain is again drawn upon. The large amount of carbonate of lime which is required by the laying Hen, is derived from chalk, mortar, or other substances containing it, which she is compelled by her instinct to eat; and if the supply of these be withheld, the eggs which she deposits are soft on their exterior,—not being destitute of shell, as commonly supposed,—but having the fibrous element of the shell (§ 181) unconsolidated by the intervening deposit of chalky particles. SIMPLEST FORMS OF DIGESTIVE APPARATUS. 261 2. Of the Digestive Apparatus, and its Actions in general. 442. It has been already pointed out, that the nature of the food of Animals is so far different from that of Plants, as to require the preparatory process of Digestion, before its nutritious parts can be taken up by the absorbent vessels and received into the system. This process may be said to have three different purposes in view:— the reduction of the alimentary matter to a fluid form, so that it may become capable of absorption; the separation of that portion of it which is fit to be assimilated or converted into organized texture, from that which cannot serve this purpose, and which is at once rejected;—and the alteration, to a certain extent when required, of the chemical constitution of the former, which prepares it for the important changes it is subsequently to undergo. The simplest con- ditions requisite for the accomplishment of these purposes are the following:—a fluid capable of performing the solution, and of effect- ing the required chemical changes;—a fluid capable of separating the excrementitious matter, by a process analogous to chemical pre- cipitation ;—and a cavity or sac in which these operations may be performed. 443. In the lowest Animals, we find this cavity formed upon a very simple plan; the digestive sac being a mere excavation in the solid tissue of the body, lined with a membrane which is an inverted continuation of the external integument, and communicating with the exterior by one orifice only, through which food is drawn in, and excrementitious matter rejected. In the little Hydra, or fresh-water Polype, the external covering of the body and the lining of the stomach correspond so closely in their structure,—their actions dif- fering only with their situation,—as to be mutually convertible; for the animal may be turned completely inside-out, without its functions being deranged. The fluid necessary to dissolve the food, known by the name of "gastric fluid," or "gastric juice," is secreted in the walls of the stomach; and, from the transparency of the tissues, the whole process may be watched. The prey is frequently, and indeed generally introduced alive, by the contractile power of the arms, which coil round it, and gradually draw it into the mouth or entrance to the stomach; and its movements may often be observed to con- tinue for some time after it has been swallowed. In a little time, however, its outline appears less distinct, and a turbid film partly conceals it; the soft parts are soon dissolved and reduced to a fluid state; and any firm indigestible portions which the body may con- tain, are rejected through the aperture by which it entered. The nutritive matter is absorbed by the walls of the stomach, every part of which appears to be endowed with equal power in this respect; and it is conveyed to the remoter parts of the arms by the simple imbibition of one part from another, without any proper circulation through vessels. 444. In Polypes of a higher conformation, however, the digestive cavity is provided with a second orifice; from the dilated cavity or 262 VARIOUS FORMS OF DIGESTIVE APPARATUS. stomach, an intestinal tube proceeds ; and this has a termination dis- tinct from the mouth, though often in its neighbourhood. The food, before entering the stomach, is submitted to a powerful triturating apparatus, resembling the gizzard of birds, by which it is broken down; and in the digestive cavity it is submitted, not merely to the action of the gastric fluid, but also to that of the bile, which is secreted in little follicles in the*walls of the stomach, and which is poured into its. cavity during the process of digestion,—-being easily recognized by its bright yellow colour. The excrementitious matter is rejected in the form of little pellets, through the intestinal tube. 445. As we ascend the Animal scale, we find the digestive appa- ratus gradually increased in complexity ; but its essential characters remain the same. Near the entrance to the stomach, we usually find an apparatus for effecting the mechanical reduction of the food, by which its subsequent solution may be rendered more easy. This may consist of a set of teeth ; either fixed in the mouth, as in Mammalia and Reptiles; or more particularly besetting the pharynx, as in Fishes; or attached to the walls of the stomach, as in Crustacea. Or it may be formed by the tongue, converted into a sort of rasp; as in the common Limpet, which thus reduces the sea-weeds that con- stitute its chief food. Or the same purpose may be answered by a gizzard, or first stomach, with dense muscular and tendinous walls; such as we find in the grain-eating Birds, and many Insects, and in certain Mollusks and Polypes. But where the food is already com- posed of very minute particles, or is received in a liquid state, (as in the case of those animals which live upon the juices of others,) or is easily acted on by the gastric juice, no such preparation is requisite. 446. Before the food reaches the true digestive stomach, it is sometimes delayed in a previous cavity, in order that it may be macerated in fluid, and may be thoroughly saturated with it. This is the purpose of the crop of Birds, and of the first stomach of Ru- minant animals. WThen this incorporation with fluid is not effected before the food is subjected to the triturating process, it usually takes place concurrently with it; and in those animals which reduce their food in the rnouth by the process of mastication, there is a special secretion of fluid into that cavity for this purpose ; this fluid is termed Saliva, and the act by which it is incorporated with the food is termed insalivation. The mechanical reduction of the aliment, and its in- corporation with fluid, constitute, as we shall hereafter see, a very important preparation for the true digestive process. 447. This process, among higher animals, takes place exclusively, or nearly so, in the stomach; the form of which varies with the cha- racter of the food. When this is of a nature to be easily acted on by the gastric fluid, the stomach is a simple enlargement of the aliment- ary canal, almost in the direct line between the oesophagus and the intestinal tube; so that there is little provision for the delay of the food in its cavity. But when the aliment is such as to be less easily reduced, and requires to be submitted to the action of the gastric fluid, for a longer period, the stomach forms a more considerable VARIOUS FORMS OF DIGESTIVE APPARATUS. 263 enlargement, and is placed more out of the direct line between the oesophagus and the commencement of the intestine. The former condition obtains in the Carnivora, and particularly in those which live more upon blood than upon flesh,—such as Weasels, Stoats, &c, in which this part of the alimentary tube is almost straight; the latter condition is found among the Herbivora, and the provision for the delay of the aliment attains its greatest complexity in the Ruminant animals. The form of the Human stomach (Fig. 70) is intermediate between that of purely carnivo- rous and purely herbivo- rous animals. As in the former, there is a direct passage from the cardiac orifice or entrance of the oesophagus, to the pyloric orifice or commencement of the intestine; but there is also a considerable di- latation or cul de sac, which is out of that line; and it appears that, during the digestive pro- cess, there is a constric- tion across the stomach, which separates the car- diac portion from the py- loric, and causes the re- tention of the food in the dilated part or large ex- tremity. The gastric fluid is still secreted in the walls of this organ, by scattered follicles which pour their products into its cavity through sepa- rate orifices; but the bile is elaborated by a distinct organ, alto- gether removed from it, which transmits its secretion by a single duct, that opens into the intestinal tube at a short distance from its commencement. 448. The action of the Stomach is restricted, in the higher animals, to the reduction of the food by the solvent powers of the gastric juice, and to the absorption (by the vessels in its walls) of those parts of it which are in a state of the most perfect solution. The change which is produced by the admixture of the bile, takes place in the intestine; and the principal part of the nutritive elements of the food are taken up by the absorbent vessels of the walls of the intestine, after that process has been accomplished. It would seem as if the preparation Fig. 70. A vertical and longitudinal section of the Human stomach and duodenum, made in such a direction as to include the two orifices of the stomach. 1. The oesophagus; upon its internal surface the plicated arrangement of the cuticular epithelium is shown. 2f. The cardiac orifice of the stomach, around which the fringed border of the cuticular epithelium is seen. 3. The great end of the stomach. 4. Its lesser or pyloric end. 5. The lesser curve 6. The greater curve. 7. The dilatation at the lesser end of the stomach, which has received from Willis the name of antrum of the pylorus. This may be regarded as the Tudiment of a second stomach. 8. The rugae of the stomach formed by the mucous mem- brane : their longitudinal direction is shown. 9. The pylo- rus. 10. The oblique portion of the duodenum. 11. The descending portion. 12. The pancreatic duct and the ductus communis choledochus close to their termination. 13. The papilla upon which the ducts open. 14. The transverse portion of the duodenum. 15. The commencement of the jejunum. In the interior of the duodenum and jejunum, the valvulae conniventes are seen. 264 DIGESTIVE APPARATUS OF MAN. of the food for absorption were not by any means completed, in this first portion of the alimentary canal; for it is still destined to pass through a long and convoluted tube, which is sometimes extended to an extraordinary degree; and in this passage it is gradually ex- hausted of its nutritious matter. The length of the intestinal canal bears a close relation to the character of the food. In the Carnivorous animals, whose aliment is easily dissolved and prepared for conver- sion into blood, the intestine is comparatively short; thus in the Lion and other Felines it is no more than three times the length of the body; and in some of the blood-sucking Bats,'it is almost straight and simple. On the other hand, in Herbivorous animals it is of enor- mous length; thus in the Sheep it is about twenty-eight times as long as the body. In animals whose diet is mixed, its length is inter- mediate between these extremes; thus in Man, the whole length of the intestinal tube is about thirty feet, or between five and six times that of the body.—The intestine is of much smaller diameter along its first portion, than it is nearer its termination; and it is consequently distinguished into the small and the large. In the small intestine, which constitutes in Man about five-sixths of the whole, the surface of the mucous membrane is greatly extended by the valvulce conni- ventes, which are folds or duplicatures, often several lines in breadth, not entirely surrounding the intestine, but extending .for about one- half, or three-fourths of its circumference. These are wanting at the lower part of the ileum. The whole surface of the mucous membrane of the small intestine, below the entrance of the biliary ducts, is thickly covered with villi, or little root-like projections, in which the proper absorbent vessels originate. No proper valvulae conniventes exist in the large intestine; the only extensions of the mucous membrane being crescentic folds at the edges of the sacculi or pouch-like dila- tations in its walls; and the villi are comparatively few in number, gradually disappearing towards the termination of the intestine. 449. The mucous membrane of the alimentary canal, through its whole course, is studded with the orifices of numerous scattered glands, which lie in its thickness, or immediately beneath it. The simplest of these are the follicles of Lieberkiihn, which are small pouches, formed by an inflexion of the mucous surface, analogous to the follicles of other mucous membranes, and Fig 7i. apparently destined for the elaboration of the protective secretion (§ 237, see Figs. 27 and 28). These follicles, in the small intestine, are very simple in their character, and not very deep; and their apertures, which are small, are situated for the most part around the bases of the villi. In the large intestine they are more pro- Mucous coat of small in- longed, especially towards the extremity of the testines as altered in fever; J- \ A r ^■ • i the follicles of Lieberkflhn rectum, where they form a distinct layer, the SecretEr? tenaci°us whUe component tubes of which are visible to the naked eye; they probably form the peculiarly thick and tenacious mucus of that part. These mucous follicles ALIMENTARY CANAL AND ITS MOVEMENTS. 265 become particularly evident when the membrane is inflamed; for they then secrete an opaque whitish matter, which is absent in the healthy state, and which distinguishes their orifices (Fig. 71).—A modified kind of these follicles, rather more complex in structure, is found abundantly in the stomach; where it is concerned in the secretion of the gastric fluid (§ 469). 450. The coats of the intestine contain other glandulae, which appear destined, not so much to elaborate fluids of use in the system, as to draw off from the blood certain products of decomposition, which are to be excreted from it. These are commonly known as the glands of Brunner, and of Peyer, after the names of their respective discoverers.—The glands of Brunner are situated in the duodenum, and lie, not in the mucous but in the sub-mucous tissue. Though their size is only about that of a hemp-seed, they are of very complex structure, consisting of several hundred follicles, clustered round the ramifications of an excretory duct, so as to resemble the Salivary glands; and each pours its secretion through a single orifice into the intestinal tube. The glands of Peyer are either solitary or agminated; the latter form large patches, which are made up of aggregations of the former. Each solitary gland consists of a closed spheroidal vesi- cle, which is half imbedded in the mucous membrane, but which also forms an elevated projection above it; and this projection is sur- rounded by a ring or zone of openings, which lead into an annular cluster of Lieberkuhnian follicles. On rupturing one of the Peyerian vesicles, its cavity is found to contain a grayish-white matter, inter- spersed with cells in various stages of development. The complete closure of this cavity would seem to render it an exception to all general rules of glandular structure ; but this is not so in reality; for it will be shown hereafter that many other glandular follicles in an early stage of their development are equally closed (Chap. IX.); and it appears that the Peyerian vesicles, when mature, discharge their contents .by an opening which then forms in the most projecting por- tion of their walls,—these contents passing at once into the cavity of the intestine, instead of being poured (as in other cases) into an excretory duct.—Of the nature of the secretions of these intestinal glandulae, nothing has been positively ascertained ; but some probable inferences from well-known facts will be stated hereafter (Chap. XI). 3. Movements of the Alimentary Canal. 451. The food which is conveyed to the mouth, is grasped with the lips, by a muscular effort, which is voluntary in the adult under ordinary circumstances, but which may be performed instinctively when the influence of the will is withdrawn; in the infant, as among the lower animals, the action seems purely instinctive, the nipple of the mother being firmly grasped by the lips when introduced between them, even after the brain has been removed.—By the act of masti- cation, which then succeeds, the food is triturated and mingled with 266 MASTICATION. the salivary secretion ; and is thus prepared for the further process of solution, to which it is to be subjected in the stomach. The degree of this preparation, and the form of the instruments by wThich it is effected, vary in different animals, according to the nature of the food. In those Carnivora, whose aliment consists exclusively of flesh, very little mastication is necessary, because this substance is very readily acted on by the gastric fluid; and we accordingly find the molar teeth raised into sharp cutting edges, and working against each other with a scissors-like action (the only one permitted by the articulation of the jaw), so as simply to divide the food. On the other hand, in those Herbivora, whose food consists of tough vegetable substances, such as the leaves of grasses, or the stems and roots of other plants, we find the molar or grinding teeth peculiarly adapted to its reduction; their surface being extended horizontally, and being kept continually rough, by the alternation of vertical plates of different degrees of hardness; and the lower jaw being so connected with the skull, that great freedom of motion is permitted. In Man we find an inter- mediate conformation, as regards both the teeth and the articulation of the jaw; for the molar teeth possess broad surfaces, which are covered with a continuous coat of enamel, but which are raised into rounded tubercles; and the articulation of the jaw allows it a degree of freedom, which is much greater than that possessed by the Carni- vora ; although inferior to that which exist in many Herbivora. The whole apparatus of Mastication is so formed in Man, as to lead to the conclusion that he is destined to live on a mixed diet, composed in part of animal flesh, and in part of vegetable substances that are sufficiently soft to be reduced by the simple act of crushing, or by the slight trituration for which the molar teeth are adapted. 452. The mechanical reduction of the food by Mastication, and the incorporation of the Salivary secretion with its substance, consti- tute a very important step in the Digestive process. We shall here- after see that the operations, to which the alimentary matter is subjected in the stomach, are of a purely Chemical nature ; and this preparation is exactly of the same character as that which the Chemist finds it advantageous to make, when he is operating on a substance of difficult solution. For nothing is so favourable to the action of the solvent, as the previous reduction of the matter to be dissolved, and its thorough incorporation with the fluid that is to act upon it. We shall hereafter see, that the relative properties of the Saliva and of the Gastric fluid are such, that, by the minute admixture of the food with the former, the latter finds access to every particle of it. Hence the practice of eating so rapidly, that Mastication and Insali- vation are insufficiently performed, is extremely injurious; since it throws more work upon the Stomach than it ought to perform, by rendering its solvent action more difficult. There can be no doubt that, by the prolonged continuance of it, a foundation is laid for the distressing complaint termed Dyspepsia, or difficulty of digestion ; and where any form of this complaint exists, too much attention cannot be paid to the efficient reduction of the food in the mouth. ACT OF DEGLUTITION. 267 453. When the aliment has been sufficiently triturated, it is con- veyed into the Pharynx by the act of Deglutition or swallowing. This act involves a great many distinct movements, into a minute descrip- tion of which we shall not here enter; but it is desirable that its general nature should be well understood. It is one of those most purely reflex in its character (§ 394), and is not capable of being per- formed or even controlled by a voluntary effort. This statement may seem inconsistent with the fact, that we swallow when we will; but it is not so in reality. The muscular movements which are concerned in deglutition, are excited by nerves that proceed from the spinal cord, not from the brain; these motor nerves are excited to action, by the contact of solid or fluid matters with the mucous surface of the fauces, —and in no other way. The impression produced by the contact is conveyed to the medulla oblongata, or that portion of the spinal cord which lies within the cranium, by afferent nerves that terminate in it; and, in immediate respondence to that impression, a motor impulse is transmitted from it, which calls the muscles into the combined action necessary to produce the movement. Now this contact also produces a sensation, provided the brain be sound and awake, because nervous fibres proceed from the mucous surface to the brain as well as to the spinal cord; but this sensation is not a necessary link in the chain of actions, by which the movement is produced ; for the act of Degluti- tion takes place during profound sleep, when all sensation is suspended, and it may be excited even after the brain has been removed. It seems to be voluntary, under ordinary circumstances, simply because it is by an act of the will, that the matter to be swallowed is carried backwards into contact with the fauces ; but that it is not so in reality, is shown by the fact, that when this impression has once been made with sufficient force, we cannot by any effort of the will, prevent the action. We have a good example of this in the following circum- stance, of no very unfrequent occurrence. The tickling of the upper part of the fauces with a feather is often practised to induce vomiting; but if the end of the feather be carried too far down, it excites the act of deglutition instead ; the feather is grasped by the pharynx and drawn downwards ; and if it be not held tenaciously between the fingers, it is drawn from them and carried downwards into the sto- mach. 454. The carrying-back of the alimentary matter, so that it reaches the fauces or upper part of the pharynx, is principally accomplished by the tongue; when it has passed the anterior palatine arches, these contract and close over the tongue, so as to prevent the return of the food into the mouth ; and at the same time the posterior palatine arches and the uvula are so drawn together, as to prevent its passage into the posterior nares. The larynx is drawn forwards beneath the root of the tongue, and the epiglottis is pressed down over the rima glot- tidis, so that nothing can enter the latter, unless drawn towards it by an act of inspiration. When fairly within the pharynx, the alimentary matter is seized by the constrictors which enclose that part of the ali- 268 MOVEMENTS OF ffiSOPHAGUS. mentary tube, and is drawn downwards by them into the oesophagus, which is the cylindrical continuation of it. The continued action of the constrictors serves to propel the food along the oesophagus; their movement being of a reflex nature, excited by the contact of the sub- stance contained in the tube, with its lining membrane,—which pro- duces an impression that is transmitted to the medulla oblongata, and is reflected back as a motor impulse to the muscles. We have here a distinct case of reflex action without sensation ; for we have no consciousness of the ordinary passage of food down the oesophagus, unless it occasion pressure on the surrounding parts through its bulk, or unduly irritate the lining membrane by its high or low temperature or its acrid qualities; and yet it may be shown by experiment, that the completeness of the nervous circle is requisite for the excitement of the movement, which will not take place when it is interrupted either by division of the nerves, or by destruction or paralysis of the medulla oblongata. 455. The progress of the food along the Oesophagus is aided by the action of the muscular coat peculiar to it. This is coraposed of the non-striated fibre; and, like that of the intestinal canal further on, it is usually stimulated to contraction by the direct contact of the stimulus, and not either by the will, or by the reflex action of the spinal cord. The movement produced by it is of the peristaltic or wave-like kind ; the contractions being limited to one portion of the tube, and being propagated along it from above downwards. This action continues after the division of all the nerves supplying the oesophagus; and it cannot, therefore, be dependent upon the brain or spinal cord. It may be observed to take place in a rhythmical man- ner (that is, at short and tolerably regular intervals), whilst a meal is being swallowed; but as the stomach becomes full, the intervals are longer and the wave-like contractions less frequent. The degree in wdrich the action of the oesophagus alone, without that of the sur- rounding muscles, is capable of propelling the food into the stomach, seems to vary in different animals. When the latter are paralyzed in the Dog, by section of the nerves that supply them, the food that has entered the oesophagus is still propelled into the stomach; but this is not the case in the Rabbit, the action of its oesophageal fibres not being sufficient to carry the food onwards to the stomach, though it will expel it from the divided extremity of the tube when it is cut across. The usual peristaltic movements of the ceosophagus are re- versed in Vomiting; and this reversion has been observed, even after the separation of the stomach from the oesophagus, as a consequence of the injection of tartar emetic into the veins. 456. At the point where the oesophagus enters the Stomach,—the cardiac orifice of the latter,—there is a sort of sphincter, or circular muscle, which is usually closed. This opens when there is a suffi- cient pressure on it, made by the accumulated food propelled by the movements of the oesophagus above ; and it then closes again, so as to retain the food in the stomach. The closure is due to reflex ac- TERMINATION OF G3S0PHAGUS IN RUMINANTS. 269 tion; for when the nerves supplying it are divided, the sphincter no longer contracts, and the food regurgitates into the oesophagus. The opening of the cardiac is one of the first acts which takes place in vomiting. 457. In Ruminating animals, there is a very remarkable conforma- tion at the lower end of the oesophagus, which is destined to regulate the passage of food into the different compartments of the stomach, according as it has been submitted to the second mastication, or not. The oesophagus does not terminate at its opening into the first stomach or paunch, but it is continued onwards as a deep groove with two lips (Fig. 73): by the closure of these lips it is made to form a tube, which serves to convey the food onwards into the third stomach; but when they separate, the food is allowed to pass either into the first or the second stomachs. When the food is first swallowed, it undergoes but very little mastication; it is consequently firm in its consistence, and is brought down to the termination of the oesophagus in dry bulky masses. These separate the lips of the groove or demi-canal, and pass into the first and second stomachs. After they have been macerated in the fluids of these cavities, they are returned to the mouth by a Fig. 72. Stomach of Sheep;—a, oesophagus; 6, paunch; c, second, or honeycomb stomach; d, third stomach, or many-plies; e, fourth stomach or reed; /, pylorus. reverse peristaltic action of the oesophagus; this return takes place in a very regular manner, the food being shaped into globular pellets by compression within a sort of mould formed by the demi-canal, and these pellets being conveyed to the mouth at regular intervals, appa- rently by a rhythmical movement of the oesophagus. It is then sub- jected to a prolonged mastication within the mouth (the " chewing of the cud"), by wdiich it is thoroughly triturated and impregnated with saliva; after which it is again swallowed in a pulpy semi-fluid state. It now passes along the groove which forms the continuation of the oesophagus, without opening its lips ; and is thus conveyed into the third stomach, whence it passes to the fourth, in which alone the true digestive process takes place. Now, that the condition of the food as to bulk and solidity, is the circumstance which determines the opening or closure of the lips of the groove, and which consequently occasions its passage into the first and second stomachs, or into the third and fourth, appears from the experiments of Flourens; who found that when the food, the first time of being swallowed, was arti- 270 MOVEMENTS OF THE STOMACH. Fig. 73. ficially reduced to a soft and pulpy condition, it passed for the most part along the demi-canal into the third stomach, as if it had been ruminated,—only a small portion finding its way into the first and second stomachs. How far the actions of this curious apparatus are dependent upon nervous influence,—or how far they are due to the exercise of the contractility of the muscular fibre, directly excited by the contact of the substances with the lining membrane of the tubes and cavities,—has not yet been clearly ascertained. 458. The food, when introduced into the Stomach, and submitted to the solvent action of its secretions, is also subjected to a peculiar movement, which is effected by the muscular walls of that organ. The purpose of this motion is obviously to keep the contents of the stomach in that state of constant agitation, which is most favourable to their chemical so- lution ; and particularly to bring every portion of the alimentary matter into contact with the walls of the stomach, so as to be sub- jected to the action of the fluid, which is "poured forth from them during the digestive process. The movement is produced by the alternate shortening and re- laxation of the various fasciculi, which are disposed in almost every direction throughout the muscular wall of the stomach ; and it seems to produce a kind of revolution of the contents of the stomach, sometimes in the direction of its length, and some- times transversely. Its result is well shown in the hair-balls, which are occasionally found in the stomachs of animals, that have swallowed hair from time to time through licking their skins; the component hairs not being pressed together confusedly, but being worked together in regular directions, and so interwoven that they cannot be readily separated. As digestion proceeds, the dissolved fluid escapes, little by little, through the pyloric orifice, which closes itself firmly against the passage of solid bodies ; and this motion continues, until the sto- mach is completely emptied ; when it ceases, until food is again in- troduced. The bulk of the alimentary mass diminishes rapidly, as Section of part of the Stomach of the Sheep, to show the demi-canal of the oesophagus ; the mucous membrane is for the most part removed, to show the arrangement of the muscular fibres. At a is seen the termination of the oesophageal tube, the cut edge of whose mucous membrane is shown at b. The lining of the first stomach is shown at c, c; and the mucous membrane of the second stomach is seen to be raised from the subjacent fibres at d. At e, e, the lips of the demi-canal are seen bounding the groove, at the lower end of which is the entrance to the third stomach or many-plies. MOVEMENTS OF THE INTESTINAL CANAL. 271 the solvent process is near its completion ; and the separation of the fluid product or chyme is aided by a peculiar action of the transverse fasciculi, which surrounds the stomach at about four inches from its pyloric extremity. These shorten in such a manner, as to produce a sort of hour-glass separation between the portions of the stomach on either side of it ; and the fluid solution, being received by the pyloric or smaller portion, is pumped away through the pylorus; whilst the solid matter yet undissolved is retained in the larger division. 459. The degree in which these movements are dependent upon the nervous system, or are under its control or direction, has not yet been clearly ascertained. Distinct movements may be excited in the stomach of a Rabbit, if it be distended with food, by irritating the par vagum soon after the death of the animal; these movements seem to commence from the cardiac orifice, and then to spread them- selves peristaltically along the walls of the stomach; but no such movements can be excited if the stomach be empty. On the other hand, there is distinct proof, that all the movements necessary to di- gestion may take place after the section of that nerve ;" although the first effect of the operation appears to be to suspend them completely. It is probable that the movements of the stomach are more regular and energetic in Herbivorous animals, whose food is difficult of di- gestion, than they are in the Carnivora, whose aliment is dissolved with comparative facility. 460. From the time that the ingested matter enters the Intestinal tube, it is propelled onwards by the peristaltic contractions of its mus- cular coat; which are excited, independently of all nervous influence, by the contact of the aliment, or by that of the secretions mingled with it in its passage along the canal. These last appear to have an important effect; for we find that, when the bile-duct is tied, so as to prevent the bile from entering the intestine, constipation always oc- curs ; whilst an increase of the biliary and other secretions, consequent upon the action of mercury or upon any other cause, produces an increased peristaltic movement, and a more rapid discharge of the excrementitious matter. During the passage of the alimentary matter along the small intestine, as we shall see hereafter, a large proportion of its fluid is removed, by the absorbent pow7er of the villi; and the residue is again brought, therefore, to a more solid consistence. This residue consists in part of those portions of the aliment, which are not capable of being dissolved or finely divided, so as to be received by the absorbents; and in part of the matters poured into the ali- mentary canal, by the various glands that discharge their contents into it, for the purpose of being carried out of the body. The feces, which are thus formed, are propelled through the large intestine, by the continued peristaltic action of its walls, until they arrive at the rectum. 461. That the ordinary peristaltic action of the intestinal canal is independent of nervous influence, is sufficiently proved by the fact, that it will continue when the tube is completely separated from all 272 DEFECATION. connection with the nervous centres; as well as by the difficulty, already adverted to (§ 353), of exciting contractions in the muscular coat by any stimulation of its nerves. All the nerves of the intestine, from its commencement at the pyloric orifice of the stomach to its ter- mination at the anus, are derived from the ganglia of the sympathetic system ; but there is evidence that those which influence its movements are really derived from the spinal cord (see Chap. XII). Although the will has no influence whatever on the peristaltic movement, yet the emotions seem to affect it; and it is probably to convey their influ- ence that the intestinal canal is supplied with motor nerves. It is also furnished with sensory nerves, which form part of the trunks of the sympathetic system, but which really pass onwards to the brain; these do not, however, make us conscious of the passage of the ali- mentary matter along the canal, so long as it is in a state of health ; but in various diseased conditions, they give rise to sensations of the most painful nature. 462. For the occasional discharge of the feces from the rectum, and for the retention of them at other times, we find the outlet or anal orifice, provided with an additional muscular apparatus, which is con- nected with the spinal system of nerves. The act of defecation is due tothe pressure upon the contents of the rectum, which is occasioned by the combined contraction of the diaphragm and the abdominal muscles; whilst, on the other hand, the retention of the feces is due to the con- tractile power of the sphincter muscle, which surrounds the anus. The action of the sphincter ani, like that of the sphincter of the cardia, is a reflex one; dependent upon the connection of the muscle, by excitor and motor nerves, with the spinal cord. If the lower portion of the cord be destroyed, or if the nerves be divided, the sphincter loses its con- tractile power, and becomes flaccid. When in proper action, however, its power is sufficient to prevent the escape of the contents of the rec- tum ; until the expulsive force becomes very strong, in consequence either of the quantity of feces which has accumulated, or the acridity of their character. In either case, the impression made upon the mu- cous merabrane of the rectum is conveyed to the spinal cord ; and, by a reflex motor impulse, the muscles of defecation are thrown into com- bined action, the resistance of the sphincter is overcome, and the feces are expelled. An unduly irritable state of the mucous membrane, or a disordered state of the excrementitious matter (resulting from the irritating character of the substances swallowed, from the acrid charac- ter of the secretions poured into the canal, or from an unusual change in the aliment during the digestive process), may occasion unduly fre- quent calls upon the muscles of defecation, which the sphincter is unable to resist. On the other hand, if the progress of the feces be delayed in the large intestines, by deficient peristaltic movement, they accumulate higher up, and the act of defecation is not excited. 463. Although the sphincter ani on the one hand, and the muscles of defecation on the other, are called into action by the reflex power of the spinal cord, and are so far involuntary in their operation, yet they are also in some degree subject (in Man at least) to the influence MUCOUS SECRETION. 273 of the will. The resistance of the sphincter may be increased by a voluntary effort, when it is desired to retain the feces in opposition to the power of the expulsors; and it is only when the latter operate with excessive force that they can overcome it. On the other hand, the expulsors may be called into action, or maybe aided, by the will, when the stimulus to their movement received through the spinal cord, would not otherwise be strong enough; and the feces may thus be evacuated by a voluntary effort, at a time when they would not other- wise be discharged. 4. Of the Secretions poured into the Alimentary Canal, and of Changes which they effect in its contents. 464. The whole Mucous Membrane of the Alimentary canal^ from the mouth to the anus, is covered during health with that peculiar viscid secretion termed mucus, of which the characters have been already described (§ 237). This is formed, partly, on the free sur- face of the membrane itself, but chiefly in the numerous follicles or depressions by which that surface is increased; and it appears de- stined for the protection of the delicate, highly vascular membrane ' from undue irritation by the contact of the substances, which are passing through the alimentary tube. When these are unusually acrid, the secretion of mucus is augmented in quantity, and is increased in vis- cidity, so as to form an effective sheath to the merabrane, which would otherwise suffer severely. When this secretion is deficient, the mem- brane is irritated by the contact of any but the blandest substances ; and the class of remedies termed demulcents are useful in coating and protecting it. 465. Duringthe mastication of the food in the mouth, the Salivary secretion is poured in, for the purpose of being mingle'd with it, and of rendering the act of mastication more easy. This secretion is formed by three pairs of glands,—the Parotid, the Sub-lingual, and the Sub- maxillary; these are composed of minute follicles, whose diameter is about l-1000th of an inch, connected together by branches of their ducts, upon which they are set like grapes upon their stalk, surrounded by a plexus of blood-vessels, and bound together by areolar tissue. Within the follicles are the true secreting cells (§ 238); by whose growth and development, the material of the secretion is separated from the blood. These salivary cells are often to be recognized in the saliva; they must not, however, be con- founded with the epithelium cells of the mucous membrane of the mouth, which are Lobule of Parotid Gland of much larger. The fluid obtained from the ^S£^«r" mouth is not pure saliva; for the mucus of 18 274 SALIVARY GLANDS AND THEIR SECRETIONS. the mouth itself is mingled with the secretion from the salivary glands. If the proportion of the former be considerable, it gives to the fluid of the mouth an acid reaction; whilst if the latter be pre- dominant (which it is directly before, and during the act of eating), the fluid of the mouth has an alkaline reaction. It may be some- times observed, that the saliva of the mouth will strike a blue colour with reddened litmus paper, whilst it turns blue litmus paper red; thus showing the presence both of an acid and an alkali in a state of imperfect neutralization. 466. The solid matter of the Salivary secretion is about 1 per cent. of the whole; and this consists in part of animal principles, and in part of saline substances. The animal matter consists of osmazome, mucus, and a peculiar substance termed ptyaline or salivary matter; which is soluble in water and insoluble in alcohol, and which is yet different from both albumen and gelatin. This substance appears to have a decided effect in producing the metamorphosis of certain ali- mentary substances, on which it acts like a ferment. Starch may be converted into sugar, and sugar into lactic acid, by its agency; and, if concentrated, it has a certain solvent power for casein, animal flesh, and other proteine-compounds. Its chemical nature has not yet been precisely determined. The saline constituents of the Saliva are nearly identical with those of the blood; the chlorides of sodium and potassium form considerably more than half; and the remainder consists chiefly of the tribasic phosphate of soda, to which the alka- line reaction of the fluid is due, with the phosphates of lime, magnesia, and iron. It is of the earthy phosphates, that the tartar which col- lects about the teeth is chiefly composed ; the particles of these being held together by about 20 per cent, of animal matter: and the compo- sition of the concretions, which occasionally obstruct the salivary ducts, is nearly the same. 467. The quantity of Saliva formed during the twenty-four hours, has been estimated at from 15 to 20 ounces ; but on this point it is impossible to speak with certainty. The secretion is by no means constantly flowing; indeed it is almost entirely suspended, when the masticator muscles and tongue are at perfect rest, unless it be excited by any mental cause; and hence it is, that the mouth becomes dry during sleep, if it be not kept closed. The flow of Saliva takes place just when it is most wanted; that is, when food has been taken into the mouth, and when the operation of mastication is going on. But it will also take place, especially in a hungry person, at the sight, or even at the idea, of savoury food; as is implied by the common ex- pression of the " mouth watering" for such an object. The influence thus exercised over it by the emotional state of the mind, is probably conveyed to the salivary glands by the Fifth pair; which contains many of the gray or organic filaments ; and which seems to take the place, in the Head, of a distinct lymphatic system. 468. Having been conveyed into the Stomach, the food is sub- mitted to the action of the Gastric Fluid, which is secreted in the GASTRIC FOLLICLES AND THEIR SECRETION. 275 walls of that organ. This fluid is not present in the empty stomach ; its secretion being excited by the presence of food, or by the irritation of the walls of the organ by some solid body. In the intervals be- tween the digestive process, the mucous membrane is of a light pink hue; but it becomes more turgid with blood, when the presence of food calls for the activity of its secreting processes. It is of a soft and velvet-like appearance ; and it is constantly covered with a very thin transparent viscid mucus, which has neither acid nor alkaline reaction. By applying aliment or other stimulants to the internal coat of the stomach, and by observing the effect through a magnifying glass, numerous minute papillae can be seen to erect themselves upon the mucous membrane, so as to rise through the coating of mucus; and from these is poured forth a pure, limpid, colourless, slightly viscid fluid, having a distinctly acid reaction, which is the Gastric juice. This fluid is secreted by follicles, which are lodged in the walls of the stomach, and which closely resemble those that else- where secrete mucus ; but they are usually of more complex structure, and are more numerous. 469. If the Mucous Membrane of the stomach be divided by a section perpendicular to its walls, it is seen to be made up, as it were, of tubular follicles closely applied to each other; their blind extremi- ties resting upon the submucous tissue, and their open ends being di- rected towards the cavity of the stomach. In some situations, these tubuli are short and straight; in other parts they are longer, and present an appearance of irregular dilatation or partial convolution (Fig. 75, 1). This is their usual character, especially near the car- diac orifice of the stomach ; but near the pyloric orifice they have a much more complex structure (Fig. 75, 2). These tubular follicles are arranged in bundles or groups, and are surrounded and bound- Fig. 75. Fig. 76. together by a fine areolar membrane ; and this also serves to convey vessels from the submucous tissue, wThich ramify among the follicles, and supply the materials for their secretion. The number of tubuli in each group is by no means constant. The follicles do not, in general, open directly upon the surface; but into the bottom of small 276 PROPERTIES OF GASTRIC FLUID AND OF PEPSINE. depressions or pits, which may be seen to cover the membrane. These pits are more or less circular in form ; and are separated from one another by membranous partitions, which vary in depth, and some- times by pointed processes, which are capable of erecting themselves in the manner just described. The diameter of these pits varies from about 1-100th to l-250th of an inch ; it is always greatest near the pylorus. The number of the gastric follicles opening into each, is usually from three to five; but there are sometimes more. 470. The chemical composition of the Gastric fluid has been a sub- ject of much discussion, and can scarcely yet be regarded as deter- mined. Possibly it may vary in its nature, according to the state of the system, and the kind of animal from which it is obtained. That of Man has been usually stated to contain a sensible quantity of un- combined Muriatic and Acetic acids, to which its acid reaction has been attributed ; whilst, on the other hand, it has been recently as- serted, that the gastric fluid of the Dog contains no free muriatic acid, and that its acid reaction is due to the presence of the superphosphate of lime. The other inorganic ingredients of the Gastric fluid are nearly the same as those of the Saliva. It contains a peculiar organic compound, which, like Ptyaline, bears a considerable resemblance to albumen, but which is not identical with it; to this the name of Pep- sine has been given. The properties of Pepsine have been principally studied in that form of it obtained from the raucous membrane of the stomach of the Pig, which bears a close resemblance to that of Man. When this membrane is digested in a large quantity of warm water, it is purified from the various soluble substances it may contain ; but the pepsine is not taken up, as it is not soluble in warm water. By continuing the digestion in cold water, the pepsine is then extracted nearly pure. When this solution is evaporated to dryness, there re- mains a brown, grayish, viscid mass, having the appearance of an extract, and the odour of glue. A similar substance may be obtained by adding strong alcohol to a fresh solution of pepsine; for the latter is then precipitated in white flocks, which may be collected on a filter, and which produce a gray compact mass when dried. Pepsine enters into chemical combination with many acids; forming compounds which still redden litmus paper; and this appears to be its condition in the gastric juice. 471. The muriate and acetate of pepsine possess a very remarkable solvent power for albuminous substances. A liquid which contains only 17 ten-thousandths of acetate of pepsine, and 6 drops of muriatic acid per ounce, possesses solvent power enough to dissolve a thin slice of coagulated albumen, in the course of six or eight hours' diges- tion. With 12 drops of muriatic acid per ounce, the same quantity of white of egg is dissolved in two hours. A liquid which contains only half a grain of acetate of pepsine, and to which muriatic acid and white of egg are alternately added, so long as the latter is dissolved, is capable of taking up 210 grains of coagulated white of egg, at a temperature between 95° and 104.° The same acid with pepsine ACTION OF GASTRIC FLUID—CHYME. 277 dissolved blood, fibrin, meat, and cheese; whilst the acid without the pepsine requires a very long time to do so at ordinary temperatures. Very dilute muriatic acid, however, at the boiling point, dissolves these albuminous substances ; and the solution has the same charac- ters as that which is made by the agency of pepsine. The horny tissues,—such as the epidermis, horn, hair, &c.,—and the yellow fibrous tissue, are not affected by the acid solution of pepsine. It appears from these experiments, that the muriatic acid is the real solvent; and that the action of the pepsine is limited to disposing the albuminous matter for solution, producing in it a change analogous to that which may be effected by heat. Hence it may be considered, like ptyaline, as a sort of ferment; its office being to produce a tend- ency to change, in the substances on which it acts, without itself entering into new combinations with any of their elements. 472. These experiments appear to afford an explanation of the properties of the gastric fluid, as ascertained by direct experiment upon it. When drawn direct from the human stomach, it is found to possess the power of dissolving various kinds of alimentary substances, whilst these are submitted to its action at a constant temperature of 100° (which is about that of the stomach), and are frequently agitated. The solution appears to be in all respects as perfect as that which naturally takes place in the stomach ; but a longer time is required to make it. This is easily accounted for by the difference of the condi- tions; for no ordinary agitation can produce the same effect with the curious movements of the stomach (§ 458); fresh gastric fluid is poured out, as it is wanted, during the natural process of digestion ; and the continual removal of the matter which has been already dis- solved by its exit through the pylorus, is of course favourable to the action of the' solvent upon the remainder. The quantity of food, which a given amount of gastric .fluid can dissolve, is limited ; pre- cisely as in the case of the acidulous solution of pepsine. The marked influence of temperature upon its action is shown by the fact, that fresh gastric fluid has scarcely any influence on the matter sub- mitted to it, when the bottle is exposed to cold air, instead of being kept at a temperature of 100°. Hence the use of a large quantity of cold water at meal-times, or of ice afterwards, must retard the diges- tive process. 473. The pulpy substance, which is the product of the reducing action of the gastric juice, is termed Chyme. Its consistence will of course vary, in some degree, with the relative quantity of solids and liquids ingested. In general it is grayish, semifluid, and homoge- neous ; and possesses a slightly acid taste, but is otherwise insipid. When the food has been of a rich oily character, the Chyme possesses a creamy aspect; but when it has contained a large proportion of fari- naceous matter, it has rather the appearance of gruel. The state in which the various alimentary principles exist in it, has not yet been accurately determined ; the following, however, may be near the truth. —The proteine-compounds, whether derived from Animal or Vege- 278 SECRETION OF GASTRIC FLUID. table food, are all reduced to the condition of Albumen; a part of which is dissolved, whilst another portion is suspended in a very finely- divided state.—Gelatin will be dissolved or not, according to its previous condition ; if it exist in a tissue from which it cannot readily be extracted, it will pass forth almost unchanged ; but when ingested in a state of solution, it remains so; and if it have been previously prepared for solution by boiling, its solution is completed in the sto- mach.—The Gummy matters of Vegetables are dissolved, when they exist in a soluble form ; as in the case of pure gum, pectine, and dex- trine or starch-gum. The degree in which Starch, when its vesicles have not been ruptured by heat, is affected by the gastric fluid, seems to differ in different animals; the Ruminants and Granivorous Birds apparently possessing the power of crushing or dissolving the enve- lops of the starch-globules, whilst they pass through the alimentary canal of other Herbivora unchanged, and may be detected entire in their excrements.—Sugar is unquestionably taken up in solution, as such, in a healthy condition of the system; but it may undergo a previous change in the stomach, in disordered states of the digestive process.— Oily matters, whether of Animal or Vegetable origin, are reduced to the condition of an emulsion ; being very finely divided, and their particles diffused through the chyme.—Most other substances, as resins, wroody fibre, horny matter, yellow fibrous tissue, &c, pass unchanged from the stomach, and undergo no subsequent alteration in the intestinal canal; so that they are discharged among the feces as completely useless. 474. We have now to notice the conditions, under which the Gas- tric fluid is secreted ; the knowledge of which is of great practical importance. We have seen that it is not poured forth, except when food is introduced into the stomach, or when its walls are irritated in some other mode ; and there is reason to believe, that it is not pre- viously secreted and stored up in the follicles, but that the act of secre- tion itself is due to the stimulus applied to the mucous membrane. The quantity of the fluid then poured into the stomach, however, is not regulated by the amount of food ingested, so much as by the wants of the system ; and as only a definite quantity of food can be acted on by a given amount of gastric juice, any superfluity remains undissolved for some time,—either continuing in the stomach until a fresh supply of the solvent is secreted, or passing into the intestinal canal in a crude state, and becoming a source of irritation, pain, and disease. The use of a small quantity of salt, pepper, mustard, or other stimulating substances, appears to produce a gently stimulating effect upon the mucous membrane, and by causing an increased afflux of blood, to augment the quantity of the gastric fluid poured forth. Any excess of these or other irritants, however, produces a disordered condition of the mucous membrane, which is very unfavourable to the digestive process. It becomes red and dry, with an insufficient secre- tion of mucus; the epithelial lining is abraded, so that the mucous coat is left entirely bare ; and irregular circumscribed patches of a PRODUCTION OF CHYME—ADMIXTURE OF BILE. 279 deeper hue, sometimes with small aphthous crusts, present them- selves here and there on the walls of the stomach. Similar results follow excess in eating. When these changes are inconsiderable, the appetite is not much impaired, the tongue does not indicate disorder, and the digestive process may be performed; but if they proceed further, dryness of the mouth, thirst, accelerated pulse, foulness of the tongue, and other symptoms of febrile irritation, manifest themselves ; and no gastric secretion can then be excited by the stimulus of food. Similar results may follow the excitement of the emotions ; and those of a depressing nature seem especially to produce a pale flaccid con- dition of the mucous membrane, which is equally unfavourable to the due secretion of gastric fluid. 475. That the amount of the secretion is ordinarily proportioned to the wants of the system,—that the introduction of any superfluous aliment into the stomach is not only useless but injurious, as giving rise to irritation,—that incipient disorder of the stomach may occur, rendering it less fit than usual for the discharge of its important duties, without manifesting itself by the condition of the tongue,—that when the tongue does indicate disorder of the stomach, such disorder is usually considerable,—and that every particle of food ingested, in such states as prevent the secretion of gastric fluid, is a source of fresh irritation,—are truths which cannot be too constantly kept in mind. There can be no doubt that the habit of taking more food than the system requires, is a very prevalent one ; and that it is persevered in because no evil result seems to follow. But when it is borne in mind that this habit must keep the stomach in a state of continual irritation, however slight, it can scarcely be doubted that the foundation is thus laid for future disorder, of a more serious kind. Two circumstances especially tend to maintain this practice in adults, independently of the mere disposition to gratify the palate. One is the habit of eating the same amount of food, as during the period of growth, when more was required by the system. The other is the custom of eating too fast; and this is injurious,—both by preventing sufficient mastication, and thus throwing on the stomach more than its proper duty,—and also by causing an over-supply of food to be ingested, before there is time for the feeling of satisfaction to replace that of hunger (§ 486). 476. The Chyme, upon quitting the stomach, passes into the duo- denum ; where it is mingled with the biliary and pancreatic secretions. The secretion of Bile is evidently a process of the highest importance in the economy; as we may judge alike from the size of the liver and the supply of blood it receives, and from the rapidly fatal effects of its suspension. Yet the chemical nature of the secretion has not yet been satisfactorily determined; and the destination of the fluid is still a matter of doubt. That a large part of it is purely excrementitious, and is poured into the intestinal tube for the purpose of being carried out of the body, can scarcely be questioned ; but there is strong evi- dence, that a part of it is destined to be absorbed again, after per- fo rraing some action of importance upon the contents of the alimentary 280 SECRETION OF BILE. canal. There is a probability that a part of its function consists in rendering the fatty matter of the aliment more soluble ; the nature of the secretion being such, as to give it in some degree the action of a soap. When fresh bile is mingled with newly-formed chyme, in a glass vessel, the mixture separates into three distinct parts;—a reddish-brown sediment at the bottom,—a whey-coloured fluid in the centre,—and a creamy pellicle at the top. The central and upper strata probably constitute the portion which is destined for absorption; whilst the sediment, partly consisting of the unreducible portion of the food, and partly of the biliary matter itself, is evidently excrementitious. 477. The composition of the Bile, and the structure of the organ which elaborates it, will be more fitly considered when the Excre- tions in general are treated of (Chap. IX.); at present we have only to consider its relation to the digestive process. In all but the very lowest animals, we find traces of a bile-secreting apparatus; and this is almost constantly situated in the immediate neighbourhood of the stomach. In many cases, the secretion is poured directly into the cavity of that organ ; but in most, it is oonveyed (as in Man) into the intestinal tube near its commencement. There are few instances in which the bile-ducts discharge themselves into the intestine low down, and still fewer in which they terminate near its outlet; and in these last, they appear also to discharge the functions of urinary organs. Hence it seems clear, from the disposition of the biliary apparatus, that it has a purpose to serve in connection with the digestive func- tion, and is not destined solely for the elaboration of a product which is to be cast out of the body; since, if the latter were the case, that product would be carried out immediately, like the urinary excretion, and would not be discharged into the alimentary canal high up.—This conclusion is confirmed by experiment; for it has been shown by the recent experiment of Schwann, that, if the bile-duct be divided, and be made to discharge its contents externally through a fistulous ori- fice in the walls of the abdomen, instead of into the intestinal canal, those animals, which survive the immediate effects of the operation, subsequently die from inanition, almost as soon as if they had been entirely deprived of food.—The observation of disease in the human subject leads to similar conclusions; for, when the biliary secretion is deficient, or its flow into the intestine is obstructed, the digestive processes are evidently disordered; the peristaltic action of the bowels is not duly performed ; the feces are white and clayey ; and there is an obvious insufficiency in the supply of nutriment prepared for the absorbent vessels. 478. On the other hand, that one great object of the secretion is to withdraw from the blood certain products of the decomposition of the tissues, which would otherwise accumulate in it, and would be dele- terious to its character, is shown by evidence yet more decisive. We find that the action of the Liver is constant, and not occasional, like that of the Salivary and Gastric glands; and that, if anything inter- SECRETION OF BILE. 281 fere with the secreting process, and thereby cause the accumulation of the elements of the bile in the blood, the effects of their presence are immediately manifested in the disorder of other functions, espe- cially those of the nervous system (§ 399); and the continued suspen- sion of the function leads to a fatal result, unless the elements of the bile are drawn off" (as sometimes happens) by the urinary organs. When the secreting action of the liver has once been performed, an obstruction to the discharge of the bile into the intestine does not seem to be so immediately injurious. The fluid accumulates, and distends the bile-ducts and the gall-bladder; and when they are completely filled, part of it is re-absorbed into the blood, apparently in a changed condition, since it does not then produce the same injurious effects as result from the accumulation of the same materials, previously to the action of the Liver upon them. The colouring-matter seems to be very readily taken back into the circulating system; and is depo- sited by it in almost every tissue of the body. 479. Although the secreting action of the Liver is constant, yet the discharge of bile into the intestine is certainly favoured by the presence of chyme in the latter. The purpose of the gall-bladder is obviously to permit the accumulation of bile, when it is not wanted in the intestine; and we find it most constantly present in those tribes of animals, which live upon animal food, and which therefore take their aliment at intervals; whilst it is more frequently absent in those herbivorous animals, in which the digestive process is almost con- stantly going on. The middle coat of the bile-ducts is clearly mus- cular, and has a peristaltic action like that of the intestinal canal; this action may be excited by galvanism, or by irritation of the branches of the Sympathetic nerve, by wrhich it is supplied. The mucous coat of the ductus choledochus is disposed in valvular folds, in such a man- ner as to prevent the reflux of the bile or of the contents of the intes- tine ; and a still further security is afforded by the valvular covering to the orifice of the duct, which is furnished by the mucous covering of the intestine itself. The flow of bile into the intestine, when its presence is needed there, is commonly imputed to the pressure of the distended Duodenum against the gall-bladder; but it is probable that the contractility of the muscular coat of the duct itself, which may be excited either through the sympathetic nerve, or by irritation at the orifice of the duct (as in the case of the Salivary glands) is the real cause of the discharge of the fluid. It is an interesting fact, which proves how much the passage of the Bile into the Intestine is depend- ent upon the presence of aliment in the latter, that the gall-bladder is almost invariably found turgid in persons who have died of starvation; the secretion having accumulated, through the want of demand for it, although there was no obstacle, to its exit. 480. The Pancreatic secretion appears to have nearly the same qualities as Saliva; the proportion of solid matter, however, being usually greater. Of its uses in the digestive process, nothing definite can be stated. 282 SENSE OF HUNGER. 481. During the passage of the contents of the Intestine, now aug- mented by the biliary secretion, along the canal, the nutritious portion is gradually withdrawn by the absorbent vessels on its walls ; and the excrementitious matter alone remains. This is increased in amount by the products of the secretion of the various glandulae, with which the mucous lining of the intestines is studded. As their function, however, is obviously to get rid of decomposing matter from the sys- tem, rather than to contribute in any way to the preparation of the nutritive materials, it will be more properly considered hereafter (Chap. XI). Many of the lower animals are furnished, at the part where the small intestine enters the large, with a ccecum, resembling that which in Man is termed the vermiform appendage of the caecum, but greatly exceeding it in size. Sometimes we find two caeca in- stead of one; and these are much prolonged, so as to form tubes of considerable length. It has been ascertained that, in herbivorous animals, a distinctly acid secretion is formed by the caecum, during the digestive process; and there is reason to believe, that the food there undergoes a second process, analogous to that to which it has been submitted in the stomach, and fitted to extract from it any un- dissolved alimentary matter it may still contain. There is no reason to believe, however, that any such process takes place in Man, whose real caecum is rudimentary,—the part of the intestine which has re- ceived the name, being merely the dilated commencement of the colon. The act of defecation, by which the excrementitious matter is discharged, has been already noticed (§ 462); the Absorption of nutritive matter will be treated of in the succeeding Chapter. 5. Of Hunger, Satiety, and Thirst. 482. The want of solid aliment is indicated by the sensation of Hunger ; and the deficiency of fluid by that of Thirst. On the other hand, the presence of a sufficiency of food or liquid in the stomach is indicated by the sense of Satiety. These sensations are intended as our guides, in regard to the amount of aliment we take in. What is the real seat of these sensatiohs, and on what conditions do they depend ? 483. The sense of Hunger is referred to the stomach, and seems immediately to depend upon a certain condition of that organ; but what that condition is, has not yet been precisely ascertained. It is not produced by mere emptiness of the stomach, as some have sup- posed ; for, if the previous meal have been sufficient, the food passes entirely from the cavity of the stomach, before a renewal of the sensa- tion is felt. It cannot be due to the action of the gastric fluid upon the coats of the stomach themselves; because this fluid is not poured into the stomach, except when the production of it is stimulated by the irritation of the secreting follicles. It has been attributed to dis- tension of the gastric follicles by the secreted fluid ; but there is no evidence that the fluid is secreted before it is wanted ; and, moreover, SENSE OF HUNGER AND SATIETY. 283 it is well known that mental emotion can dissipate in a moment the keenest appetite, and it is difficult to imagine how this can occasion the emptying of the follicles. Perhaps the most satisfactory view is that, which attributes the sense of hunger to a determination of blood to the stomach, preparing it for the secretion of gastric fluid; since this is quite adequate to account for the impression made upon the nerves; and it accords with what has just been stated of the influence of mental emotions, since we know that these have a powerful effect upon the circulation of blood in the minute vessels (§ 603). 484. Although the sense of Hunger is immediately dependent, in great part at least, upon the condition of the stomach, yet it is also indicative of the condition of the general system; being extremely strong, when the body has undergone an unusual waste without a due supply of food, even though the stomach be in a state of distension; whilst it is not experienced, if, through the general inactivity of the system, the last supply has not been exhausted, even though the sto- mach has been long empty. It is well known, that when food is defi- cient, the attempt to allay the pangs of hunger by filling the stomach with non-nutritious substances, is only temporarily successful; the feeling soon returning with increased violence, though it has received a temporary check. The reason for this is obviously, that the general system has received no satisfaction, although the stomach has been caused to secrete gastric fluid by the contact of solid matter with its walls, so that the state on which hunger immediately depends, has been for a time relieved. This state is soon renewed, unless the solid matter introduced into the stomach be of an alimentary character, and be dissolved and carried into the system. 485. When the food is nutritious in its character, but of small bulk, experience has shown the advantage of mixing it with non-nutritious substances, in order to give it bulk and solidity; for if this be not done, it does not exert its due stimulating influence upon the stomach; the gastric juice is not poured forth in proper quantity; and the result is, that neither is the sense of hunger relieved, nor are the wants of the body satisfied. Thus the Kamschatdales are in the habit of mixing earth or saw-dust with the train-oil, on which alone they are frequently reduced to live. The Veddahs or wild hunters of Ceylon, on the same principle, mingle the pounded fibres of soft and decayed wood with the honey on which they feed when meat is not to be had ; and on one of them being asked the reason of the practice, he replied, " I cannot tell you, but I know that the belly must be filled." It has been found that soups and fluid diet are not more readily converted into chyme than solid aliment, and are not alone fit for the support of the body in health; and it is often to be observed, in disordered states of the stomach, that it can retain a small quantity of easily-digested solid food, when a thin broth would be rejected. 486. The sense of Satiety is the opposite of Hunger; and like it, depends on two sets of conditions,—the state of the stomach, and that of the general system. It is produced in the first instance by the 284 HUNGER, SATIETY, AND THIRST. ingestion of solid matter into the stomach, which gives rise to the feeling of fullness ; but this is only a part of the sensation which ought to be experienced ; and it is only when the act of digestion is being duly performed, and nutritive matter is being absorbed into the ves- sels, that the peculiar feeling of satisfaction is excited, which indicates that the wants of the system at large are being supplied.—It has been very justly remarked by Dr. Beaumont, that the cessation of the de- mand set up by the system, rather than the positive feeling of satiety, should be the guide in regulating the quantity of food taken into the stomach. The sense of satiety is beyond the point of healthful indulg- ence ; and is Nature's earliest indication of an abuse and overburdening of her powers to replenish the system. The proper intimation is the pleasurable sensation w:hich is experienced, when the cravings of the appetite are first allayed; since, if the stomach be sufficiently distended with wholesome food, for this to be the case, it is next to certain that the digestion of that food will supply what is required for the nutrition of the body. It is only when the substance with which the stomach is distended, is not of a digestible character, that the feelings excited by the state of that organ are anything but a correct index of the wants of the system. 487. The Par Vagum is evidently the nerve, which conveys to the sensorium the impression of the state of the stomach, and which is therefore the immediate excitor of the sensation of hunger, or of the feeling of satiety. But it is evident from experiments upon animals, that it is not the only source, through which they are incited to take food, and are informed when they have ingested enough; and it is probable that the Sympathetic nerve is the channel, through which the wants of the system are made known, and through which, in particular, the feeling of general exhaustion is excited, that is expe- rienced when there has been an unusual waste, or when the proper supply has been too long withheld. 488. The conditions of the sense of Thirst are very analogous to those of hunger; that is, it indicates the deficiency of fluid in the body at large ; but the immediate seat of the feeling is a part of the alimentary canal,—not the stomach, however, but the fauces. It is relieved by the introduction of fluid into the circulating system, through any channel; whilst the mere contact of fluids with the sur- face to which the sensation is referred, produces only a temporary effect, unless absorption take place. If liquids be introduced into the stomach by an oesophagus-tube, they are just as effectual in allaying thirst, as if they were swallowed in the ordinary manner; and the same result follows the injection of fluid into the veins, (as was most remarkably the case when this method of treatment was practised in the Asiatic Cholera,) or the absorption of fluid through the skin or the lower part of the alimentary canal. The deficiency of fluid in the body may arise,—and Thirst may consequently be induced,—either by an unusually small supply of fluid, or by excessive loss of the fluids of the body, as by perspiration, diarrhoea, &c. But it may also be ABSORPTION FROM THE DIGESTIVE CAVITY. 285 occasioned by the impression made by particular kinds of food or drink upon the alimentary canal; thus salted or highly-spiced meat, fermented liquors when too little diluted, and other similar irritating agents, excite thirst; the purpose of which sensation is evidently to cause the ingestion of fluid, by which these substances may be diluted, and their irritating action prevented. CHAPTER V. ABSORPTION AND SANGUIFICATION. 1. Absorption from the Digestive Cavity. 489. So long as the Alimentary matter is contained in the digestive cavity, it is as far from being conducive to the nutrition of the sys- tem, a£ if it were in contact with the external surface. It is only when absorbed into the vessels, and carried by the circulating current into the remote portions of the body, that it really becomes useful in maintaining the vigour of the system, by replacing that which has decayed, and by affording the materials for the various organic pro- cesses which are continually going on. Among the Invertebrated animals, we find the reception of alimentary matter into thecircula- ting system to be entirely accomplished through the medium of the veins, which are distributed upon the walls of the digestive cavity. We not unfrequently observe, that the intestinal tube is completely enclosed within a large venous sinus, so that its whole external sur- face is bathed with blood ; and into this sinus, the alimentary mate- rials would appear to transude, through the walls of the intestinal canal, to become mingled with the blood, and to be conveyed with its current into the remote portions of the body. Among the Verte- brata, we find an additional set of vessels, interposed between the walls of the intestine and the sanguiferous system, for the purpose, as it would seem, of taking up that portion of the nutritive matter which is not in a state of perfect solution, and of preparing it for being introduced into the current of the blood. These vessels are the lacteals or absorbents. They are very copiously distributed upon the walls of the small intestine, commencing near the entrance of the biliary and pancreatic ducts; the walls of the large intestine are less abundantly supplied with them, and they are not to be met with at all on the walls of the stomach. 490. Nevertheless it is quite certain, that substances may pass into the current of the circulation, which have been prevented from pass- ing further than the stomach; thus, if a solution of Epsom-salts be introduced into the stomach of an animal, and its passage into the 286 ABSORPTION BY ENDOSMOSE. intestine be prevented by a ligature around the pylorus, its purgative action will be exerted nearly as soon, as if the communication between the stomach and intestines had been left quite free; or if a solution of prussiate of potash be introduced into the stomach under similar circumstances, the presence of that salt in the blood may be speedily demonstrated by chemical tests. 491. This passage of substances in a state of perfect solution, from the stomach into the blood-vessels, is probably due to the operation of that peculiar modification of Capillary Attraction, which is called Endosmose. When two fluids differing in density are separated by a thin animal or vegetable membrane, there is a tendency to mutual admixture through the pores of the membrane; but the less dense fluid will transude with much greater facility than the more dense ; and consequently there will be a considerable increase on the side of the denser fluid; whilst very little of this, in comparison, will have passed towards the less dense. When one of the fluids is contained in a sac or cavity, the flow of the other towards it is termed Endos- mose, or flow-inwards; whilst the contrary current is termed Exos- mose or flow-outwards. Thus if the caecum of a fowl, filled with syrup or gum-water, be tied to the end of a tube, and be immersed in pure water, the latter will penetrate the caecum by Endosmose, and will so increase the volume of its contents, as to cause the fluid to rise to a considerable height in the attached tube. On the other hand, a small proportion of the gum or syrup will find its way into the sur- rounding fluid by Exosmose. But if the caecum were filled with water, and were immersed in a solution of gum or sugar, it would soon be nearly emptied,—the Exosmose being much stronger than the En- dosmose. It is in this manner that we may cause the flattened corpus- cles of the blood to be distended into spheres, by treating them with water; or may empty them almost completely, by immersing them in syrup (§ 216); since their contents are more dense than the surround- ing fluid in the first case, so that they will be augmented by Endos- mose ; whilst they are less dense in the second, so as to be diminished by Exosmose. 492. Now it seems to be in this manner, that substances contained in the cavity of the stomach, and perfectly dissolved by its fluids, are received into the blood-vessels; for as the blood is the fluid of greater density, it will have a tendency to draw towards it, by Endosmose, the saline and other matters, which are in a state of perfect solution in the stomach. The mucous membrane, which forms the inner wall of that organ, is most copiously supplied with blood-vessels; partly, indeed, that they may afford the materials of the gastric secretion ; but partly, also, that they may take up the substances, which are capable of entering the circulating current by this direct channel. That this act of absorption is principally effected by the veins rather than by the arteries, appears from several considerations. The walls of the former are much thinner than those of the latter. Moreover, the for- mer are usually distributed nearer to the surface ; whilst the latter are ABSORPTION OF SOLUBLE MATTERS INTO THE VEINS. 287 more deeply-seated. And the direction of the passage of the blood in the former is favourable to the act of absorption, whilst in the latter it is the reverse ; for the blood in the arteries is passing from large trunks, through which it flows with facility, into the innumerable sub- divisions and ramifications of the capillary system, in which the resist- ance to its flow is very much increased; whilst in the veins, the numerous streams flowing through the capillaries are uniting and con- verging into main trunks of greatly-increased capacity, so that the resistance is greatly diminished; and it is easily shown on Physical principles, that the former condition presents a direct obstacle to absorption, whilst the latter as directly favours it. For if a current of fluid be made to pass through a horizontal tube, which undergoes an enlargement at one part of its course, so that the fluid passes from the smaller to the larger portion, and if a small tube be made to open into the enlarged part, and to dip down vertically into a basin below, the fluid of that basin will be caused to rise in the small tube, so, as to be drawn into the current that is flowing through the horizontal pipe,—and this with a force proportional to the amount of its enlarge- ment, and to the rapidity of the current that is flowing through it. 493. Although it is difficult to speak with certainty on the point, yet there appears a strong probability that, both in the stomach and intestinal tube, the absorption of nutritive matters in a state of per- fect solution,—such as gum, sugar, pectine, gelatin, and soluble albumen,—is thus accomplished through the medium of the veins; which also take up the chief supply of water that is required by the system. It is difficult else to see the purpose of the extraordi- nary vascularity of the mucous mem- brane, and in particular of those fila- ments or narrow folds, termed villi, which so thickly cover its surface. Each of these villi is furnished with a plexus of minute blood-vessels, of which the larger branches may even be seen with the naked eye, when they are distended with blood or with a coloured injection. By these villi, the vascular surface of the mucous mem- brane is enormously extended. In Man, they are commonly cylin- drical or nearly so, and are from about a quarter of a line to a line and a half in length; but in many of the lower animals they are spread out into broader laminae at the base, and are connected together so as to form ridges or folds.—It appears from the experi- ments of MM. Tiedemann and Graelin, that when various substances were mingled with the food, which, by their colour, odour, or chemi- cal properties, might be easily detected,—such as gamboge, madder, rhubarb, camphor, musk, assafetida, and saline compounds,—they were seldom found in the chyle, though many of them were detected in the blood and in the urine. The colouring matter appeared to be Fig. 77. Distribution of Capillaries in the Villi of the Intestine. 288 ABSORPTION INTO THE LACTEALS. seldom absorbed at all; the odorous substances wrere generally de- tected in the venous blood and in the urine, but not in the chyle; whilst, of the saline substances, many were found in the blood and in the urine, and only a very few in the chyle. 494. Every one of the intestinal Villi, however, also contains the commencement of a proper absorbent vessel; and this system of ves- sels has received the name of lacteal, on account of the milky aspect of the fluid which is found within it. The accompanying figure represents the appearance offered by the incipient lacteals in a villus of the jejunum of a young man, who had been hung soon after taking a full meal of farinaceous food. The trunk that F'g-7& issues from the villus is formed by the conflu- ence of several smaller branches, whose origin it is difficult to trace; but it is probable that they form loops by anastomosis with each other, so that there is no proper free extremity in any case. It is quite certain that the lacteals never open by free orifices upon the surface of the intestine, as was formerly imagined. From the researches of Mr. Goodsir, already referred to, it appears that these loops are imbedded in a one of the intestinal viin, mass of cells, which are the real agents in the with the commencement of , .• c ,1 , • 1 ,, , , ,• , , a lacteal. selection of the materials that are destined to be conveyed into the lacteals; and that the growth of these cells is the first stage of the process, by which the nutritive matters that are in a state of very fine division, but not in perfect solution, are received into the system (§ 241). When these cells have completed their office, and have passed through the terra of their lives, they yield their contents to the absorbent vessels, either by bursting or by deliquescence ; and thus the substances which they have selected and combined by their own processes of growth, are delivered to the current in which they are to undergo further trans- formations, and to be made subservient to the nutrition of the general system. 495. It is particularly important to keep in view the difference between the two modes by which alimentary substances are intro- duced into the system, when we are treating those disordered states in which the digestive process is imperfectly performed, or altogether suspended. There can be little doubt, that the immediate cause of death, in many diseases of exhaustion, is the want of power to main- tain the heat of the body; the stomach not being able to dissolve food, and the functions of the lacteals being altogether suspended, by the non-development of the absorbing cells, so that the inanition is as complete as if food were altogether withheld. Now under such circumstances, it becomes a matter of greatest importance to present a supply of combustible matter, in such a form that it may be intro- duced into the circulating system by simple Endosmose; and the value which experience has assigned to weak broths and thin fari- ABSORPTION OF ALCOHOL, ETC.—MESENTERIC GLANDS. 289 naceous solutions, and still more, to diluted alcoholic drinks, fre- quently repeated, under such circumstances, seems to depend in great part upon the facility with which they may be thus absorbed. The good effects of alcohol, cautiously administered, are no doubt owing in part to its specific influence upon the nervous system ; but that they are also due to its heat-producing power, appears from the results of the administration of frequently-repeated doses, in states of utter exhaustion,—the temperature of the body being kept up so long as they are continued, and falling when they are inter- mitted. As the alcohol is thus burned off, nearly as fast as it is introduced, it never accumulates in sufficient quantity to produce its usual violently-stimulating effects upon the nervous system. 2. Passage of the Chyle along the Lacteals, and its admixture with the Lymph collected from the general System. 496. The Lacteal vessels, which commence on the surface of the intestines, run together on their walls, and form larger trunks, which converge and unite with each other in the mesentery; and the main trunks thus formed then enter certain bodies, which are commonly known as the " mesenteric glands." Their structure, however, does not seem to correspond with that of the proper glands; as they are 79. Fig. 80. Diagram of a lymphatic gland, showing the in- tra-glandular network, and the transition from the scale-like epithelia of the extra-glandular lymphatics, to the nucleated cells of the intra- glandular. Portion of intra-glandular lymphatic, showing alongthe. lower edge the thickness of the germi- nal membrane, and, upon it, the thick layer of glandular epithelial cells. simply composed of lacteal trunks, convoluted into knots, and dilated into larger cavities, amongst which blood-vessels are minutely dis- tributed. These blood-vessels have no direct communication with the interior of the lacteals; but are separated from them by the mem- branous walls of both sets of tubes. The epithelium, which lines the absorbent vessel, undergoes a marked change where the vessel enters the gland, and becomes more like that of the proper glandular fol- licles in its character. Instead of being flat and scale-like, and form- ing a single layer in close apposition with the basement-membrane, as it does in the lacteal tubes before they enter the gland and after they have emerged from it, we find it coraposed, within the gland, of numerous layers of spherical nucleated cells (Figs. 79 and 80); of which the superficial ones are easily detached, and appear to be 19 290 TERMINATION OF LACTEALS IN THORACIC DUCT. identical with the cells that are found floating in the chyle. The purpose of these cells will be presently inquired into. 497. After emerging from Fig-si. the mesenteric glands, the lacteal trunks converge, with occasional union, until they discharge their contents into the receptaculum chyli, which is situated at the front of the body of the second lumbar vertebra. Into the same cav- ity are poured the contents of a part of the other division of the Absorbent system; which is distributed through the body in general, and which, from the transparency of the fluid or lymph it con- tains, is termed the lymphatic system. From the recepta- culum chyli arises the thora- cic duct; which passes up- wards in front of the spine, receiving other lymphatic trunks in its course, to ter- minate at the junction of the left subclavian and jugular veins; where it delivers its contents into the sanguiferous system. A smaller duct re- ceives some of the lymphatics of the right side, and there terminates at a corresponding part of the venous system ; but it does not receive any of the contents of the lac- teals. 498. The lymphatic sys- tem is evidently allied very closely to the lacteal, in its general purposes; and makes its first appearance in the same class of ani- mals, namely, in Fishes. The vessels of which it is composed are distributed through most of the softer tissues of the body, and are particularly abundant in the skin. They have never been found to commence by closed or open extremities; but seem to The course and termination of the thoracic duct. 1. The arch of the aorta. 2. The thoracic aorta. 3. The abdominal aorta; showing its principal branches di- vided near their origin. 4. The arteria innominata, di- viding into the right carotid and right subclavian arteries. 5. The left carotid. 6. The left subclavian. 7. The su- perior cava, formed by the union of, 8, the two vinae in- nominata; ; and these by the junction, 9, of the internal jugular and subclavian vein at each side. 10. The greater vena azygos 11. The termination of the lesser in the greater vena azygos. 12. The receptaculum chyli; seve- ral lymphatic trunks are seen opening into it. 13. The tho- racic duct, dividing opposite the middle of the dorsal ver- tebrae into two branches, which soon reunite; the course of the duct behind the arch of the aorta and left subclavian artery is shown by a dotted line. 14. The duct making its turn at the root of the neck, and receiving several lym- phatic trunks previously to terminating in the posterior aspect of the junction of the internal jugular and subcla- vian vein. 15. The termination of the trunk of the ductus lymphalicus dexter. LACTEAL AND LYMPHATIC SYSTEMS. 291 form a network, from which the trunks arise. In their course they pass through glands, disposed in different parts of the body, which exactly resemble in structure those which are found upon the lacteals in the mesentery. And they at last terminate, as already shown, in the same general receptacle with the lacteals. Hence it cannot be reasonably doubted, that the fluid which they absorb from the various tissues of the body, is destined to become again subservient to nutri- tion ; being poured back into the current of the blood, along with the new materials, which are now for the first time being introduced into it. That the special Absorbent apparatus of Vertebrated animals has for part of its functions to effect a change in the materials absorbed, and thus to aid in fitting them for introduction into the blood, seems apparent from the facts of Comparative Anatomy ; which show that, the more distinct the blood is from the chyle and lymph, the more marked is the provision for delaying the latter in the absorbent sys- tem, and for subjecting it to preliminary change. 499. The course of the lymphatic and lacteal vessels in Fishes is short and simple ; they are not furnished with glands; and they pour their contents into the blood-vessels at several different parts of the body. In this class the blood contains fewer red corpuscles, and its coagulating power is feebler than in any other Vertebrata. And in the lowest tribes, in which the Vertebrated character is almost entirely wanting, and in which the blood is almost pale, no special absorbent system has yet been discovered. In Reptiles, the length of the ab- sorbent vessels is remarkably increased by their doublings and convo- lutions ; so that the system appears to be more highly developed than in either of the warm-blooded classes. But this superiority is not real; for there is yet no trace of the glands, which concentrate, as it were, the assimilating power of a long series of vessels. Moreover, we often find the lymphatics of this class furnished with pulsating dila- tations, or lymphatic hearts; which have for their office to propel the lymph into the venous system. In the Frog there are two pairs of these; one situated just beneath the skin (through which its pul- sations are readily seen in the living animal), immediately behind the hip-joinj ; the other pair being more deeply seated at the upper part of the chest. The former receive the lymph of the posterior part of the body, and pour it into the veins proceeding from the same part; the latter collect that which is transmitted from the anterior part of the body and head, and empty their contents into the jugular vein. Their pulsations are totally independent of the action of the heart, and of the respiratory movements; since they continue after the removal of the former, and for an hour or two subsequently to the death and complete dismemberment of the animal. They usually take place at the rate of about sixty in the minute ; but they are by no means regular, and are not synchronous on the two sides. 500. In Birds, we find the absorbent system existing in a more perfect form ; its diffused plexuses and convolutions being replaced by glands; in which the contained fluid is brought into closer proxi- 292 ABSORPTION BY THE LYMPHATICS. mity with the blood ; and in which it is subjected to the influence of assimilating cells. These, however, are not very numerous; being principally found on the lymphatics of the upper extremities. The absorbents, in this class, terminate principally by two thoracic ducts, one on each side, which enter the jugular veins by several orifices. There are, however, two other entrances, as in Reptiles, into the veins of the lower extremity ; and these are connected with two large dilatations of the lymphatics, which are evidently analogous to the lymphatic hearts of Reptiles, but which have little or no power of spontaneous contraction.—In Mammalia, the absorbent system pre- sents itself in its most developed and concentrated state. The vessels possess firmer walls, and are more copiously provided with valves, than in the classes beneath; and the glands are much more numerous, particularly upon the vessels that receive or imbibe substances from without,—as those of the digestive cavity, the skin, and the lungs. The terminations of the absorbents in the veins are usually restricted, as in Man, to the single point of entrance of the thoracic duct on either side; but they are sometimes more numerous; and certain variations in the arrangement of the thoracic ducts, which occasion- ally present themselves as irregularities in Man, are the ordinary con- ditions of these parts in some of the lower Mammalia. 501. With regard to the source of the matters absorbed by the lymphatics, it is difficult to speak with certainty. We shall presently see that their contents bear a close resemblance to the fluid element of the blood, or " liquor sanguinis," in a state of dilution ; and it is very probable that they partly consist of the residual fluid, which, having escaped from the blood-vessels into the tissues, has furnished the latter with the materials of their nutrition, and is now to be re- turned to the former. But they may include, also, those particles of the solid frame-work, which have lost their vital powers, and which are not fit therefore to be retained as components of the living system, but which have not undergone a degree of decay which prevents them from serving, like matter derived from the dead bodies of other animals, as a material for reconstruction, when it has been again sub- jected to the organizing process. 502. It was formerly supposed (and the doctrine was particularly inculcated by the celebrated John Hunter), that the office of the Lym- phatic system is to take up and remove all the effete matter, that is to be cast out of the body, being no longer fit for its nutrition. But for such a supposition there is no adequate foundation. It seems absurd to imagine, that this effete matter would be mingled with the newly-ingested aliment, and would be poured back with it into the general current of the circulation, instead of being at once carried out of the system And the idea is directly negatived, as we shall presently see, by the actual composition of the lymph drawn from these vessels ; the solid matter of which consists, in great part at least of substances of a nutritive character. It is true that other substances are occasionally found in the lymphatics; thus, when the gall-bladder MOVEMENT OF FLUID IN ABSORBENTS. 293 and bile-ducts are over-distended with bile, in consequence of some obstruction to its exit, the lymphatics of the liver are found to contain a biliary fluid. In like manner, the lymphatics in the neighbourhood of a large abscess have been found to contain pus. When the limb of an animal, round the upper part of which a bandage is tied, is kept for some hours in tepid milk, the lymphatics of the skin are found distended with that fluid. And when saline solutions are applied to the skin, they are usually detected more readily in the lymphatics, than in the veins. But these facts only prove, that the lymphatics very readily imbibe soluble substances with which they are in proxi- mity; and this imbibition seems to take place on the same physical principles, as the imbibition of soluble substances by the veins of the intestinal canal. 503. The more ready absorption of such substances by the lympha- tics, than by the veins, of the cutaneous surfaces,—contrary to what obtains in the alimentary canal,—is easily accounted for, by the very abundant distribution of the lymphatics in the skin, and the ready access which fluids can obtain to their walls. In other tissues it is different; thus it appears that saline matters injected into the lungs are detected much sooner in the serum of the blood than they are in the lymph ; and make their appearance earlier in the left cavities of the heart, to which they would be conveyed by the pulmonary vein, than in the right, which they would reach through the thoracic duct and descending cava. This is obviously due to the minute distribu- tion of the blood-vessels upon the walls of the air-cells; which makes them far more ready channels for the imbibition of fluid, than the lymphatics could be.—In regard to the occasional absorption of pus from the cavity of an abscess or of an open ulcer, by the lymphatics, it is to be remarked that the absorbent vessels must themselves pro- bably be laid open by ulceration ; since in no other way can we un- derstand the entrance of globules, so large as those of pus, into their interior. 504. In regard to the cause of the movement of the chyle and lymph along the absorbent vessels, from their commencement to their termination in the central receptacle, no very definite account can be given. The middle coat of these vessels has a fibrous texture ; and the fibres bear some resemblance to that of the non-striated muscle. In the thoracic duct, this fibrous structure is more evident; and dis- tinct contractions have been excited in it, by irritating the sympathe- tic trunks from which it receives its nerves, and the roots of the spinal nerves with which those trunks are connected. Hence it seems pro- bable, that there is a sort of peristaltic contraction of the walls of the absorbents, analogous to that which takes place in the intestinal tube, serving to drive their contents slowly onwards ; their reflux being prevented by the valves, with which they are copiously furnished. Moreover, it is probable that the general movements of the body may concur with the contractile power of the absorbent vessels themselves, to urge their contents onwards; for almost every change in position 294 STRUCTURE AND FUNCTIONS OF THE SPLEEN. must occasion increased pressure on some portion of them, which will propel the fluid contents in the sole direction permitted by the valves, and thus give them an additional impulse towards the trunks, in which they are collected for delivery into the blood-vessels. 3. Of the Spleen, and other Glandular Appendages to the Lymphatic System. 505. The structure and functions of the Spleen, and of certain other organs allied to it in character, have been among the most obscure subjects in Anatomy and Physiology; and they are far from having been yet fully elucidated. There seems sufficient evidence, however, for regarding them in the light of appendages to the Lym- phatic system, and as concerned, like it, in the process of Sanguifica- tion, or the preparation of Blood. Hence this appears to be the most appropriate place, for such a brief notice of them, as the present state of our knowledge admits. 506. The Spleen is certainly to be regarded as an organ of com- pound structure, having at least two sets of functions to fulfil. It is essentially composed of a fibrous membrane, which constitutes its exterior envelop, and which sends prolongations in all directions across its interior, so as to divide it into a number of minute cavities or follicles of irregular form. These splenic follicles communicate freely with each other, and with the splenic vein, and they are lined by a continuation of the lining membrane of the latter. The partitions between the follicles are formed, not only by these membranes, but by the peculiar parenchyma of the Spleen ; and this seems to be made up of reticulations of blood-vessels and lymphatics, with a large quantity of minute globules or incipient cells, of about half the dia- meter of blood-corpuscles, which lie in the meshes of the capillary network, and which seem to be in intimate connection with the lym- phatics. Lying in the midst of this parenchyma, there are found a large number of bodies, about one-third of a line in diameter, which are known as the Malpighian bodies of the Spleen. These resemble lymphatic glands in miniature, being composed of convoluted masses of blood-vessels and lymphatics, united by elastic tissue ; and the lymph they contain is rendered somewhat milky by the large number of the lymph-corpuscles that float in them, although the fluid of the afferent lymphatics is quite clear;—so that the correspondence both in structure and function seems to be exact. 507. The parenchymatous and the cellated structures do not seem to bear any constant proportion to each other; thus the former prevails most in Man, and the latter in the Herbivora. The walls of the fol- licles are so elastic, that their cavities may be greatly distended with a very moderate force,—the Spleen of the sheep, which weighs about 4 oz., being easily made to contain about 30 oz. of water. This pecu- liar distensibility evidently points to the Spleen as a kind of reser- voir, connected with the Portal circulation, for the purpose of reliev- STRUCTURE AND FUNCTIONS OF THE SPLEEN. 295 ing the portal vessels from undue pressure or distension, under a great variety of circumstances. The portal system is well known to be destitute of valves, so that the splenic vein communicates freely with the whole of it; and thus, if any obstruction exist to the flow of blood through the liver, or any peculiar pressure elsewhere prevents the mesenteric veins from dilating to their full extent, the general cir- culation is not disturbed,—the Spleen affording a kind of safety-valve. That any cause of congestion of the Portal system peculiarly affects the Spleen, has been proved by experiment; for, after the portal vein has been tied, the spleen of an animal that previously weighed only 2 oz., has been found to increase 20 oz. Further, in Asphyxia, when the circulation of blood is checked in the Lungs, and when the stagnation extends itself backwards to the right side of the heart, to the vena cava, and thence to the portal system, the Spleen is often found after death to be enormously distended with blood. And in the cold stage of intermittent fever, in which a great quantity of blood is driven from the surface towards the internal organs, the Spleen receives a large portion of it, so that its increased size becomes quite perceptible; and in cases of confirmed Ague, the Spleen becomes permanently enlarged, forming what is popularly known as the " ague- cake."—Again, the Spleen appears to serve as a reservoir, into which superfluous blood may be carried, during the digestive process. When the alimentary canal is distended with food, and a great afflux of arterial blood takes place to the mucous membrane, the veins of the portal system will be liable to increased pressure from without, whilst their contents will be augmented by the quantity of fluid newly absorbed from the alimentary canal. In this, as in the preceding cases, the distensibility of the spleen makes it a kind of safety-valve, by which undue distension of the portal system is relieved. It has been ascertained that its maximum volume is attained about five hours after a meal, when the process of chymification is at an end, and that of absorption is taking place with activity ; and the increase is pro- portional rather to the amount of the fluids ingested, than to that of the solids. 508. But besides this safety-valve function, there can be little ques- tion that the Spleen performs another, which corresponds with the function of the lymphatic glands in general. The identity in struc- ture between its Malpighian bodies, and the ordinary lymphatic glands, is such as clearly points to this inference; and it is confirmed by this remarkable fact, which has been ascertained by recent experiments,— that after the spleen has been extirpated, the lymphatic glands of the neighbourhood increase in size, and cluster together as they enlarge, so as to form an organ that at least equals the original spleen in volume. This circumstance explains the reason of the almost invariable negative result of the extirpation of the spleen; for although the operation has been frequently practised, with the view of determining the functions of the organ by the symptoms presented by the animals after its re- 296 SUPRA-RENAL CAPSULES. moval, no decided change in the ordinary course of their vital pheno- mena has ever been observed, and the health, if at all disturbed for a time, is afterwards completely regained. Now if the functions of the Spleen,—putting aside the safety-valve action of its distensible cavities,—be the same with that of the lymphatic glands in general, it is easy to understand how its loss may be at once compensated by an increased action on their part, and how it may be permanently replaced by an increased development of certain of those bodies.— Thus, then, we may fairly regard the Spleen as concurring with the glands of the absorbent system, in the assimilating process, by which the crude nutritive materials are rendered fit to circulate in the blood; and as the latter operate upon those which are taken up by the lac- teals, so may the former exert their influence upon those, which have been received into the veins,—separating them from the mass of the blood, and delivering them to the lymphatics to be further elabo- rated. 509. It is worthy of remark, that a Spleen is found in all Verte- brated animals, which have a distinct Absorbent system; but that no organ exactly corresponding with it exists in the Invertebrata, which are destitute of that system,—although the distensible cellated cavi- ties, apparently destined to perform its safety-valve function, exist in some of the higher among them. This is an additional reason for regarding its parenchymatous portion as essentially a part of the assi- milating apparatus of the Absorbent system. 510. The Supra-Renal Capsules seem to correspond with the Spleen in their general structure, and in their connection with the Lymphatic system ; whilst in the arrangement of their component parts, they bear more resemblance to the Kidney. Their exterior or cortical portion is formed of straight arteries, which divide into a minute capillary net- work ; and from this arise venous branches, which form a minute plexus, pouring its contents into a large central cavity, which is the dilated commencement of the supra-renal vein. No apparatus of secreting tubes or vesicles can be detected in it; but the interspaces of the venous plexus are filled up with a sort of pulp consisting of minute spherules, averaging about 1-10,000th of an inch in diameter, but varying from nearly twice that size to less than half. These bodies appear to be the nuclei of cells, the full development of which is checked; but in the Ruminant animals, and occasionally in the Human subject, the cells are more or less developed, and. then re- semble the ordinary lymph-corpuscles in size and appearance. The Lymphatics are of large size, like those of the Spleen ; and probably convey away the matter which has been elaborated by these organs, that it may be mingled with that which is being taken up and pre- pared by other parts of the Absorbent system. The Supra-Renal capsules attain a very large size early in fcetal life, surpassing the true Kidneys in dimension, up to the tenth or twelfth week; but they afterwards diminish relatively to the latter, and are evidently subor- THYMUS GLAND. 297 dinate organs, during the whole remainder of life. It does not seem unlikely that these bodies, like the Spleen, have a double function; and that, besides participating in the general actions of the Absorb- ent glandulae, they may serve as a diverticulum for the Renal circu- lation, when from any cause the secreting function of the Kidneys is retarded or checked, and the movement of blood through them is stagnated. 511. The Thymus Gland is another body, which seems referable to the same group; having all the essential characters of a true gland (§ 714), save an excretory duct; and its function being evidently connected, during the early period of life at least, with the elaboration of nutritive matter, which is to be re-introduced into the circulating current. Its elementary structure may be best understood from the simple form it presents, when it is first capable of being distinguished in the embryo. It then consists of a single tube, closed at both ends, and filled with granular matter; and its subsequent development con- sists in the lateral growth of branching off-shoots from this central tubular axis. In its mature state, therefore, it consists of an assem- blage of glandular follicles, which are surrounded by a plexus of blood-vessels; and these follicles all communicate with the central reservoir, from which, however, there is no outlet. The cavities of the follicles contain a fluid, in which a number of corpuscles are found, giving it a granular appearance. These corpuscles are for the most part in the condition of nuclei; but fully developed cells are found among them, at the period when the function of this body seems most active. The chemical nature of the contents at this period, closely resembles that of the ordinary proteine-compounds.— It has been commonly stated, that the Thymus attains its greatest development, in relation to the rest of the body, during the latter part of fcetal life; and it has been considered as an organ peculiarly connected with the embryonic condition. But this is a mistake ; for the greatest activity in the growth of this organ manifests itself, in the Human infant, soon after birth; and it is then, too, that its functional energy seems the greatest. This rapid state of growth, however, soon subsides into one of less activity, which merely serves to keep up its proportion to the rest of the body; and its increase usually ceases altogether at the age of about two years. From that time, during a variable number of years, it remains stationary in point of size; but, if the individual be adequately nourished, it gradually assumes the character of a mass of fat, by the development of the corpuscles of its interior into fat-cells, which secrete adipose matter from the blood. This change in its function is most remarkable in hybernating Mam- mals ; in which the development of the organ continues, even in an increasing ratio, until the animal reaches adult age, when it includes a large quantity of fatty matter. The same is the case, generally speaking, among Reptiles. It is an important fact in the history of this organ, that it is not to be detected in Fishes; and does not appear to exist, either in the tadpole state of the Batrachian reptiles, or in the 298 THYMUS GLAND.—THYROID GLAND. Perennibranchiate group; so that we may regard it as essentially con- nected with pulmonic respiration.* 512. Various facts lead to the conclusion, that the function of the Thymus, at the period of its highest development, is that of elabo- rating and storing up nutritive materials, to supply the demand which is peculiarly active during the early period of extra-uterine life. The elaborating action probably corresponds with that, which is exerted by the glands of the Absorbent system; and the product, as in the preceding cases, seems to be conveyed away by the lymphatics. The provision of a store of nutritive matter seems a most valuable one, under the circumstances in which it is met with; the frvaste being more rapid and variable than in adults, and the supply not constant. Thus it has been noticed that, in over-driven lambs, the thymus soon shrinks remarkably; but that it becomes as quickly distended again during rest and plentiful nourishment. As the demand becomes less energetic, and as the supplies furnished by other organs become more adequate to meet it, the Thymus diminishes in size, and no longer performs the same function. It then obviously serves to provide a store of material, not for the nutrition of the body, but for the respi- ratory process, when this has to be carried on for long periods—as in hybernating Mammals and in Reptiles—without a fresh supply of food.—It is possible that the Thymus gland may further stand in the same relation to the Lungs, as the Spleen to the Liver, and the Supra- Renal capsules to the Kidneys; that is, as a diverticulum for the blood transmitted through the bronchial arteries (which are the nutritive vessels of the Lungs), before the Lungs acquire their full development in comparison with other organs, or when any cause subsequently obstructs the circulation through their capillaries. 513. The Thyroid Gland bears a general analogy to the Thymus; but its vesicles are distinct from each other, and do not communicate with any common reservoir. They are surrounded, like the vesicles of the true glands, with a minute capillary plexus; and in the fluid they contain, numerous corpuscles are found suspended, which appear to be cell-nuclei, in a state of more or less advanced development. This body is supplied with arteries of considerable size; and with peculiarly large lymphatics. Though proportionably larger in the fcetus than in the adult, it remains of considerable size during the whole of life. It appears, from the recent inquiries of Mr. Simon,f that a Thyroid gland, or some organ representing it in place and office, exists in all Vertebrated animals. It presents its simplest form in the class of Fishes; in some of which it appears to consist merely of a plexus of capillary vessels, connected with the origin of the cerebral vessels, and capable, by its distensibility, of relieving the latter, in case of any obstruction to the proper movement of blood through them. In the higher forms of this organ, the glandular structure,—consisting of the closed vesicles over which the capillary * See Mr. Simon's admirable Prize Essay on the Thymus Gland. ■j- Philosophical Transactions, 1844. CHARACTERS OF CHYLE AND LYMPH. 299 plexus is distributed, and of their cellular contents,—is superadded : and the organ then appears, like the Spleen, to be destined for two different uses; namely, to serve as a diverticulum to the Cerebral circulation; and to aid in the elaboration of nutritive matter, which is taken up by the Absorbent system, and which is again poured by it into the general current of the circulation. 514. Thus the Spleen, the Supra-Renal Capsules, the Thymus Gland, and the Thyroid Gland, all seem to share in the preparation of the nutritive materials of the blood, along with the ordinary glan- dulae of the Absorbent system. In fact, we may regard them all as together constituting an apparatus, which is precisely analogous to that of the ordinary glands, but of which the elementary parts are scattered through the body, instead of being collected into one com- pact structure. Thus if we could imagine any tubular gland, such as the Kidney or the Testis, to be unraveled, and its convoluted tubuli to be spread through the system, yet all discharging their con- tents by a common outlet, we should have no unapt representation of the Lymphatic portion of the Absorbent system. Its function appears to be, to separate the crude Albuminous matter from the blood, to subject it to an elaborating action performed by the epithelium-cells lining the tubes, and then to pour forth this elaborated product,—not as an excretion to be carried out of the body,—but (in conjunction with that, which has been newly taken in by the Lacteal portion of the system, and which has undergone elaboration by its glandulae), into the blood-vessels, which are to convey it to the different parts of the body where it is to be appropriated. The four bodies we have been just considering, appear to be, so far as their glandular function is concerned, appendages to this system. Their uses as diverticula, to the circulation through other organs, render them liable to occasional distension with blood; and it seems determined that this blood shall not lie useless, but shall be subservient to the action in question; the gland-cells that line the cavities of the organ withdrawing certain constituents of the blood, to restore them, through the Lymphatic system, in a state of more complete preparation for the operations of Nutrition. Their function is very probably vicarious; that is, the determination of blood is greatest (through the state of the other organs) at one time to one of these bodies, and at another time to another. Hence the effects of the loss of any one of them are not serious ; as the others are enabled in great degree to discharge its duty. 4. Composition and properties of the Chyle and Lymph. 515. The chief chemical difference between the Chyle and the Lymph, consists in the much smaller proportion of solid matter in the latter, and in the almost entire absence of fat, which is an important constituent of the former. This is well shown in the following com- parative analyses, performed by Dr. G. 0. Rees, of the fluids obtained 300 CHARACTERS OF CHYLE AND LYMPH. from the lacteal and lymphatic vessels of a donkey, previously to their entrance into the thoracic duct; the animal having had a full meal seven hours before its death. Chyle. Water - 90-237 Albuminous matter (coagulable by heat) 3-516 Fibrinous matter (spontaneously coagulable) 0-370 Animal extractive matter, soluble in water and alcohol - 0-332 Animal extractive matter, soluble in water only - 1233 Fatty matter - - - 3-601 Salts ;—Alkaline chloride, sulphate and car- bonate, with traces of alkaline phosphate, oxide of iron ... 0-711 100-000 The Lymph obtained from the neck of a horse has been recently analyzed by Nasse, with nearly the same result. He found it to con- tain 95 per cent, of water; and the 5 per cent, of solid matter was chiefly composed of albumen and fibrin, with watery extractive,— scarcely a trace of fat being to be found. The proportions of saline matter were found to be remarkably coincident with those, which exist in the serum of the blood ; as might be expected from the fact, that the fluid portion of the lymph must have its origin in that which has transuded through the blood-vessels: the absolute quantity, however, is rather less.—A similar analysis of the Chyle of a cat by Nasse, has given results very closely correspondent with that of Dr. Rees ; for the proportion of water was 90-5 per cent.; and of the 9-5 parts of solid matter, the albumen, fibrin, and extractive amounted to more than 5, and the fat to more than 3 parts.—Dr. Rees has also analyzed the fluid of the Thoracic duct of Man; which consists of chyle with an admixture of lymph ; and he found this to contain about 90-5 per cent, of water, 7 parts of albumen and fibrin, 1 part of aqueous alcoholic extractive, and not quite 1 part of fatty matter with about \ per cent, of salines. The composition of this fluid more resembles that of the lymph than that of the chyle; the proportion of the fatty to the albuminous matter being small. This was probably due to the circumstance, that the subject from which it was obtained (an executed criminal) had eaten but little for some hours before his death. 516. The characters of the Chyle are not the same in every part of the Lacteal system; for the fluid undergoes a very important series of changes in its characters, in its transit from the walls of the intestines to the receptaculum chyli. The fluid drawn from the lacteals that traverse the intestinal walls, has no power of spontaneous coagula- tion ; whence we may infer that it contains little or no Fibrin. It contains Albumen in a state of complete solution, as we may ascer- Lvmph. 95-536 1-200 0-120 0-240 1-319 a trace. 0-585 100-000 PROGRESSIVE ALTERATIONS IN THE CHYLE. 301 tain by the influence of heat or acids in producing coagulation. And it includes a quantity of fatty matter, which is not dissolved, but suspended in the form of globules of variable size. The quantity of this evidently varies with the character of the food ; it is more abund- ant, for instance, in the chyle of Man and the Carnivora, than in that of the Herbivora. It is generally supposed that the milky colour of the chyle is owing to the oil-globules ; but Mr. Gulliver has pointed out that it is really due to an immense multitude of far more minute particles, which he has described under the name of the molecular base of the chyle. These molecules are most abundant in rich, milky, opaque chyle; whilst in poorer chyle, which is semi-transparent, the particles float separately, and often exhibit the vivid motions common to the most minute molecules of various substances. Such is their minuteness, that, even with the best instruments, it is impossible to determine either their form or their dimensions with exactness; they seem, however, to be generally spherical; and their diameter may be estimated at between l-36,000th and l-24,000th of an inch. Their chemical nature is as yet uncertain; they are remarkable for their unchangeableness, when submitted to the action of numerous re-agents, which quickly affect the proper Chyle-corpuscles; whilst their ready solubility in Ether would seem to indicate that they are of an oily or fatty nature. 517. The milky aspect which the serum of blood sometimes ex- hibits, is due to an admixture of this molecular base. It may be par- ticularly noticed, when blood is drawn a few hours after a full meal, that has been preceded by a long fast. By recent experiments it has been found, that the serum begins to show this turbidity, about half an hour after the meal has been taken ; and that the turbidity increases for some hours subsequently, after which it disappears. The period at which the discoloration is greatest, and the length of time during which it continues, vary according to the digestibility of the food. When the serum is allowed to remain at rest, the opaque matter rises to the surface, presenting very much the appearance of cream; and when separately examined, it has been found to contain a proteine- compound, mingled with oily matter,—the relative amount of the two appearing to depend in part upon the characters of the food in- gested. Hence it would not seem improbable, that the molecular base of the chyle is partly derived from albuminous matter of the food, which has not been completely dissolved in the digestive pro- cess, but which has been reduced to a state of exceedingly minute division (§ 473). The gradual disappearance of the turbidity of the serum indicates that the substance which occasioned it no longer exists as such in the circulating current; being either drawn off by the nutritive or secretory operations, or being converted by the as- similatino- process into the ordinary constituents of the blood. 518. During the passage of the Chyle along the lacteals, towards the Mesenteric glands, it undergoes two important changes ; the pre- sence of Fibrin begins to manifest itself by the spontaneous coagula- 302 PROGRESSIVE ALTERATIONS IN THE CHYLE. bility of the fluid ; and the oil-globules diminish in proportional amount. The fibrin appears to be formed at the expense of the albu- men ; as this latter ingredient undergoes a slight diminution. It is in the chyle drawn from the neighbourhood of the mesenteric glands, that we first meet with the peculiar floating cells or chyle-corpuscles, formerly adverted to (§ 212), in any number. The average diameter of these is about l-4600th of an inch; but they vary from about l-700th to l-2600th,—that is, from a diameter about half that of the human blood-corpuscles, to a size about a third larger. This varia- tion probably depends in great part upon the period of their growth. They are usually minutely granulated on the surface, seldom exhibit- ing any regular nuclei, even when treated with acetic acid ; but three or four central particles may sometimes be distinguished in the larger ones. These corpuscles are particularly abundant in the chyle obtained by puncturing the mesenteric glands themselves; and there can be little doubt, that they are identical writh the altered epithelium-cells, which line the lacteal tubes in their course through those bodies (§ 496). 519. The glandular character of these cells, and their continued presence in the circulating fluid, seem to indicate that they have an important concern in the process of Assimilation,—that is, in the conversion of the crude elements derived from the food, into the organizable matter adapted to the nutrition of the body; in other words, in the conversion of Albumen into Fibrin ; which change would seem to take place to a considerable extent in the Mesenteric glands. For it is only in the Chyle which is drawn from the lacteals intervening between the mesenteric glands and the receptaculum chyli, that the spontaneous coagulability of the fluid is so complete as to produce a perfect separation into clot and serum. The former is a consistent mass, which, when examined with the microscope, is found to include many of the chyle-corpuscles, each of them being sur- rounded with a delicate film of oil; the latter bears a close resemblance to the serum of the blood, but has some of the chyle-corpuscles sus- pended in it. Considerable differences present themselves, however, both in the perfection of the coagulation, and in its duration. Some- times the chyle sets into a jelly-like mass; which, without any sepa- ration into coagulum and serum, liquefies again at the end of half an hour, and remains in this state. The coagulation is usually most complete in the fluid drawn from the receptaculum chyli and thoracic duct; and here the resemblance between the floating cells, and the white or colourless corpuscles of the blood, becomes very striking. 520. The Lymph, or fluid of the Lymphatics, differs from the Chyle, as already remarked, in its comparative transparency : its want of the opacity or opalescence, which is characteristic of the latter, being due to the absence, not merely of oil-globules, but also of the "molecular base." It contains floating cells, which bear a close resemblance to those of the Chyle on the one hand, and to the colourless corpuscles of the Blood on the other; and these, as in the preceding case, are most numerous in the fluid, which is drawn from ABSORPTION FROM EXTERNAL AND PULMONARY SURFACE. 303 the lymphatics that have passed through the glands, and in that ob- tained from the glands themselves. Lymph coagulates like chyle; a colourless clot being formed, which encloses the greater part of the corpuscles. The Lacteals may be regarded as the Lymphatics of the intestinal walls and mesentery; performing the function of inter- stitial absorption, as well as effecting the introduction of alimentary substances from without. During the intervals of digestion, they contain a fluid, which is in all respects conformable to the lymph of the lymphatic trunks. 521. Thus by the admixture of the aliment newly introduced from without, with the matter which has been taken up in the various parts of the system, and by the preparation which these undergo in their course towards the thoracic duct, a fluid is prepared, which bears a strong resemblance to blood in every particular, save the presence of red corpuscles. Even these may sometimes be found in the contents of the thoracic duct, in sufficient amount to communi- cate to them a perceptible red tinge ; but there can be little doubt that they have found their way thither accidentally,—some of the lymphatic or lacteal trunks, which have been divided in the dissec- tion necessary to expose the duct, having taken up blood by their open mouths, and rapidly transmitted it into the general receptacle. The fluid of the thoracic duct may be compared to the blood of Invertebrated animals; from which the red corpuscles are almost or altogether absent; but which contains white or colourless corpuscles, and which possesses but a slight coagulating power, in consequence of its small proportion of fibrin. And we hence see, why these animals should require no special absorbent system; since the blood- vessels convey a fluid, which is itself so analogous to the chyle and lymph to be absorbed, that the latter may be at once introduced into it, without injuring its qualities. 5. Absorption from the External and Pulmonary Surface. 522. Although the Mucous Membrane of the Alimentary Canal is the special channel for the introduction of nutritive or other sub- stances into the system, it is by no means the only one. The Skin covering the body, and the Mucous Membrane prolonged into the Lungs, are also capable of absorbing liquids and vapours, and of introducing them into the Circulation; although they serve this pur- pose less in Man and the higher animals, than in some of the lower. Their utility in this respect is best shown, when, from peculiar cir- cumstances, the function of the digestive cavity cannot be properly performed; and when, therefore, the system has been more than usually drained of its fluids, and stands in need of a fresh supply.— Thus shipwrecked sailors, and others, who are suffering from thirst, owing to the want of fresh water, find it greatly alleviated, or alto- gether relieved, by dipping their clothes into the sea, and putting them on whilst still wet, or by frequently immersing their own 304 CO:\fPOSITION OF THE BLOOD. bodies. In a case of dysphagia, in which neither solid nor fluid nutriment could be introduced into the stomach, the patient was kept alive for a considerable time, and his sufferings greatly alleviated, by the administration of nutritive clysters, and by the imraersion of his body in a bath of tepid milk and water, night and morning. Under this system, the weight of the body, which had previously been rapidly diminishing, remained stationary, although the amount of the excretions was increased; and the use of the bath had a special influence in assuaging the thirst, which was previously distressing. It appeared that the water of the urinary excretion, amounting to from 24 oz. to 36 oz. per day, must have been entirely supplied from this latter source. Again, a man who had lost nearly 3 lbs. by per- spiration, during an hour and a quarter's labour in a very hot atmo- sphere, regained 8 oz. by immersion in a warm bath at 95° for half an hour.—In these cases it appears probable, from the experiments already noticed (§ 502), that the Lymphatics, rather than the blood- vessels, are the chief agents in the absorbing process ; not, however, from any powers peculiar to them, but merely on account of the thinness of their walls, and their very copious distribution in the skin. 523. Absorption may also take place from an atmosphere saturated with watery vapour. Of this we have a very curious proof in the Frog; whose urinary bladder (which serves as a sort of reservoir of water) has been observed to be refilled, after having been emptied, by placing the animal in an atmosphere loaded with watery vapour. Numerous instances are on record, which prove that such absorption may take place in Man, to a very considerable extent; though the proportion introduced through the Skin, and through the Lungs, cannot be exactly ascertained. The ready introduction of volatile matter into the system, through the latter channel, is a matter of familiar experience; thus if we breathe an atmosphere through which the vapour of turpentine is diffused, it soon produces the character- istic odour of violets in the urinary secretion. And it is probably in this manner, that a large number of those poisonous miasmata are introduced, which are such fertile causes of disease. 6. Of the Composition and Properties of the Blood. 524. Having traced the steps by which the blood is elaborated, and prepared for circulation through the body, and having (in the former part of the volume) inquired into the characters of its chief constitu- ents, we have now to consider the fluid as a whole, to study the usual proportions of these constituents, and the properties which they im- part to it. 525. The Blood, whilst circulating in the living vessels, may be seen to consist of a transparent, nearly colourless fluid, termed liquor sanguinis; in which the corpuscles, to which the blood owes its red hue, as well as the white or colourless corpuscles, are freely suspended COMPOSITION OF THE BLOOD. 305 and carried along by the current.—On the other hand, when the blood has been drawn from the body, and is allowed to remain at rest, a spontaneous coagulation takes place, separating it into clot and serum. The clot is composed of a network of Fibrin, in the meshes of which the Corpuscles, both red and colourless, are involved; and the serum is the same with the liquor sanguinis deprived of its Fibrin. When the Serum is heated, it coagulates, showing the presence of Albumen. And if it be exposed to a high temperature, sufficient to decompose the animal matter, a considerable amount of earthy and alkaline salts remains.—Thus we have four principal components in the Blood ;— namely, Fibrin, Albumen, Corpuscles, and Saline matter. In the circulating Blood they are thus combined :— Fibrin } Albumen V In solution, forming Liquor Sanguinis. Salts ) Red Corpuscles,—Suspended in Liquor Sanguinis. But in coagulated blood they are thus combined:— t, , n -, } Crassamentum or Clot. Red Corpuscles ) Albumen ) t> • • 1 *• c • o ^ , > Remaining in solution, forming Serum. A certain amount of Serum, however, is involved in the Crassa- mentum ; and can only be separated by cutting the clot into thin slices, and carefully washing it. 526. The components of the Blood may be separated, and their amount estimated, in various ways. Thus, if fresh-drawn blood be continually stirred with a stick, or be " whipped," with a bunch of twics, the Fibrin coagulates in the form of strings, which adhere to the wood, and may thus be withdrawn ; whilst the red corpuscles then remain suspended in the serum, gradually sinking to the bottom in virtue of their greater specific gravity.—On the other hand, the Red Corpuscles may be separated, in those animals in wdiich they are large enough, by passing the blood through a filter; having previously min- gled with it some substance, which retards, but does not prevent its coagulation* (§ 185). The liquor sanguinis is thus separated from the blood-discs; and the former coagulates, whilst the blood-discs are retained upon the filter. This experiment convincingly proves, that the act of coagulation is not due to the red corpuscles, as was at one time imagined. The ordinary act of coagulation, by withdraw- ing the Fibrin and Corpuscles, makes it easy to estimate the propor- tion of Albumen and of Saline matter in the Blood, when due allow- ance is made for the quantity of Serum retained in the Clot; and the relative proportions of these may be determined, by evaporating the * This experiment cannot be performed with Human blood, because the corpuscles are small enough to pass through the pores of any filter that allows the liquor san- guinis to permeate it; but it answers very well with Frog's blood. 20 306 COMPOSITION OF THE BLOOD. fluid, so as to obtain the whole amount of solid matter it contains, and by then calcining the residuum, so as to ascertain how much of this is a mineral ash,—the remainder being chiefly Albumen.—The solid matter of the blood also contains various Fatty substances, which may be removed from it by ether. Some of these appear to corre- spond with the constituents of ordinary Fat (§261); whilst another contains phosphorus, and seems allied to the peculiar fatty acids of Neurine (§ 383); and another has some of the properties of Choles- terine, the fatty matter of the Bile (§ 724).—Besides these, there are certain substances known under the name of Extractive; one group of which is soluble in water and another in Alcohol. Of the precise nature of these, little is known. They have, been aptly termed " ill- defined" animal principles ; and it is probable that they may include various substances in a state of change or disintegration, which are being eliminated from the Blood by the processes of Excretion. 527. The general result of numerous recent analyses of the Blood may be thus stated:—The whole amount of solid matter is greater in the Male than in the Female. The higher proportion extends to all its components except the Albumen; and this is almost invariably present in an amount, which is absolutely greater in 1000 parts of female blood, than in 1000 parts of that of the male, and which is considerably greater in proportion to the other solid matters. The proportion of Albumen seems more constant than that of the other constituents of Blood; seldom varying beyond 5 or 6 parts either more or less than 70 in 1000.—The quantity of Corpuscles appears liable to considerably greater variation; the superiority on the side of the Male, however, being very strongly marked in the maximum and minimum, as well as in the average. We may regard its average in the Male at about 140 in 1000 parts of blood ; but it may fall to 110-5 parts, without the health being seriously affected; whilst, on the other hand, it may arise to 186 without any manifestation of dis- ease. In the Female, its average may be about 112 parts in 1000; but it may fall to as little as 71-4, and may rise to 167, consistently with ordinary health. The range of variation is thus much greater in the Female than in the Male; the minimum being considerably less in the former, than half the maximum ; whilst in the latter, it is much more. This is probably due in part to the fact, that the loss by the Catamenial discharge may produce a great temporary depression in the proportion of the corpuscles.—The average proportion of Fibrin seems to be no more than 2-2 in the Male; and though it may rise to as much as 3-5 or even 4, without disordering the system, it does not seem to fall below 2, in the state of ordinary health. The average in the Female is probably about 2; the proportion may rise to 3, or fall to 1-8; but the variation seems less considerable in the Female than in the Male.—Much is probably yet to be learned, regarding the influ- ence of different kinds of food recently taken, on the proportion of these constituents of the blood ; and it does not seem unlikely, from what has been already stated (§ 517), that the quantity of fatty matter COMPOSITION OF THE BLOOD. 307 is especially liable to variation, in accordance with the amount con- tained in the food, and the time which has elapsed since the last meal. 528. The Saline constituents of the blood, obtained by drying and incinerating the whole mass, usually amount to between 6 and 7 parts in 1000. More than half of their total quantity is composed of the Chlorides of Sodium and Potassium ; and the remainder is made up of the tribasic Phosphate of Soda, the Phosphates of Lime and Mag- nesia, Sulphate of Soda, and a little Phosphate and Oxide of Iron. Of these the chief part are dissolved in the Serum ; but the Earthy Phosphates, which are insoluble by themselves, are probably com- bined with the proteine-compounds (§ 175); and the iron is contained, chiefly or entirely, in the red corpuscles. It is difficult to speak with certainty, from the examination of the ashes of the blood, as to the state of the saline constituents of the circulating fluid. Thus, the Serum has an alkaline reaction ; and this has been supposed to be due to the presence of alkaline Carbonates. Moreover, the presence of the Lactates of potash and soda has been usually asserted. On the other hand, some recent analyses wTould indicate, that the alkaline reaction is entirely due to the presence of the tribasic Phosphate of soda ; and that no alkaline carbonates or lactates exist in the blood. This discrepancy seems partly due to the mode of analysis employed ; for it has been lately pointed out by Dr. G. 0. Rees, that although the ashes of the entire mass of blood do not effervesce on the addition of an acid, effervescence takes place, wThen acid is added to the ashes of the Serum ; showing the existence in it, either of alkaline Carbon- ates, or of Lactates which have been reduced to the state of Carbon- ates by incineration. It appears that when the entire mass of blood is incinerated, enough phosphoric acid is produced from the phos- phorized fats, to neutralize the alkaline carbonates, and thus to pre- vent their presence from being recognized. There can be no doubt, however, that the tribasic phosphate of soda exists as such in the blood, and contributes to its alkaline reaction; and it appears to con- fer upon the serum a special power of absorbing carbonic acid. 529. The following appear, from the considerations stated in the preceding part of the Volume, to be the chief uses of the principal constituents of the Blood, in the general economy. The Fibrin is the material, which is most completely prepared for organization, and which supplies what is requisite for the nutrition of the larger pro- portion of the solid tissues of the body. It is, therefore, being con- tinually withdrawn from the blood by the nutritive operations; and the demand appears to be supplied, in part by the influx of Fibrin that has been prepared in the Absorbent system, and in part by the continued transformation of Albumen, which takes place during the Circulation of the Blood. If a proper amount of Fibrin be not pre- sent in the Blood, its physical properties are so far altered, by the diminution of its viscidity, that it will not circulate through the capil- laries as readily as before,—a certain degree of viscidity having been experimentally found to be favourable to the movement of fluid 308 COMPOSITION OF THE BLOOD. through glass or metallic tubes of small bore.—The Albumen of the blood is the raw material, at the expense of which not only the Fi- brin, but many other substances are generated during the nutritive process. All the Albuminous compounds of the Secretions, the Horny matter of the Epidermic tissues, the Gelatin of the simple fibrous tissues, the solid materials of the Red Corpuscles, and other substances, may be regarded as almost certainly produced by the transformation of the Albumen of the Blood; and a continual supply of this from the food is therefore requisite, to preserve the due pro- portion in the circulating fluid.—The Red Corpuscles, which (it will be remembered) are almost exclusively confined to Vertebrated ani- mals, appear to be more connected with the function of Respiration, than with that of Nutrition ; and the stimulating action of Arterial blood, especially upon the Muscular and Nervous tissues, appears chiefly to depend upon their presence. It has been observed in particular, that their presence is more effectual in stimulating the heart's action, than is that of either of the other constituents of the blood. In addition to what has been already stated (§ 219), in re- ference to their continual disintegration and renewal, it may be men- tioned, that when the blood of one animal was injected by Majendie into the veins of another having discs of very different size and form, the original Red corpuscles soon disappeared, and were replaced by those characteristic of the species, in whose veins the fluid was cir- culating. 530. The use of the Saline matter is evidently in part to prevent decomposition in the circulating Blood ; but also to supply the mineral materials, requisite for the generation of the tissues, and entering into the composition of the secretions. It is by the saline and albuminous matters in conjunction, that the specific gravity of the Liquor Sangui- nis is kept up to the point, at which it is equivalent to that of the contents of the Red Corpuscles; and it is only in this condition, that the formation of the latter can duly take place.—The Fatty matters of the Blood are evidently derived from the food, either directly, or by a transformation of its farinaceous ingredients (§ 430); and they are chiefly appropriated to the maintenance of the combustive process. That which may be superfluous is either deposited in the cells of Adi- pose Tissue, or it is eliminated by the Liver, the Sebaceous follicles of the Skin, and, in the female when nursing, by the Mammary glands. The blood appears to contain, ready formed, the peculiar azotized and phosphorized fat of Nervous matter; but how this is generated,— whether by the combination of azotized and phosphorized ingredients with ordinary fat, or by the metamorphosis of albuminous matter,— cannot be said to be yet determined. 531. The proportion of these components of the Blood is liable to undergo changes in disease, Vhich extend far beyond the widest limits which have been mentioned as consistent with health. Thus, the quantity of Fibrin exhibits a remarkable increase in Inflammation ; the amount then found in the blood being from 5 or 6 parts in 1000 COMPOSITION OF BLOOD IN DISEASE. 309 to 9, 10, or even 10^, according to the extent and intensity of the disease. On the other hand, it presents a remarkable diminution in Typhoid fevers; the quantity being sometimes as little as 0-9. If any decided Inflammation should develop itself, however, in the course of the Fever, the proportion of Fibrin rises accordingly. A deficiency of Fibrin in the blood predisposes to Hemorrhages, Con- gestions, &c, either into the substance of the tissues, or on the surface of membranes ; and these conditions are well known to be of frequent occurrence as complications of febrile disorders. An excess of Fibrin is not much affected by copious bleeding, even if this be frequently repeated; but there is reason to think, that the administration of Mercury has a tendency to restrain its production. 532. It is difficult to say what amount of Red Corpuscles should be regarded as excessive; since, as we have seen, they may augment to a great degree, without disturbing the health. When they are present in an amount much above the average, they seem concerned in pro- ducing the condition termed Plethora ; which marks a " high condi- tion" of the system, and which borders upon various diseases, espe- cially those of Congestion, and Hemorrhages. To these a peculiar liability then exists ; because, although the proportion of Fibrin in the blood is not absolutely low, it is low in reference to that of the Red Corpuscles. Plethoric persons do not seem more liable to In- flammation, than are those of weakly constitution. The quantity of the Red Corpuscles is rapidly diminished by frequent bleeding; and hence it is lowered by repeated Hemorrhages. On the other hand, it is speedily restored to its usual standard under the influence of nutritious diet, if the digestive powers have not been too much weakened to make use of this.—The proportion of Red Corpuscles undergoes a marked diminution in various forms of Anaemia; and particularly in Chlorosis. In severe cases of this latter disease, it has been found as low as 27 in 1000 ; and it not unfrequently sinks to 40 or 50. The marked influence of the administration of Iron, in favouring the repro- duction of Red Corpuscles, has been already noticed (§ 219). 533. The proportion of Albumen in the Blood seems less liable to change, except in the condition termed Albuminuria, in which a large quantity of Albumen appears in the Urine. When this condition is permanently established, it is indicative of the existence of serious organic disease of the kidney ; but it may occur for a short time under the influence of simple congestion of that organ, which causes an escape of the albuminous part of the blood, together with the water which is filtered-off (as it were) in this gland (§ 728). Now when Albuminuria is fully established, there is a marked diminution in the quantity of Albumen in the serum of the blood; and this diminution is constantly proportional to the amount of Albumen present in the Urine. The proportion of Saline matter appears to undergo less altera- tion in disease than that of the other constituents of the Blood; and has not been found to have a regular correspondence, either in the way of excess or diminution, with any particular morbid state. 310 COAGULATION OF BLOOD. 534. The condition of the Blood may be affected, not merely by alteration in the proportions of its normal ingredients, but by the pre- sence of other substances;—either such as are generated in it, and are constantly being eliminated from it in health, but have accumu- lated to an abnormal degree ;—or such as have found their way into it from without. Thus, Carbonic Acid, Urea and Lithic Acid^ Cho- lesterine and other elements of Bile, and other matters which it is the office of the Excreting organs to remove, may accumulate in the blood, and may become fertile sources of disease, by their injurious influence. The introduction of various Mineral substances, by absorption from without, changes the composition of the normal elements of the Blood, and thus affects their vital properties; thus strong saline solutions diminish or destroy the coagulating power of the Fibrin. But the most remarkable cases of depravation of the Blood, by the introduc- tion of matters from without, are those which result from the action of ferments,—exciting such Chemical changes in the constitution of the fluid, that its whole character is speedily changed, and its vital pro- perties are altogether destroyed. Of such an occurrence we have a marked example in the various forms of malignant fevers ; in which the introduction of a very minute quantity of noxious matter into the blood, either through the lungs or through the skin, produces a speedy alteration in the characters of the whole mass of the blood, the func- tion of every organ in the body is disordered, and decomposition of the solids and fluids takes place to a considerable extent, even before circulation ceases, and whilst consciousness yet remains. The train of symptoms produced by the bite of venomous Serpents, and of rabid animals, appears referable to the same cause,—the alteration in the condition of the whole current of blood, by the introduction of a minute quantity of a substance that acts as a ferment. 535. The Coagulation of the Blood, as already explained, depends upon the passage of its Fibrin from the fluid state to the solid (§ 184); consequently, if the fibrin be separated from the other elements, no coagulation takes place. On the other hand, if the amount of Fibrin be larger than ordinary, the coagulum possesses an unusual degree of firmness. The length of time which elapses before coagulation, and the degree in which the Clot solidifies, vary considerably; in general they are in the inverse proportion to each other. Thus, if a large quantity of blood be withdrawn from the vessels of an animal at the same time, or within short intervals, the portions that last flow coagulate much more rapidly, but much less firmly, than those first obtained, in consequence of the diminished proportion of fibrin. On the other hand, when the fibrin is in excess, its coagulation is un- usually delajjied. From this delay an important change results, in the mode in which the coagulation takes place; for the red corpus- cles, instead of being uniformly diffused through the coagulum, have time to sink to the bottom, in virtue of their greater specific gravity; and the upper part of the clot is consequently made up of Fibrin, almost exclusively, whilst the lower is chiefly formed by the aggre- BUFFY COAT. 311 gation of the red corpuscles. Hence the upper layer is almost desti- tute of colour, (whence it has received the name of buffy coat,) and is remarkably tenacious in its character; whilst the lower is very deep in hue, and very friable in consistence. When the fibrillated network forming the buffy coat undergoes the slow contraction, which is characteristic of highly-elaborated fibrin subsequently to its coagu- lation, it draws in the edges of the upper surface of the clot, giving it a cupped appearance. 536. The Buffy Coat may present itself under a great variety of conditions; and it can no longer, therefore, be regarded as it formerly was, a sign of the Inflammatory state. It is most fully developed when acute Inflammation exists ; because in that condition all the cir- cumstances which favour it are present. That it may be produced by any cause, which occasions delay in the coagulation of the blood, is evident from the fact, that healthy blood may be made to exhibit it, by adding a solution of a neutral salt, which retards, but does not prevent its coagulation. But the blood may coagulate with its ordi- nary rapidity, or even more speedily than usual; and may yet exhibit the Buffy Coat. And, moreover, the separation of the Fibrin and the Red Corpusdes may take place in films of blood so thin, as not to admit of a stratum of one being laid over the other; the two elements separating from each other laterally, and the films acquiring a speckled or mottled appearance, equally characteristic of the In- flammatory condition with the Buffy coat itself. Hence the separa- tion must be due in such cases to other causes than gravity, and recent observations have ac- counted for it, by showing that the Red Corpuscles have an un- usual attraction for one another in the Inflammatory state, causing their coalescence in piles and masses; wdiilst the particles of Fibrin have also a peculiarly strong attraction for each other. Thus there is a powerful tend- ency, that draws together the components of each kind, and consequently tends to separate them from the others; and when this separation takes place, the difference in the specific gravity of the two elements decides their respective situations.—The peculiar tendency of the Red corpuscles to unite, in the In- flammatory state, serves to dis- tinguish this condition, even in a single drop of blood; and it is then that the White corpuscles may be most easily distinguished, as Fig. £2. The microscopic appearance of a drop of blood in the inflammatory condil^i. The red corpus- cles lose their circular f >rm, and adhere" together; the white corpuscles remain apart, and are more abundant than usual. 312 CIRCULATION OF BLOOD. they are seen apart from the rest of the mass, having no tendency to unite with it. In fact, the white corpuscles are not found in com- pany with the red, in the ordinary coagulum, but rather with the fibrinous portion; and when they are peculiarly abundant, as they usually are in Inflammatory blood, they may form a considerable pro- portion of the buffy coat. 537. The Buffy Coat may present itself, without the least increase in the normal quantity of Fibrin, and without any approach to the Inflammatory state; simply because the Fibrin is present in exces- sive amount, in relation to the amount of Red corpuscles, the latter being much below their usual proportion. Thus in severe Chlorosis, the buffy coat is almost as strongly marked as in the severest Inflam- mation ; but the two conditions are at once distinguished by the rela- tive proportions of solid matter in the blood, as indicated by the size of the Coagulum. For in Chlorosis the coagulum is very small, in consequence of the reduced proportion of Corpuscles, and is almost invariably found floating in the serum; whilst in the ordinary Inflam- matory condition, it is of full size, frequently adhering to the side of the vessel. CHAPTER VI. OF THE CIRCULATION OF THE BLOOD. 1. Nature and Objects of the Circulation of Nutrient Fluid. 538. The nutritive fluid,—the elements of which are thus chiefly taken up by the Absorbent system, and are there prepared, as by glandular apparatus diffused through the whole body, to be mingled with the general mass of the previously-formed Blood,—is carried into the various parts of the system, by the act of Circulation. This move- ment answers various purposes. It furnishes all the tissues, which are to derive nutriment from the Blood, with a constantly renewed supply of the materials which they severally require ; and in this manner it is subservient to the growth, not only of those tissues which form part of the solid structure of the body, but also of those various cells, covering its free surfaces, which are being continually cast off and renewed, and which, in the course of their development, separate from the blood the products that are to perform ulterior purposes in the economy, or are to be cast off as altogether effete. Thus the Circu- lation is subservient to the functions of Nutrition and Secretion. In the exercise of these functions, different materials are drawn from the blood by the several tissues it supplies. Thus the nutrition of the muscle requires fibrin ; that of the nerve requires fatty-matter; that of CIRCULATION OF BLOOD. 313 the bone draws off gelatin and earthy salts; that of the hepatic cells removes the fatty matter and other elements of bile ; that of the milk- cells (during lactation) separates albuminous, fatty, and saccharine substances ;—and so on. Thus various portions of the blood, when returning from the several organs through which they have been trans- mitted, have undergone very different changes by the nutritive and secreting processes, according to the function of the organs which they have supplied ; and if the same portion of the circulating fluid were constantly being transmitted to each organ, and returned from it, its composition would speedily undergo a change that would render it no longer fit for its purposes. By the union of the different local circu- lations, however, into one general circulation, this change is pre- vented, and the wdiole mass of the blood is maintained in its normal or regular condition; for as its composition is such, as to supply all parts of the body, in a state of health, with the proportions of nutri- tive material which they respectively need, and as the returning cur- rents are all mingled together in the vessels, before being again dis- tributed to the system, each part supplies what the other has been deprived of, and thus the normal proportion of ingredients in the whole mass of the blood is constantly kept up, whilst in each of its separate streams it is undergoing an alteration of a different kind. 539. But these processes alone might be carried on by the aid of a much less rapid Circulation, than that which exists in Man and the higher animals. We do, in fact, occasionally meet with examples, in- which they continue for some time, under an almost total stagnation of the current. There are others, however, which require a mucta more rapid and uninterrupted movement of the circulating fluid. We have already seen that, for the action of the Nervous and Muscular tissues, oxygen is necessary; and the amount of that gas contained in the blood circulating through these tissues would be very speedily exhausted, if it were not continually renewed ; whilst the carbonic acid, which is formed at the expense of that oxygen, would speedily accumulate to an injurious degree, if it were not carried off as fast as it is produced. Hence we find that in all Animals, the maintenance of the Respiration, by carrying Oxygen from the respiratory surface into the different parts of the system, and by conveying back Carbonic acid to be thrown off at the Respiratory surface, is one of the great purposes of the Circulation of the blood ; and its extreme importance is shown by the very speedy check, w-hich the interruption of this function produces in the movement of the blood, in warm-blooded animals. Thus in a Bird or Mammal, completely cut off from Oxygen, the circulation in the lungs will come to a stop, which stop- page will necessarily extend itself over the whole body, in little more than three minutes. We find, then, that the rate of the Circulation* in different animals, bears a relation to the energy of their Respiration ; and this energy is closely connected with the general activity of their functions, but particularly with that of the Nervous and Muscular systems, which are most dependent for the exercise of their powers. 314 ASCENT OF SAP IN PLANTS. upon a continually fresh supply of oxygen, and upon the unceasing removal of the carbonic acid which is generated in their substance. 2. Different forms of the Circulating Apparatus. 540. It is desirable that the Circulating apparatus should be first studied in its very simplest form,—that which it possesses in Plants and in the lowest tribes of Animals; as in this way alone can the forces, which are concerned in the movement of the fluid, be rightly appreciated. There are, in all the higher Plants, two distinct cur- rents, that of the ascending, and that of the descending sap. The former of these fluids should be compared rather with the chyle than with the blood of Animals ; for it is a crude fluid, not yet prepared to take part in the nutrition and extension of the structure. But there are some circumstances attending its movement, which throw light upon other more complicated phenomena. The ascending sap con- sists principally of water; which is imbibed, together with various substances which it holds in solution, by the delicate tissue at the soft extremities of the root-fibres, or spongioles. The power of forcing upwards a column of sap, which exists in these bodies, and which seems due to Endosmose (§ 491), is shown by very simple experi- ments. If the stem of a Vine, or of any tree in which the sap rises rapidly, be cut across when in full leaf, the sap continues to flow from the lower extremity; and this with such force, as to distend with vio- lence, or even to burst, a bladder tied firmly over the cut surface. If instead of a bladder, a bent tube be attached to this, and mercury be poured into it so as to indicate the pressure exerted, it is found that the rise of the sap takes place with a force equal to the pressure of from one to three atmospheres (from 15 to 45 lbs. upon the square inch)—or even more. Thus the ascent of the sap is partly due to a powerful vis a tergo, or impelling influence, derived from the point where the absorption takes place. 541. But, on the other hand, if the upper extremity be placed with the cut surface of the stem in water, a continued absorption of that fluid will take place, as is evidenced by the withdrawal of the water from the vessel; the fluid which is thus taken up, however, is not retained within the stem and branches, but is carried into the leaves, and is thence dissipated by exhalation. It is obvious, then, that the vis a tergo is not the sole cause of the ascent of the sap; but that a vis afronte also exists, by which the fluid is drawn towards the parts in which it is to be employed. This is further made apparent by a few simple experiments. If a branch, when thus actively absorbing fluid, be carried into a dark room, the absorption and ascent of fluid immediately cease almost completely; and are renewed again, so soon as the leaves are again exposed to light. Now wre know, from other experiments, that light stimulates the exhaling process (§ 87), whilst darkness checks it; and the cessation of the demand in the leaves thus produces a cessation in the absorption at the lower extremity of ASCENT AND DESCENT OF SAP IN PLANTS. 315 the stem. And this is the case also, in the natural condition of the plant; as is easily shown by immersing the roots in water, and ob- serving the respective quantities which are'removed by absorption during sunshine, shade, and darkness. On the other hand, the move- ment of the sap may be excited, when it would not otherwise take place, by the production of a demand at the extremities of the branches; thus if a branch of a vine growing in the open air, be introduced into a hot-house, and be subjected to artificial heat during winter, its buds will be developed, its leaves will expand, and these will draw fluid to themselves through the roots and stem, which are still inactive as regards the remainder of the tree. And the natural commencement of the movement of the ascending sap, which takes place with the returning warmth of spring, has been experimentally shown to occur, in the first instance, not in the neighbourhood of the roots, but nearest the extremities of the branches; the exhalation of fluid from the expanding buds being the first process, and a demand for fluid being thus created, which is supplied by the flow that is thus excited in the lower part of the stem,—this, again, being supplied from the roots, which are thus caused to recommence their absorbent function. 542. Thus we see that, in the ascending sap, the movement is en- tirely regulated by the demand for fluid, occasioned by the actions of the leaves; even though it is in great part dependent on the vis a tergo, which has its seat in the spongioles. Not even this force, however, —powerful as it has been shown to be,—can produce the continu- ance of the upward flow, when the exhalation from the leaves is checked by darkness, and when the demand occasioned by the action of these organs is consequently suspended. 543. The movement of the descending sap offers numerous points, which deserve to be carefully considered. This fluid is strictly com- parable to the blood of animals; having undergone a preparation or elaboration in the leaves, which adapts it to the nutrition and exten- sion of the structure, and to the formation of the various secretions of the plant. A great part of the fluid of the ascending sap has been lost by exhalation ; and the remainder, thus concentrated, receives a large additional supply of solid matter through the agency of the green cells of the leafy parts, which take in carbon from the atmosphere ' (§ 83); so that it now includes a considerable amount of gummy matter, in the state prepared for being converted into solid tissue; as well as numerous other compounds. Now this elaborated sap is then conveyed into the various parts of the system, through the agency of a network of vessels, which takes its origin in the leaves, and extends along the branches to the stem and roots, chiefly in the bark of those parts. These vessels are strictly analogous to the capillaries or small- est blood-vessels of Animals; but they differ from them in this,— that the capillary network of Animals communicates on either side ♦ with larger trunks, being formed, in fact, by the interlacement or anastomosis of their minutest branches,—whilst the network of nutri- 316 CIRCULATION OF ELABORATED SAP. tive vessels in Plants is everywhere continuous with itself, not having any communication with large vessels, so that the fluid prepared in the leaves commences a circulation there, which is continued on the same plan, until it has found its way to its remote destination in the roots. 544. The natural movement of the elaborated sap through these vessels may be studied, under favourable circumstances, with the assistance of the microscope ; the requisite conditions being, that the part should be sufficiently transparent for the vessels to be distinctly seen, that the sap shall contain globules in sufficient number to allow its movement to be distinguished by their means, and that the circu- lation should be observed without the separation of the organ exam- ined from the rest of the Plant, which would produce irregular move- ments, by the escape of the sap from the wounded part. These conditions may be attained in many Plants;—most conveniently, perhaps, in the stipules of the Ficus elastica, one of the trees which affords the largest supplies of Caoutchouc; and it is then found that the movement takes place in the following manner. Distinct currents are seen, passing along the straightest and most continuous vessels, and crossing by the lateral connecting branches of the network. These currents follow no determinate direction ; some proceeding up, and others down; some to the left, and others to the right: not un- frequently a complete stoppage is seen in one or more of the channels, without any obvious obstruction; and the movement then recom- mences, perhaps, in the opposite direction. The influence of a force, developed by the act of circulation, which determines the direction of the movement, appears from this; that if a tube be cut off, so as to give its contents an equally free exit at both ends, the sap only flows out at one extremity. The movement is retarded by lowering the temperature of the surrounding air, and it is completely checked by extreme cold ; it is capable of being renewed by moderate warmth ; and a further addition of heat increases its rapidity. By a strong electric shock, the force by which the latex is propelled seems to be altogether destroyed ; for the movement then ceases entirely. 545. Now it is quite certain that this circulation cannot be due to any vis a tergo; both because it is not constant in its direction in particular vessels; and because there is no organ in which any pro- pelling force, that could extend itself through such a complex system of vessels, may be developed. Nor can it be in any wTay due to the force of gravity; for although this may assist the descent of the fluid through the stem, it is totally opposed to its ascent from the ends of its branches towards their origin, when, as often happens, the latter are at the higher level. Moreover, it may be noticed that this circulation takes place most readily, in parts that are undergoing a rapid development; and that its energy corresponds with the vitality of the part. Further, it may be observed to continue for some time in parts that have been completely detached from the rest; and on which neither vis a tergo, nor vis a fronte, can have any influence. CIRCULATION OF ELABORATED SAP. 317 It is evident, then, that the force,—whatever be its nature,—by which this continued movement is kept up, must be developed by the pro- cesses to which that movement is subservient; in other words, that the changes involved in the acts of nutrition and secretion are the real source of the motor power. The manner in which they become so, is the next object of our inquiry; and on this subject, some new views have recently been put forth by Prof. Draper, which seem to account well for the phenomena. 546. It is capable of being shown, by experiments on inorganic bodies, that, if two liquids communicate with each other through a capillary tube, for the walls of which they both have an affinity, but this affinity is stronger in the one liquid than in the other, a move- ment will ensue ; the liquid which has the greatest affinity being absorbed most energetically into the tube, and driving the other before it. The same result occurs when the fluid is drawn, not into a single tube, but into a network of tubes, permeating a solid struc- ture ; for if this porous structure be previously saturated with the fluid, for which it has the less degree of attraction, this will be driven out and replaced by that for which it has the greater affinity, when it is permitted to absorb this. Now if, in its passage through the porous solid, the liquid undergoes such a change, that its affinity be dimin- ished, it is obvious that, according to the principle just explained, it must be driven out by a fresh supply of the original liquid, and that thus a continual movement in the same direction would be produced. 547. Now this is precisely that which seems to take place in the organized tissue, permeated by nutritious fluid. The particles of this fluid, and the solid matter through which it is distributed, have a cer- tain affinity for each other ; which is exercised in the nutritive changes, to which the fluid becomes subservient during the course of its circu- lation. Certain matters are drawn from it, in one part, for the sup- port and increase of the woody tissue ; in another part, the secreting cells demand the materials which are requisite for their growth,—as starch, oil, resin, &c; and thus in every part that is traversed by the vessels, there are certain affinities between the solids and the fluids, wThich are continually being newly developed by acts of growth, as fast as those which previously existed are satisfied or neutralized by the changes that have already occurred. Thus in the circulation of the elaborated sap, there is a constant attraction of its particles towards the walls of the vessels, and a continual series of changes produced in the fluid, as the result of that attraction. The fluid, which has given up to a certain tissue some of its materials, no longer has the same attraction for that tissue ; and it is consequently driven from it by the superior attraction, then possessed by the tissue for another portion of the fluid, w7hich is ready to undergo the same changes, to be in its turn rejected for a fresh supply. Thus in a growing part, there is a constantly renewed attraction for the nutritive fluid, which has not yet traversed it; whilst, on the other hand, there is a diminished attraction for the fluid, which has yielded up the nu- 318 CIRCULATION IN PLANTS AND LOWER ANIMALS. tritive materials required by the particular tissues of the part; and thus the former is continually driving the latter before it. 548. But the fluid which is thus repelled from one part, may still be attracted towards another; because that portion of its contents, which the latter requires, may not yet have been removed from it. And in this manner, it Svould seem, the flow of sap is maintained, through the whole capillary network, until it is altogether exhausted of its nutritive matter. The source of the movement is thus entirely to be looked for in the changes, which take place in the act of growth ; and the influence of heat, cold, and other agents, upon the movement, is exercised through their power of accelerating or retarding those changes.—The fluid which thus descends through the stem and roots, seems to be at last almost entirely exhausted ; a portion of it appears to find its way into the ascending current, and to be mingled with the ascending current; but all the rest seems to have been entirely appropriated by the different tissues, through which it has circulated. Thus there is no need of any general receptacle, into which it may be collected, and from which it may take a fresh departure ; such as is afforded by the heart of Animals. And as the purpose of this circu- lation is only to supply the nutritive materials, and not to convey oxygen,—this element being but little required in the vegetative pro- cesses, and being supplied by other means,—the same energy and rapidity are not required in it, as need to be provided for in the higher Animals. 549. A condition of the Circulating system very similar to this, exists in several of the lower Animals, as well as in the embryo-state of the higher. In the very lowest, no blood-vessels are required, for the same reason that no sap-vessels exist in the lowest Plants;— namely, because every part absorbs and assimilates nutritious fluid for itself, so that it does not require a supply from vessels. As, in the Sea-Weeds, the whole substance is nourished by direct absorption from the fluid in contact with the external surface, every part of which seems endowed with the same absorbent power, so in the Polypes do we find, that the whole substance is nourished by direct absorption from the internal surface, which forms the lining of the digestive cavity. In the same manner, the Aeration of the animal fluids,—or the exposure of them to the air contained in water, by which they may part with carbonic acid and imbibe oxygen,—is provided for, riot by any special respiratory organs, but by the contact of water with every part of the soft external and internal surfaces. Further, as the sub- stance of their body is nearly of the same kind in every part, they do not require the continual interchange of the fluid distributed to its several portions. Thus no circulation is necessary, in these simple animals, either for the nutrition of their tissues, or for the aeration of the fluids. The same is the case with others of the lower tribes ; as well as with the embryo of the higher Animals, at the earliest periods of their development. Thus the lower Entozoa, or parasitic worms, have a digestive cavity channeled out, as it were, in their soft gela- CIRCULATION IN LOWER ANIMALS, AND IN EMBRYO. 319 tinous tissues; and from the walls of this, the nourishment is drawn by the several component parts of those tissues, without the mediation of vessels. And the embryo even of Man, in its early condition, consists of an aggregation of cells, each of which absorbs for itself from the nutritious fluid with which it is surrounded, and goes through all its functions independently of the rest. 550. Proceeding a little higher, we find the first appearance of proper vessels in the higher Entozoa, and in the Acalephce or Jelly- fish. These vessels take up the nutritive fluid from the walls of the digestive cavity, on which they are spread out, just as the roots of Plants do from the soil. They then unite into trunks, by which the fluid is conveyed to the more distant parts of the structure, in the same manner as the ascending sap is conveyed to the leaves by the vessels of the stem and branches; and these trunks again subdivide, and form a network of capillary vessels, which are dispersed through the several parts of the fabric; some of them being very abundantly distributed upon a portion of the surface, which is particularly destined to perform the respiratory function. Through these capillary vessels, the fluid seems to move in very much the same manner, as through the system of anastomosing vessels in Plants;—that is, its motion is due, rather to forces which are developed during its circulation, than to any vis a tergo derived from the contractile power of a propelling organ. But there is this difference; that, after having traversed the minute vessels, and yielded up to the tissues a part of the solid matter which it contains, the fluid is collected again by other trunks, which convey it back to the point from which it started ; there it is min- gled with the fluid that has been newly absorbed, and with that which has undergone aeration ; and it is then distributed, as be- fore, through the general capil- lary network of the body. 551. Now this is very much the condition of the human em- bryo, at the time when vessels are first developed in its sub- stance. These vessels are form- ed by the coalescence of cells; and from the contents of these cells, which have been imbibed from the yelk, the first blood seems to be derived. The first formation of blood-vessels takes place, not in that part of the em- bryonic structure w-hich is to be developed into the perfect animal, but in a membranous expansion from it, which surrounds the yelk, and which answers the purpose of Vascular Area of Fowl's egg, at the beginning of the third day of incubation;—a, a, yelk; b,b,b,b, venous sinus bounding the area: c, aorta; d, punc- tum saliens, or incipient heart; e, c, area pellucida; /,/, arteries, of the vascular area; g, g, veins; h, eye. 320 CIRCULATION IN LOWER ANIMALS. a temporary stomach. A capillary network is formed in a limited portion of this membrane, termed the vascular area (Fig. 83); and this not by the branching of larger trunks, these trunks being subse- quently formed by the reunion of the capillaries. The first movement of the blood is towards the central spot, in which the organs of the permanent structure are being evolved; and it takes place before the incipient heart has acquired any muscularity, so that it must be quite independent of any contractile force exerted by that organ. Here too, then, we perceive that the circulation is essentially capillary; and that it is sustained by forces very different from those, of which the action is most evident to us in the higher animals. 552. As we ascend the animal scale, however, we find that pro- vision is made for a more regular and vigorous Circulation of the Blood, than that which exists in the lowest classes. Thus in the class of Echinodermata (including the Star-fish and Sea-Urchin), a portion of the principal vessel is peculiarly endowed with contractile power; and this may be seen in constant pulsation, like the heart of the higher animals,—alternately contracting, to propel the fluid it contains, through the vessels that issue from it,—and then dilating, to receive a fresh supply from the vessels that pour their contents into it. A similar provision is observable in the lower tribes of Worms, in which this contractile vessel lies along the back ; pro- pelling the blood forwards, by a sort of peristaltic movement, through trunks which pass out at its anterior termination ; and receiving it again, after it has circulated through the system, by vessels which enter at its posterior extremity. In the higher orders of Worms, in the Myriapoda or Centipede tribe, and in Insects, we find this dorsal vessel divided by transverse partitions containing valves, into separate cavities which answer to the different segments of the body. Each of these is, to a certain extent, the heart of its own segment, receivinop and propelling blood by trunks which open into it; but they all par- ticipate in the more general circulation just described, a large portion of the blood being poured into the hindermost segment, transmitted forwards from cavity to cavity through the valves which separate them, and at last propelled through trunks that issue from the most anterior segment. In some instances we find that two or three of these trunks, on either side, pass round the oesophagus, and reunite below it, so as to enclose it in a sort of collar; and they form a main trunk by this union, which runs backwards along the under surface of the body, and which distributes the blood to its different organs by lateral branches. These subdivide into a capillary network ; and the returning vessels, which originate in this network, pour the blood which has circulated through it into the posterior cavity of the dorsal vessel.—Still it is very evident, from the observation of the circulation in those transparent species, in which the whole process can be dis- tinctly watched under the Microscope, that the contractile power of the dorsal vessel is far from sufficient of itself to sustain the Circula- tion; and that the movement of the blood through the capillary net- CIRCULATION IN ARTICULATED CLASSES. 321 work is in part due to forces developed during its progress,—being often retarded or accelerated in particular spots, without any visible change in the propelling force of the central organ. 553. In most of these animals, there are distinct organs of Respi- ration, confined to some one part of the body; and we often find that the vessels which convey blood to them, are furnished with distinct contractile portions, like so many supplementary hearts, for the pur- pose of propelling the blood through them more energetically. In proportion as we ascend the series of Articulated animals, do we find, for the most part, a more vigorous and regular circulation, both for the nutrition of the system, and for the transmission of the blood through the respiratory organs ; but there is an exception in the case of Insects, which deserves special notice. In this class, the circula- tion is much less vigorous than it is in other Articulated animals of similar complexity of structure; though it might have been antici- pated, that the extraordinary activity of their movements would ne- cessitate a corresponding rapidity in the circulating current, espe- cially for the purpose of conveying an extraordinary supply of oxygen to the nervous and muscular systems. But this is provided for in another wiy; the air being conveyed to these tissues, not through the blood, but by direct transmission through the minute ramifications of the air-tubes or tracheae, which penetrate the very smallest organs of the body. 554. The condition of the Circulating apparatus in the Embryo of higher animals, at a period a little advanced beyond that just alluded to, presents a striking analogy with that last described ; for the heart, at the time of its first formation, seems like a mere dilatation of the principal vascular trunk, having thickened walls, in which, after a time, muscular fibre begins to be developed, and the contractile power manifests itself. The pulsation of this heart, however, does not seem to extend its influence immediately through the vascular area; the capillary circulation in which, remains for some time in great degree independent of it. There is no resemblance in form, how- ever, between the dorsal vessel of Insects, and the incipient heart of the higher animals; since the latter is never much prolonged, and speedily becomes doubled (as it were) upon itself; and its first division into distinct cavities is merely for the purpose of separating its receiv- ing portion, or auricle, from its propelling portion, or ventricle. But the general condition of the Circulating system is much the same in the two cases ; and it is further alike in this,—that it is not always easy to show that the vessels have distinct walls, as they frequently seem: like mere channels excavated in the tissues. 555. We may next turn our attention briefly to the condition of the Circulating apparatus in the Molluscous classes, which has lately been found to present some very peculiar characters. In these it would seem as if the moving power were more concentrated in the heart, than in the preceding; for this organ seems no longer like a mere dilatation of the vascular trunk, but is a distinct sac wTith mus* 21 322 CIRCULATION IN MOLLUSKS. cular walls, usually having at least two cavities, an auricle and a ven- tricle. The usual course of the circulation is the following. The blood, expelled from the ventricle of the heart, passes along the main systemic artery, or aorta ; which distributes it to the body at large. It is then collected again, and transmitted to the respiratory organs ; in which it is exposed, either to the air contained in the surrounding water, or (in the terrestrial Mollusks) more directly to the atmosphere; and from these it is returned to the heart, to be again transmitted to the system. Thus we see that the heart of these animals receives and impels aerated blood ; and that its office is to send that blood to the capillaries of the general system. Hence it may be called a sys- 556. The blood, in the first part of its course, passes through dis- tinct vessels : it has been lately shown, however, that in the Mollusks in general, the blood which has passed through the systemic capilla- ries, and is on its way to the respiratory organs, is no longer thus confined, but that it meanders through passages or sinuses, which are channeled out in the tissues, and which even communicate freely with the abdominal cavity in which the viscera lie ; so that their whole exterior is bathed by the circulating fluid. It is perhaps in this part of its course, that it most readily takes up the fresh nutrient mate- rials, which have been prepared by the digestive process, and which would, under such circumstances, find their way with comparative facility from the inner surface of their walls to the outer.—After being thus diffused, in its venous or carbonized state, through the substance of the tissues and through the visceral cavity, it is again collected into distinct trunks ; and these convey it to the respiratory organs.— Now although it cannot be doubted, that the impelling power of the heart is the chief cause of the movement of the blood through the sys- temic vessels, yet it would seem impossible to suppose, that this power can be exerted over the unrestrained currents, in which it is diffused through the body, after passing through the systemic capil- laries ; and it can scarcely be doubted, that its passage through the capillaries of the respiratory organs is due to the power which is developed in themselves, under the conditions already alluded to. 557. There is a very curious phenomenon to be observed in the circulation of some of the lowest Mollusks; namely, the continual reversal of the course of the current. The heart, in these animals, is much less perfectly formed, than in the higher tribes; and seems more like the mere contractile dilatation of the principal trunk, which is the sole representative of that organ in the Echinodermata. The circulating fluid is sometimes transmitted first to the system ; and, after being distributed to its different parts by the ramifications of the main artery, it meanders through the channels excavated in its tis- sues • and then flows towards the respiratory surface, after passing over which it returns to the heart. But after a certain duration of its flow in this direction, the current stops, and then recommences in the contrary direction,—proceeding first to the respiratory organs, and CIRCULATION IN FISHES. 323 then to the system in general. It would seem as if in this, one of the lowest forms of animals possessing a distinct Circulation, the central power were not yet sufficiently strong, to determine the course which the fluid is to take ; so that it undergoes continual vacillations. In a group of Compound Polypes, to which this class of Mollusks has many points of affinity, there is a movement of fluid through the stem and branches, which in like manner continually changes its direction. This movement, however, can scarcely be regarded in the light of a proper Circulation ; since the tubes in which it occurs are in direct communication with the digestive cavities of the Polypes. But the flow seems altogether independent of any mechanical propulsion; and takes place most energetically and regularly towards parts in which new growth is going on. 558. We have now to consider the chief forms in wdiich the Cir- culating apparatus presents itself in the Vertebrated classes ; and first in that of Fishes. We have here, as in Mollusks, a heart with two cavities, an auricle and a ventricle; this heart, however, is not placed at the commencement of the systemic circulation, but at the origin of the respiratory vessels. The blood which it receives and propels, is venous or carbonized ; this is transmitted along a main trunk, which speedily subdivides into lateral branches or arches; and these distribute it to the fringes of gills, that hang on the sides of the neck. By the action of the water on the gills, the blood is aerated in its passage through them ; and it is then collected by a series of converg- ing vessels, which reunite to form the great systemic artery, or aorta. By the ramifica- tions of this artery, the blood, now aerated, is distributed through the system, and affords the requisite nourishment and stimulation to its tissues. Returning from the systemic capil- laries in a venous state, the blood of the head and anterior portion of the body finds its way at once into the great systemic vein, or vena cava, by which it is conveyed back to the au- ricle of the heart; but that which has traversed the capillaries of the posterior part of the body, and of the abdominal viscera, is conveyed by a distinct system of veins to the liver and the kidneys. In these organs, the veins again subdivide into a network of capillaries, which is distributed through the secreting structure, and which serves to afford to the secreting cells the materials of their development. This is termed the portal system of vessels. From the capillaries of the liver and kidneys, the blood is finally collected by the hepatic and renal veins, which convey it into the vena cava; Fig. 84. Diagram of the Circulating Apparatus of Fishes;—a, the auricle ; b, the ventricle; e, the trunk supplying the bran- chial arteries, d, the aSrated blood returning from the gills is conveyed by e, e. the bran- chial veins, to f, the aorta, which distributes it to the sys- tem; thence it is collected, and returned to the auricle, by the veins which unite in the vena cava, g. 324 CIRCULATION IN FISHES.—LANCELOT. where it is mingled with the blood that has not passed through those organs, and is thus conveyed to the heart. 559. The heart of Fishes, then, belongs to the respiratory circula- tion. It propels venous blood to the capillaries of the gills, in which it is aerated; returning from these, the aerated blood is transmitted through a second set of capillaries, those of the system, in which it again becomes venous; whilst a portion of this blood is made to tra- verse a third set of capillaries, those of the liver and kidneys, before it is again subjected to the propelling power of the heart. Now as the heart, instead of being stronger than it is in animals with the com- plete double circulation presently to be described,—in which the greater part of the blood propelled by it only traverses one set of capillaries, and never more than two,—is much weaker in proportion, it is evident that here, too, a supplementary power must exist, by which the flow of blood through the capillaries is aided, and on which, indeed, the portal circulation must greatly depend. 560. It is requisite that, in the class of Fishes, the whole of the venous blood returned from the system should pass through the respi- ratory organs before being again transmitted to the body; since the aerating action of the small quantity of air diffused through the water, would otherwise be insufficient for its renovation. But in Reptiles, all of which breathe air during their adult condition, the case is very different; for if the whole current of their blood were exposed to the atmosphere, before being again sent to the body, the quantity of oxy- gen conveyed into the tissues would be too great, and would have an over-stimulating effect. The plan of the Circulation is, therefore, differently arranged in Reptiles. We find the heart to consist of three cavities ; two auricles and one ventricle. From the ventricle issues a single trunk, which speedily subdivides; some of its branches pro- ceeding to the lungs, and others to the body. The blood which is transmitted through this trunk, is of a mixed character, as we shall presently see ; being neither fully aerated, nor yet highly carbonized. It contains sufficient oxygen, to stimulate the nervous and muscular systems of these comparatively inert animals; whilst it also contains enough of carbonic acid, to require being exposed to the atmosphere through the medium of the lungs. The blood which has passed through the systemic capillaries, and which has been thereby ren- dered completely venous, is returned to one of the auricles__the systemic—by the vena cava. On the other hand, the blood which has passed through the capillaries of the lungs, and which has been thereby rendered completely arterial, is returned through the pulmonary vein to the other auricle,—the pulmonary. Thus one of the auricles exclusively receives aerated, and the other carbonated blood ; and as both pour their contents into the common ventricle, the blood which that cavity contains and propels is of a mixed cha- racter. 561. An extremely interesting aspect of the circulating apparatus is presented by the Amphioxus or Lancelot; an animal which presents CIRCULATION IN REPTILES. 325 the general form of a Fish, and which can scarcely be referred to any other group ; but in which the characters of the Vertebrated series are degraded (as it were) to the level of the lowest Molluscous and Ver- miform classes. The blood, which is white, moves through distinct vessels, but there is no proper heart; and the vascular trunks present several dilatations, in different parts, which have muscular walls, and show contractile power. Thus the circulation is carried on, not through the agency of a central impelling organ, as in Fishes ; but by power which is scattered or diffused through various parts of the system of blood-vessels, as in the lower Invertebrata.—The respira- tory apparatus, also, is formed upon a type much lower than that of Fishes; for it consists simply of a dilatation of the first part of the alimentary canal, or pharynx, upon the walls of which the blood is distributed in divided streams, its cavity being filled with water, which serves to aerate the blood. This is precisely the type, on which the respiration is effected, in those lowest Mollusks, of which mention has just been made, as exhibiting alternations-in the direction of the circulating current (§ 557). In other respects, however, the arrangement of the vascular system in this extraordinary animal, cor- responds with that which obtains in Fishes. 562. Various modifications of this form of Circulating apparatus exist in the different groups of Reptiles. In the lowest among them, which breathe permanently by gills like Fishes, besides possessing imperfectly developed lungs, the apparatus exhibits a blending of both plans; for a small portion of the blood, which is propelled by each contraction of the ventricle, passes directly to the lungs; the principal part of it being at once distributed to the gills as in Fishes. After passing through these, it is transmitted to the general system; and on returning thence, in a completely venous state, it is mingled with the blood which has been arterialized in the lungs. This latter, however, bears so small a proportion to the rest, that, if the aeration were not partly effected by the gills, it would be insufficient for the wants of the animal.—The tadpoles of the common Frog and Water Newt, as well as of other species which, like them, begin life in the general condition of Fish, present a similar condition at one period of their change. At first, the whole aeration is effected by means of gills, the lungs being in a rudimentary or undeveloped state; and the entire circulation is carried on as in Fishes, the pulmonary vessels being scarcely traceable. As the lungs begin to be developed, how- ever, a portion of the blood is sent to them ; and at the same time, communicating passages which previously existed, between the ves- sels that convey blood to the gills, and those that return it from them, are increased in size; so that a certain proportion of the blood is trans- mitted to the system, without having passed through the gills at all. By a further increase in the diameter of these, the whole current of blood takes this direction, the gills being no longer serviceable; and as, at the same time, the lungs are attaining their full development, the aeration which they effect in the blood transmitted to them be- 326 CIRCULATION IN MAMMALS. Fig. 85. comes sufficient, and the whole circulation is thus permanently estab- lished on the Reptile type. 563. On the other hand, among the higher Reptiles, we find the circulating ap- paratus presenting approaches to the form it possesses in Birds and Mammals. For the ventricle is divided, more or less com- pletely, into two cavities, one of which propels aerated blood to the system, whilst the other transmits venous blood to the lungs. A certain amount of mixture of arterial and venous blood always takes place, however, either in the heart itself, or in the vessels ; so that the blood which the body receives is never purely arterial. But this mixture is sometimes effected in such a manner, that pure arterial blood is sent to the head and anterior extremities; though the remainder of the body receives a half-aerated fluid. This is accomplished in the Crocodile, by a provision very simi- lar to that which exists in the foetus of warm-blooded animals (Chap. XI). The portal circulation in Reptiles is carried on nearly upon the same plan as in Fishes. It receives the blood from the posterior ex- tremities and from the tail, as well as from the abdominal viscera; and this blood is distributed by the portal capillaries, not only through the liver, but also through the kidneys, although the latter also receive arterial branches from the aorta. The fact that the kidneys are sup- plied from the general portal circulation, in Fishes and Reptiles, has an important bearing on the difference in the arrangement of their own vessels, which will be hereafter shown to exist, between the kid- neys of these animals, and those of Birds and Mammals (§ 727). 564. In the warm-blooded division of the Vertebrated series, which includes the classes of Birds and Mammals, we find the whole circu- lation possessed of a greatly increased energy ; but the distinguishing peculiarity of the apparatus in these animals, is that conformation of the heart and vessels, which secures a complete double circulation of the blood;—that is, which provides for the aeration of every particle of the venous blood which has returned from the system, before it is again sent into the tissues. The heart may be regarded as consisting of two distinct parts,—a systemic heart, like that of the Mollusks, forming its left side,—and a respiratory heart, like that of Fishes, constituting its right. Each of these parts has a receiving cavity or auricle, and an impelling cavity or ventricle. The cavities of the two sides are completely separated from one another, in the adult state at least; though their walls are united, for economy of material. Diagram of the Circulation in Reptiles :—a, single ventricle, re- ceiving the aerated blood from b, the pulmonary auricle, and venous blood from c, the systemic auricle; and propelling part of this mixed fluid to the pulmonary capillaries d, and part to the systemic capil- laries, e. CIRCULATION IN MAMMALS. 327 It is obvious that much is saved in this manner ; since, as the con^ tractions of the auricles and of the ven- tricles on the two sides occur simultane- ously, the pressure of blood in the one is partly antagonized by that on the other, wherever it acts on the wall that is com- mon to both. This antagonism is not complete, however ; since the systemic ventricle contracts with far greater force than the pulmonary ; and the wall be- tween them must be capable of resisting the difference of pressure on its two sides thus occasioned.—The blood which is returned from the system, in a venous state, through the vena cava to the right auricle, and which is poured by it into the right ventricle, is impelled by the latter through the capillaries of the lungs, where it undergoes aeration. Returning thence, in an arterialized state, it is con- veyed into the left auricle, and thence flows into the left ventricle ; by which it is propelled through the great systemic artery or aorta, and through its ramifica- tions to the general system. 565. The greater part of the blood, which has been rendered venous by pas- sing through the systemic capillaries,is col- lected bythe systemic veins, and is returned directly to the heart through the vena cava. But a portion is still employed for the distinct circula- tion, which is destined to supply the materials for the secreting action of the liver. The blood that has traversed the capillaries of the walls of the alimentary canal, and of the other viscera concerned in diges- tion, is collected again by the converging veins into a large venous trunk, the vena porta, by which it is distributed through the liver. This vessel, although formed by the convergence of veins, and con- veying venous blood, has really the character of an artery in an equal degree; for it subdivides and ramifies after its entrance into the liver, so as to form a network of capillaries, from which the blood is again collected, and thence transmitted by the hepatic vein to the vena cava. —Thus that portion of blood, which supplies the liver with the mate- rials of its secreting action, passes through two sets of capillaries, between the time of its leaving the heart and its return to it. The portal circulation in Birds, as in Reptiles and Fishes, receives the blood from the posterior part of the body, and from the extremities; but the portal blood is only conveyed to the liver; the kidneys being supplied by the renal artery. Fig. 86. Diagram of the Circulating Ap- paratus in Mammals and Birds :—a, the heart containing four cavities; 6, vena cava, delivering venous blood into c, the right auricle; d, the right ventricle, propelling venous blood through e, the pulmonary artery, to/, the capillaries of the lungs; g, the left auricle, receiving the ae'rated blood from the pulmonary vein, and delivering it to the left ventricle, h, which propels it through the aorta t, to the systemic capillaries, _;, whence it is collected by the veins, and car- ried back to the heart through the vena cava, b. 328 CIRCULATION IN* MAMMALS. Fig. 87. Anatomy of the human heart and lungs. 1. The right ventricle; the vessels to the right of the figure are the middle coronary artery and veins; and those to its left, the anterior coronary artery and veins. 2. The left ventricle. 3. The right auricle. 4. The left auricle. 5 The pulmonary artery. 6. The right pulmonary artery. 7. The left pulmonary artery. 8. The remains of the ductus arte- riosus 9. The arch of the aorta. 10. The superior vena cava. 11. The right arteria innominata, and in front of it the vena innominata. 12. The right subclavian vein, and behind it its corresponding artery. 13. The right common carotid artery and vein. 14. The left vena innominata. 15. The left carotid artery and vein. 16. The left subclavian vein and artery. 17. The trachea. 16. The right bronchus. 19. The left bronchus. 20,20. The pulmonary veins; IS, 20, form the root of the right lung; and, 7,19, 20, the root of the left. 21. The superior lobe of the right lung. 22. Its middle lobe. 23. Its inferior lobe. 24. The superior lobe of the left lung. 25. Its inferior lobe. 566. This perfect form of the Circulating apparatus is only attained, in the warm-blood animal, after a series of transformations, which strongly remind us of the permanent forms presented by the vascular system in Fishes and Reptiles. Thus in the embryo of the Chick at about the 60th hour, and in that of the Dog at about the 21st day, the curved and dilated tube, of which the heart previously consisted, (§ 554,) is found to be distinctly divided into an auricle and a ven- tricle. From the latter originates the main arterial trunk, which divides into four pairs of lateral branches; and these pass round the pharynx precisely in the position and direction of the arteries of the gills of Fishes. They do not, however, distribute the blood to gill- tufts ; for none such are developed in the embryo of the warm-blooded animal: but they meet again below the pharynx, to form a trunk, which supplies the general circulation.—Within a short period, how- ever, the whole plan of the circulation undergoes a change. The auricle and the ventricle are each divided by a partition, that is de- veloped in the middle of the heart; and thus the two auricles and the two ventricles are formed. Whilst this is going on, a change takes place also in the vessels that arise from the heart; for the arterial trunk, that was previously single, undergoes a division into two dis- tinct tubes; one of which is connected with the left ventricle, and MOVING POWERS OF THE CIRCULATION. 329 becomes the aorta, whilst the other originates in the right ventricle, and becomes the pulmonary artery. Of the four pairs of branchial arches, some are subsequently obliterated ; whilst others undergo changes that end in their becoming the arch of the aorta, the right and left pulmonary arteries, and the right and left subclavians. 567. The muscular power of the heart is much greater in the warm- blooded than in the cold-blooded Vertebrata, in proportion to the extent of the circulation which it is concerned in maintaining; and it is evidently destined to take a much larger share in the propulsion of the fluid, than it is in the lower tribes. Many Physiologists, indeed, are of opinion that the movement of the blood is entirely due to the action of the heart; and this view appears to be supported by the results of numerous experiments upon the circulation. But it is very difficult, if not impossible, to make experiments that shall be really satisfactory upon this point; and it appears safer to trust to the " expe- riments ready prepared for us by Nature," as Cuvier termed them,— namely, those lower forms of animated being, in which various diver- sities of structure present themselves, and in w7hich we can study the regular and undisturbed effects of these.—Thus we have seen that, in Plants and the lowest Animals, which have no central impelling cavity, the movement of the nutritive fluid is entirely dependent upon the power that is diffused through the network of vessels in which it circulates. As we ascend the series, we find an organ of impulsion developed upon a certain part of the vascular system, whose object it is to give increased energy and regularity to the movement And ascending still higher, we find the moving power gradually con- centrated, as it were, in this organ; yet it is not altogether with- drawn from the capillary network, as we shall see from several facts to be presently adduced. The particular actions of the Heart, the Arteries, the Capillaries, and the Veins, will now be considered in more detail. 3. Action of the Heart. 568. The Heart is a hollow muscle, endowed in an eminent degree with the property of irritability ; by which is meant, the capability of being easily excited to movements of contraction alternating with re- laxation (§ 347). At first sight, its actions seem different from that of the muscles, which are called into action by the impulse of the will; for in these there is apparently no such alternation, the state of con- traction being kept up as long as the will operates. But it has been already explained that, even in these, the individual fibres are proba- bly in a state of continual alternation of contraction and relaxation, during their active condition,—one set taking up the action, whilst another is returning to the state of relaxation. Hence the chief pecu- liarity in the Heart*s action consists in this,—that the whole mass of fibres of each division of the organ contract and relax together. The contraction of the two ventricles is perfectly synchronous, as is that of 330 MOVEMENTS OF THE HEART. the two auricles; but the contraction of the auricles is synchronous with the dilatation of the ventricles, and vice versa. The regularity of this alternation, however, is somewhat disturbed, when the irrita- bility of the heart is becoming exhausted ; and both sets of movements will continue, when the auricle and ventricle have been separated from one another. Their regular succession, in the natural state, is doubtless in part due to the fact, that the transmission of blood from the auricle into the ventricle, by the contraction of the former, is the stimulus which most effectually excites the latter to contraction ; whilst the ventricle is contracting, the auricle, now free to dilate, is distended by the flow of blood from the veins that open into it; and this flow stimulates it to renewed contraction, just at the time when the con- traction of the ventricle has been completed, and its state of relaxa- tion enables it to receive the blood poured in through the orifice lead- ing from the auricles. 569. In the living animal, the auricular and ventricular movements succeed one another with great regularity; and when the circulation is proceeding with vigour, scarcely any appreciable pause can be dis- covered between the different acts. The contraction or systole of the Auricle takes place precisely at the same moment w*ith the dilatation or diastole of the Ventricles; and, as soon as the latter are full, and the former are empty, the diastole of the Auricles, and the systole of the Ventricles, immediately succeed. The systole of the Ventricles occasions the propulsion of blood into the arterial system; and this action produces the pulse, as will be explained hereafter. And it also corresponds with the impulse or stroke of the heart against the parietes of the chest. This impulse is not produced, as some have supposed, by the swinging of the entire heart forwards; but by the peculiar mode in which the Ventricular systole takes place. In the contraction of its walls, every dimension is lessened ; but shortening is the most perceptible change, the vertical diameter of the Ventricle being the greatest. Owing to the peculiar spiral disposition of the fibres of the heart, its apex is not simply drawn upwards by their contraction, but it is made to describe a spiral movement, from right to left, and from behind forwards; and it is in this manner, that it is caused to strike against the side of the chest. 570. The systole of the Ventricles is immediately followed by their diastole ; but the commencement of this has been observed to occur at a small interval previous to the contraction of the Auricles; and sometimes a brief interval of repose may be noticed, separating the first stage of the Ventricular diastole, which may be partily due to the simple elasticity of the walls of the Ventricles, from the second, which is accompanied by the systole of the Auricles, and in which the blood of the latter is forcibly propelled into them. When the circulation is being carried on regularly, the blood is propelled into the Ventricles with sufficient force to dilate them strongly ; so that the hand closed upon the heart is opened with violence. Even the auricles dilate with more force than it seems easy to account for by the vis a tergo MOVEMENTS AND SOUNDS OF THE HEART. 331 of the blood in the venous system; which is small compared with that which the fluid possesses in the arteries. 57]. The natural movements of the Heart are accompanied by cer- tain sounds, which are heard when the ear is applied over the cardiac region; and an acquaintance with these sounds and with their causes is of much importance, since the alterations which they undergo in disease, afford us some of our most accurate information in regard to the nature of the morbid affection. Concurrently with the impulse of the heart against the chest, a dull and prolonged sound is heard ; this, which is termed the first sound, marks the ventricular systole, and is synchronous with the pulsation in the arteries. The second sound, which is short and sharp, follows immediately upon the conclusion of the first; and it must therefore be produced during the first stage of the Ventricular diastole, before the systole of the Auricles has com- menced. It is followed by a brief interval of repose, which occurs during the remainder of the Ventricular diastole and the Auricular systole; and this is succeeded by a recurrence of the first sound. If the whole period between two successive pulsations be divided into four parts, it is estimated that the first sound usually occupies two of these ; and the second sound, and the interval, one part each. 572. Now in order to understand the causes of these sounds, it is necessary to study the course of the blood through the heart a little more in detail. When the Ventricles, distended with blood, are con- tracting upon their contents, they eject them forcibly through the nar- row orifices of the aorta and pulmonary artery; and the semilunar valves, which guard these orifices, are thrown back against the walls of the arteries. The regurgitation of the blood into the auricles is prevented by the action of the mitral and tricuspid valves; but the flaps of these do not suddenly fall against each other, when the blood first begins to press them together; being restrained by the chorda tendinece. The connection of these with the cameo: columnce, which form part of the ventricular walls, and contract simultaneously with them, appears to have this use :—that the flaps of the valves, which are completely thrown back during the preceding rush of blood from the auricles to the ventricles, may be drawn into a favourable posi- tion, for the blood to get behind them and bring them together, so as completely to close the orifice. As soon as the Ventricular diastole begins to take place (even before the contraction of the auricles has commenced) there will be a tendency of the blood, that has just been propelled into the aorta and pulmonary artery, to flow back to the heart; but this regurgitation is completely prevented by the semilu- nar valves of these orifices, which are immediately filled out by this backward tendency of the blood, and which meet in such a manner as completely to close the orifices. This closure is much more sudden than that of the mitral and tricuspid valves, being altogether unre- strained. 573. The^rs^ sound is certainly in part due to the impulse of the heart against the thoracic parietes; as is proved by the fact, that when 332 SOUNDS OF THE HEART. the impulse is prevented, the sound is much diminished in intensity; and also by the circumstance, that, when the ventricles contract with vigour, the greatest intensity of the sound is over the point of percus- sion. But that it is not entirely due to this cause, is also sufficiently evident from two circumstances;—its prolonged character, which could scarcely be given by a momentary impulse ;—and its continu- ance, though with diminished intensity, when the parietes of the chest are wanting, and even after the complete removal of the heart from the body. Moreover, the duration of the first sound is much increased by any morbid state of the orifices of the ventricles, which obstructs the exit of the blood. Much discussion has taken place as to the cause of that part of it, which is not due to the impulse ; some having attributed it to the muscular contraction of the walls of the ventricles, others to the flow of blood over the irregular surfaces of their interior, and others to the rush of the fluid through the narrow orifices leading to the aorta and pulmonary artery. There can be little doubt, that the first and last of these causes are both concerned in producing the sound. For as a sound may be distinctly heard by means of the stethoscope, when the heart is contracting vigorously out of the body, and when no blood is propelled by it, nothing else than muscular contraction can be then regarded as its source; and there is other evidence that sound may be produced by this cause, since the vigorous contraction of any other large muscle gives rise to a continued tingling, which may be heard through the stethoscope. But when the heart is contracting in its natural position, and is pro- pelling the blood with its ordinary vigour, the sound is heard in its greatest intensity at the base of the heart, i. e., at the origin of the great arteries; and since any obstruction to the exit of the blood through them increases the intensity, as well as the length of the sound, it can scarcely be doubted that it is partly due to the rush of the blood through the contracted entrances of these vessels. A very similar sound, known as the " bruit de soufflet," or bellows-sound, may be heard through the stethoscope, over any large artery, when it is compressed, so as to permit the passage of blood less readily than usual. Thus the ordinary first sound may be regarded as composite in its nature ; being made up of the sound produced by the impulse of the heart against the parietes of the chest, of the muscular sound occasioned by the forcible contraction of the thick walls of the ven- tricles, and of the sound generated by the friction of the particles of blood against each other, and against the boundaries of the narrowing orifices which lead into the vessels. 574. The cause of the second sound is simpler, and more easily understood. It is due to the sudden filling-out of the semilunar valves with blood, at the moment when the ventricular systole has ceased, and when the commencing diastole produces a tendency to the regur- gitation of blood from the aorta and pulmonary artery. The sudden passage of the valves, from a state of complete relaxation to one of complete tension, occasions a sort of click; which is the second sound SOUNDS OF THE HEART—THICKNESS OF ITS WALLS. 333 of the heart. That this is the real cause, has now been fully demon- strated. If one of the valves be hooked back against the side of the artery, by the introduction of a curved needle, so that a reflux of blood is permitted, the sound is entirely suppressed. And if the complete closure of the valves be prevented by disease, so that their tension is diminished, and a certain amount of regurgitation takes place, the second sound is no longer heard in its proper intensity; whilst, on the other hand, a sound analogous to the first, and some- times prolonged over the whole interval of repose, indicates the reflux of the blood into the ventricles. When the semilunar valves are thickened by morbid deposit, their surface roughened, and their open- ing narrowed, the first sound becomes harsher and sharper ; and the second sound acquires the same character,—the backward as well as the forward flow of the blood being affected by this cause. 575. The natural movements of the mitral and tricuspid valves ap- pear to be accomplished with perfect freedom from sound; for the size of the orifices which they guard prevents any considerable friction of the blood, in its flow from one cavity to the other; and their closure, when the ventricular systole begins, does not take place with the rapidity and suddenness of that of the semilunar valves. But when their structure is changed by disease, their action is not so noiseless; and they give rise to various morbid sounds, which are heard in addi- tion to the ordinary sounds, and which may even obscure them alto- gether—In the same manner the ordinary movements of the heart do not produce any audible friction sound, between the two surfaces of the pericardium, that which covers the heart and that which lines the pericardial sac. These surfaces are kept moist, in health, by the serous fluid constantly exhaling from them; and they are extremely smooth; so that they glide over one another noiselessly. But if they become dry, as in the first stage of inflammation, a slight creaking is heard, accompanying both the ordinary sounds of the heart, and somewhat resembling the rustling of paper. And if they are roughened by the deposit of inflammatory exudations, this " to and fro" sound becomes of a harsher character. 576. The walls of the left Ventricle are considerably thicker than those of the right; and the contractile power is greater. This differ- ence is obviously required, by the difference in length between the systemic and the pulmonary vessels; the amount of force necessary to drive the blood through the latter being far inferior to that which is requisite to propel it through the former. The average thickness of the walls of the left Ventricle is about 4^ lines; being somewhat greater than this at the middle of the heart, and less at its apex. The average thickness of the walls of the right ventricle is not more than 1 \ line; being a little greater than this at the base, and less at the apex of the heart. The left auricle is somewhat thicker than the right.—The ca- pacities of all the four cavities are nearly equal; each of them, in the full-sized heart, holding about two ounces of fluid. The Ventricles are, perhaps, a little larger than their respective Auricles; but there 334 RATE OF CIRCULATION. is no very positive difference in capacity, between the Ventricles and Auricles of the two sides. 577. The quantity of blood which is propelled at each Ventricular systole, cannot, therefore, exceed two ounces; and it is probably somewhat less, as the ventricles do not seem to empty themselves completely at each contraction. Now the whole quantity of the blood seems to be about one-fifth of the entire weight of the body; so that it will amount to about 28 lbs. in an individual of 140 lbs. weight. Allowing'75 pulsations to a rainute, 150 ozs. (or 91bs. 6 ozs.) of blood would pass through each ventricle of the heart in that time; consequently nearly three minutes would be required for the passage of the entire mass of the blood through the whole circle of its move- ment, if its rate be entirely determined by the impulses it receives from this central organ.—But it appears, from various experiments, that the rate of circulation is much more rapid than this. For if a solution of any salt easily detectible in the blood, be injected into one of the large veins near the heart, it may be traced in the arterial circulation in from 15 to 20 seconds afterwards; during which inter- val it must have traversed the whole pulmonary system of vessels and passed through both sides of the heart. And if the salt be one which acts powerfully on the heart itself,—as is the case with Nitrate of Baryta or Nitrate of Potass,—this action is manifested almost at the same moment with the appearance of the salt in the arteries of other parts; thus showing that it has been conveyed by the coronary arteries into the capillaries of the heart itself. The period required for the transmission of a saline substance from the veins of the upper part of the body to those of the lower,—which can scarcely be accom- plished through any more direct channel than the current that returns to the heart, then passes through the lungs back to the heart again, and then flows through the systemic arteries and capillaries to the veins,—is accomplished in little more than 20 seconds, even in an animal so large as a Horse. It appears, then, that even the vigorous and constant action of the Heart is not alone sufficient to maintain the circulation at its ordinary rate ; and we are not justified, therefore, in excluding those sources of movement in the higher animals, which evidently exert so important an influence in the lower. 578. The force with which the heart propels the blood is such, that if a vertical pipe be inserted into the Carotid artery of a horse, the blood will sometimes rise in it to a height of 10 feet. From comparative experiments upon other animals, it has been estimated that the vigor- ous action of the heart in Man would sustain a column of blood in his aorta about 1\ feet high; or, in other words, that the force with which the heart ordinarily propels the blood through the aorta, is equal to that which would be generated by the weight of a column of blood of the same size, and 1\ feet high; which weight would be about \\ lbs. But the force which must be exerted by the heart to sustain such a column, may be shown, upon physical principles, to be as much greater than this as the area of a plane passing through CIRCUMSTANCES AFFECTING RATE OF PULSE. 335 the base and apex of the left ventricle is greater than that of the trans- verse section of the aorta; and as the proportion of these area? is about 3-1, the real force of the heart may be stated at about 13 lbs. 579. The number of contractions of the heart, in a given time, is liable to great variations within the limits of health, from several causes ; the chief of which are diversities of Age and Sex, amount of Muscular exertion, the condition of the Mind, the state of the Diges- tive system, and the period of the Day.—The following are the points of greatest importance, in regard to the action of these several influ- ences : Age.—The pulse of the newly-born infant averages from 130 to 140 per minute; and this rate gradually diminishes, until, in adult age, the pulse averages from 70 to 80 ; and in the decline of life from 50 to 65. Sex.—The pulse of the adult female exceeds that of the adult male in frequency, by about 10 or 12 beats in a minute; and it is also more liable to disturbance from other causes. Muscular exertion.—The effect of this in accelerating the pulse is well known; but as the amount of change depends upon the degree of exertion, no general statement can be made on the subject. The continued influence of a moderate degree of muscular exertion, is shown by the effect of posture upon the pulse. Thus the pulse is on the average from 7 to 10 beats faster (per minute) in the standing than in the sitting posture ; and 4 or 5 beats faster in the sitting than in the recumbent posture. This amount of variation is temporarily increased by the muscular effort required for the change of posture; but this soon subsides into the continued rate, which the permanent maintenance of the new posture involves. There are certain states of the system, in which the heart's action is increased to a most vio- lent degree, by a simple change of posture; and in which, therefore, it is necessary that even this slight movement should be made with gentleness and caution. Mental Condition. The action of the heart is peculiarly influenced, as every one is aware, by the excitement of the emotions. This is a fact to which, howrever familiar, the medical practitioner should con- stantly direct his attention. The trifling agitation occasioned by the entrance of the medical man will produce, in many patients, such an acceleration of the pulse, as would be very alarming, if its true cause were not known. And the real rate of the pulse cannot be ascertained, until time has been permitted for the agitation to subside; which is favoured, also, by the influence of a gentle manner and tranquilizing conversation. The operation of the intellectual powers does not seem to affect the rate of the heart's movement in any other way, than by inducing a general state of feverishness, if it be too long or too ener- getically kept up. State of the Digestive System.—The pulse is quickened during the digestion of a meal; but no exact numerical statement can be made on this subject. Period of the Day.—The frequency of the pulse appears to be some- 336 INFLUENCE OF NERVES UPON HEART'S ACTION. what greater in the morning than it is in the evening ; and the tem- porary action of any of the preceding causes more quickly subsides in the evening than in the morning. 580. The movements of the heart have been supposed to depend upon a constant supply of nervous influence, generated by the cerebro- spinal system, and transmitted through the sympathetic nerve, the branches of which are copiously distributed to it. And this idea seemed to derive support from the fact, that, when the brain and spinal cord are removed, or when large portions of them are suddenly destroyed, by crushing or by the breaking up of their substance in any other mode, the movements of the heart are arrested. But it has been shown that the brain and spinal cord may be gradually removed, without any such consequence ; and the occasional production of foe- tuses destitute of those centres, but possessing a regularly-pulsating heart, is another proof that the movements of this organ do not depend upon a supply of nervous influence derived from them. Still they are capable of being influenced by impressions transmitted through the nerves. It has been ascertained by Valentin, that, after the heart has ceased to beat, its contractions may be re-excited by stimulating the roots of the Spinal Accessory nerve, or of the first four Cervical nerves; the influence of that stimulation being conveyed to the heart by the Sympathetic system, the cardiac portion of which communicates with these nerves. Irritation of the Par Vagum, also, has a tendency to accelerate the heart's action, or to re-excite it when it has ceased ; but the complete severance of both its trunks produces little disturb- ance in the regularity of the movement. The action of the heart may be also affected more directly through the sympathetic system ; thus it is excited by irritation of the cervical ganglia, especially the first; whilst continued pressure upon the cardiac nerve, by an enlarged bronchial gland, has appeared to be the cause of its occasional sus- pension. It is without doubt through its nervous connections, and probably though the sympathetic system, that the heart receives the influence of mental emotions. 581. The movements of the heart maybe suspended, or altogether checked, by sudden and violent impressions on the nervous centres, even though these do not occasion any perceptible breach of substance. Thus in concussion of the brain, there is not merely insensibility, but also a complete suspension of the circulation, occasioned by a failure of the heart's power. This suspension may be permanent, so that animation cannot be restored ; or it may be temporary, as in ordinary fainting. The well-known influence of blows upon the epigastrium, in producing sudden death, is probably to be attributed to a similar cause,—namely, the shock thus communicated to the extensive plexus of ganglionic nerves, radiating from the semilunar ganglia, and pro- ceeding to the abdominal viscera. Violent impressions upon other nervous expansions may produce a dangerous weakening of the heart's contractile power; this is the case, for example, with exten- sive burns, which may produce faintness, and even death, especially EQUALIZATION OF FLOW IN THE ARTERIES. 337 in children, by the depression which they induce. Many other causes of sudden suspension of the heart's action might be enumerated ; but they may be generally traced to a strong impression upon the nervous system ; though of the mode in which this operates we know nothing. 4. Movement of the Blood in the Arteries. 582. The Blood, thus propelled from the Heart into the Arteries by a series of interrupted jets, would continue to flow in the same manner, if it were not for the equalization of its movement, effected by the properties of the arterial walls. This influence is exerted by the middle or fibrous coat, which consists in part of yellow elastic tissue (§ 189), and in part of non-striated muscular fibre (§ 337). The proportion of these two components varies in arteries of different calibre; the muscular tissue being thicker in the smaller branches, and the elastic tissue being found in larger amount in the main trunks. 583. It is chiefly to the simple physical property of Elasticity, thus possessed by the Arterial tubes, that we owe the equalization of the flow of blood; and we may hence understand the reason, why the trunks that are in nearest connection with the heart, should be those most endowed with it. If a forcing-pump were to inject water, by successive strokes, into a system of tubes with perfectly unyielding walls, the flow of fluid at the farther extremities of these tubes would be as much interrupted, as its entrance into them. But if the pump be connected with an air-vessel (as in the common fire-engine), so that a part of the force of each stroke is expended in compressing the air, the expansion of this, during the interval between the successive strokes, produces a continuous flow of water along the tubes. Or if the tubes themselves were endowed with a certain degree of elasticity, which should allow them to dilate near their commencement, so as to receive the new charge of fluid, and which should occasion a con- tinued pressure upon the fluid during the intervals of the stroke, the same equalizing effect would be produced. This is precisely the case with the Arterial system; the intermittent jets, by which the blood is propelled from the heart, are speedily converted into a con- tinued stream ; so that, at even a moderate distance from the heart, the only indication of its interrupted action is presented by the greater or less rapidity of the flow; and this gives rise, when an artery is divided, to an alternate rise and fall of the jet of blood, and, in the ordinary circulation, to the phenomenon called the pulse. This is due to an increase in the dimensions of the arterial tube, both in length and breadth, with each additional ingress of blood ; the increase in length is the more considerable of the two effects, and causes the artery to be somewhat lifted from its seat. During the intervals, a quantity of blood corresponding to that which had entered, escapes by the further extremity of the tube; and thus the artery is enabled to contract to its previous dimensions, and to return to its bed. We 22 338 EFFECTS OF MUSCULARITY OF ARTERIES. may compare the pulse, therefore, to a wave, which commences in the heart, and travels onwards through the arterial system. 584. In the large arteries near the heart, the pulsation is always precisely synchronous with the ventricular systole; but it takes place somewhat later in the arteries at a distance from the heart; the time required for the transmission of the wave being proportioned to the degree in which the walls of the arteries yield to it. If they were quite rigid, the egress at one extremity must take place at the precise moment that the fluid is forced into the other. On the other hand, if the walls be too easily distensible, they yield to the propelling force in such a degree that it is entirely expended upon them; and the fluid is not moved onwards at all, or but very slowly. In the healthy state of the arterial walls, they should contract upon their contents, with sufficient force to equalize the flow of blood, and to prevent the pulse- wave from occupying more than one-sixth or one-seventh of a second, in its propagation to the remotest arteries of the system; and the pulse should be full, producing a prolonged but gentle elevation beneath the finger, and capable of resisting moderate pressure. This condition is dependent in great part upon the due tonicity of the muscular coat of the arteries (§ 365). When this tonicity is in excess, the walls of the arteries are too rigid; the pulse at the wrist is felt to occur exactly at the same time with the ventricular systole; and its character is that of strength, incompressibility, and sustained power, though it may be even slower than usual. This is the case in what is commonly termed "high condition" of the system; which predisposes to inflammatory disorders, but which renders it less susceptible than usual to the influ- ence of malaria, contagious miasmata, or other causes of a depressing character. On the other hand, when the tonicity of the arteries is less than it should be, their walls yield too much to the pulse-wave; so that the pulse at the wrist is often felt even after the second sound is heard; and the pulse itself is jerking, unsteady, and too easily compressible. This loose relaxed state of the vessels is the most unfavourable that can be to regularity and vigour of the circulation ; and it manifests its ill effects in the general condition of the system, which is then peculiarly prone to suffer from the agency of malaria, infectious miasmata, or any other depressing causes. 585. Although many Physiologists have denied that the Arteries possess real Muscular Contractility in any degree, yet there can be no longer any doubt on the subject; since numerous experimenters have succeeded in producing distinct contraction in their walls, by the application of those stimuli which act upon muscular fibre in general. Moreover it has been ascertained, that when an artery is dilated by the blood injected into it from the heart, it reacts with a force superior to the impulse to which it yielded; and that, if a portion of an artery from an animal recently dead, in which the vital properties are still preserved, and a similar portion from an animal that has been dead some days, in which nothing but the elasticity remains, be distended with equal force, the former contracts to a much greater degree than EFFECTS OF MUSCULARITY OF ARTERIES. 339 the latter, after the distending force is withdrawn.—One use of "this contractile power may very probably be, to assist the Heart in main- taining the flow of blood ; for if the Arterial walls yield readily to the ingress of blood, and then contract upon their contents with a force greater than that which distended them, the current must necessarily be propelled onwards with greater force. This supplementary pro- pelling force, on the part of the arteries, may serve as a compensation to that diminution of the heart's power, which must result from the increased friction of the blood against the walls of the vessels occa- sioned by their subdivision ; and we thus observe, even in the highest animals, some traces of that diffused agency, on which the Circulation is so much more dependent in the lower tribes. 586. It seems probable, however, that one chief use of the Mus- cularity of the Arterial walls consists in its regulation of the diameter of the tubes, in accordance with the quantity of blood to be con- ducted through them to any part; the proper amount being deter- mined by circumstances at the time. Such local changes may form a part of the regular series of actions of the human body, as when the Uterine and Mammary arteries undergo enlargement, at the periods of pregnancy and parturition; and they occur still more fre- quently in diseases, which are attended by increased action of par- ticular organs. In such cases, it cannot be vis a tergo of the Heart, that occasions the enlargement of certain arterial trunks, and of no others; since any increase in its propulsive power would affect all alike. It must be, therefore, through a power inherent in themselves, that the dilatation takes place; and there seems much reason for attributing to the Sympathetic system of nerves a control over this power, and consequently the office of regulating the local distribution of blood, in accordance with the wants of the different parts. It is well known that the nerves of this system are copiously distributed upon the arterial walls ; and it has been experimentally shown, that they have the power of producing contractions in the larger arteries. Moreover, there is every reason to believe, that the diameter of the Capillary blood-vessels, and the rate of the movement of blood through them, are much influenced by these nerves (§ 603); and it seems highly probable, therefore, that they should have a correspond- ing influence upon the size of the trunks, from which these capilla- ries are derived. 587. The Arterial system possesses nearly the same relative capa- city in every part: that is, if a section could be made through all the systemic arteries at a certain distance from the heart, the united areas would be found equal to that of the aorta; and those of the branches of the pulmonary arteries would equal those of their trunk. This results from the fact, that, at every subdivision, the united areas of the branches are almost precisely equal to that of the trunk from which they proceeded; although the united diameters of the former far exceed that of the latter. According to a well-known mathemati- cal law, the areas of circles are as the squares of the diameters; con- 340 RAMIFICATION AND ANASTOMOSIS OF ARTERIES. seqnently, in making such comparisons, it is necessary to square the diameters of the trunk and those of the branches, and to contrast the former with the sum of the latter. Thus a trunk whose diameter is 7, may subdivide into two branches, each having a diameter of nearly 5; for the square of 7 is 49, and twice the square of 5 is 50. Or a trunk whose diameter is 17 may subdivide into three branches, whose diameters are 10, 10, and 9^ (making 29^ as the sum of the diame- ters) ; for the square of the diameter of the trunk is 289, whilst the sum of the squares of those of the branches is 290^. It appears from Mr. Paget's recent admeasurements, that there is seldom an exact equality betwreen the area of the trunk and that of its branches; the area sometimes increasing, and sometimes diminishing. The former seems the general rule in the upper extremities; the latter in the lower. Thus the area of the trunk of the external carotid is to that of its branches, as 100 to 119; whilst the area of the abdominal aorta, just before its final division, is to that of its branches as 100 to 89. 588. In almost every part of their course, the ramifications of the arteries communicate freely with each other, by anastomosis; and this communication is most important, as affording the means by which the circulation is sustained, when the current through the main trunk is obstructed. There is scarcely an artery in the body, except the aorta, which may not be tied, with the certainty that the blood will still be conducted to its destination, by the collateral circulation. At first, the quantity which thus passes is very insignificant, and is by no means sufficient to supply what is needed ; thus, when the femoral artery has been tied for popliteal aneurism, the limb becomes cold, and the sensibility of its surface and its muscular power are alike diminished. In a few hours, however, its warmth returns, and its sensibility and muscular power are restored; indicating that its circulation has been already re-established through the collateral branches. And where an opportunity presents itself at a subsequent period for examining the state of the vessels in such a limb, it is found that an extraordinary enlargement has taken place in arteries that were previously of insignificant size, which form a communica- tion between the branches that issued above and below the inter- ruption. Moreover, it is commonly found, that the main trunk has become completely impervious above the part where it was oblite- rated by the ligature, up to the point at which the nearest lateral branch is given off.—Even the abdominal aorta has been tied in dogs, without fatal results; the circulation in the posterior part of the body, and in the hinder extremities, being then maintained ■chiefly by the inosculation of the external mammary artery with the epigastric, upon the parietes of the abdomen. 5. Movement of Blood in the Capillaries. 589. The ultimate ramifications of the Arteries pass so insensibly into those of the Veins, that no definite line of demarkation between ARRANGEMENT OF CAPILLARY VESSELS. 341 them can be drawn; and although we are in the habit of speaking of the "Capillaries" as a distinct system of vessels, yet it ought to be strictly borne in mind, that they differ only in size from the vessels, from which they receive their blood on the one side, and into w^hich they pour it on the other. It was at one time supposed that they are merely channels or passages, excavated in the tissues, having no definite walls of their own. This is probably true of them in the lower tribes of_Animals; and it may also be the case at an early stage of their development in the higher. But when their formation is complete, they undoubtedly possess walls of a fibrous texture, as distinct as those of the arteries and veins, though of extreme thin- ness. From the occasional appearance of bodies resembling cell- nuclei, in the substance of the walls of the capillaries, it has been thought that their tubes are formed, in the first instance, by the coalescence of cells arranged in a linear direction; and this idea receives confirmation from the fact, that the ducts of Plants are undoubtedly formed in this manner, and not by the mere retirement of the tissues on either side leaving an intervening channel. The closely-reticulated structure usually formed by the capillaries, has commonly been regarded as distinguishing them both from the arte- ries and the veins; and it is not uncommon to speak of the arteries as delivering the blood into the "capillary network," and the veins as receiving the fluid that has traversed this. Such expressions are not incorrect as implying the simple fact, that between the arteries and the veins is a network of minute vessels, through which the blood has to travel when proceeding from one to the other ; but these vessels must not be regarded as belonging to a distinct class, being nothing else than the minutest subdivisions of the veins and arteries, which commonly inosculate freely with each other. 590. The degree of this inosculation, and the consequent form of the capillary network, are subject, however, to very great variations; and these may be generally shown to have a relation to the form of the ulti- mate elements of the tissues, which are traversed by the capillaries. Thus we see in the capillaries of Muscle, that the major part run parallel to the course of the fibres, lying in the minute interspaces between them (Fig. 88); a few trans- verse branches serving to connect them with each other. A similar distribution capillary network of Muscle. prevails in the capillaries of the Nervous trunks ; but those of the Nervous centres are arranged in the form of a minute network, so as completely to traverse every part of the struc- ture (Fig. 89). Again, we observe that the capillaries of Glands form a rainute network around the secreting follicles (Fig. 90); and a simi- lar arrangement prevails in the capillaries of the air-cells of the lungs, which are set so closely together, that it would seem as if the purpose 342 DISTRIBUTION OF THE CAPILLARIES. were to cover the surface with blood as completely as possible, con- sistently with its being retained within vessels, and not spread out Capillary Network of Nervous Centres. Fig. 90. Capillary Network around the follicles of Parotid Gland. into a continuous film (Fig. 102). A network of very much the same character is found in the villi of the mucous membrane (Fig. 77), on the ordinary surface of simple mucous membrane (Fig. 91), and on Fig. 91. Fig. 92. Capillary Network in simple mu- cous membrane of palpebral con- junctiva. Capillary Network in choroid coat of the eye. that of the choroid coat of the eye (Fig. 92). Where the surface of the mucous membrane is depressed into follicles, the arrangement of the capillaries has an evident reference to these (Fig. 93); whilst on Fig. 94. Distribution of Capillaries around follicles of Mucous Membrane. Distribution of Capillaries at the sur- face of the skin of the finger. VARYING SIZE OF THE CAPILLARIES. 343 the other hand, where the surface of the skin is raised up into sensory papillae, the capillary network sends looped prolongations into these, which are found accompanying their nerves (Figs. 94 and 95.) 591. It cannot be supposed that the ar- rangement of the vessels has any further Flg'95" influence upon the function of the part they supply, than that which it derives from the regulation of the supply of blood afforded to each individual portion of the structure. The form of the capillary network is evi- dently determined by that of the elements of the tissues permeated by it; these are the real operative instruments in every y ^^ of ^^ part; and the distribution ot the blood- papiiia of the tongue. vessels is so arranged, as to afford them precisely the amount of nourishment they respectively require. Thus we have seen, that there are many living parts, possessing most im- portant functions, in the human body, which are not in any direct relation with blood-vessels, and which yet derive their whole nutri- ment, and the materials of their functional operations, from the blood. This is the case, for example, with the whole of epithelial and epi- dermic cells ; and also with the articular cartilages, and the substance of the teeth. Even in bone, the islets between the Haversian canals, which are completely unpenetrated by vessels, are of considerable size. Such islets must everywhere exist, between the meshes of the capillary network; so that the question of the vascularity or non-vas- cularity of a tissue is one of degree onty;—the ultimate fibre of muscle or nerve, and the cells and fibres of other tissues, being as completely non-vascular, as the entire substance of a tooth or of an articular cartilage; the latter being nourished, like the former, by imbibition from the surrounding vessels. 592. The term " Capillary" may be employed in an extended or a restricted sense; in the former it includes all the minute vessels, which pass between the arteries and the veins; in the latter it is ap- plied only to those, which admit no more than a single file of blood- discs at once, and excludes those, which admit two, three, or even four rows, even although they establish a direct communication from one side of the network to the other. The former application of the term is the most convenient, although perhaps not the most strictly accurate; and it will be therefore here employed in its extended sense. And this is rendered more correct by the fact, that the size ot the individual capillaries is by no means permanent; an enlargement often taking place in one, and a contraction in others, at the same time: so that vessels, which were previously true capillaries, no longer remain such; and passages, which were previously ot tar greater calibre, are reduced to the average diameter. g 593. The opinion was long entertained, that there are vessels adapted to the supply of the white or colourless tissues; carrying 344 VARYING SIZE OF THE CAPILLARIES. from the arteries only the fluid portion of the blood, or liquor sangui- nis, and leaving the rest behind. No other such vessels have been really observed, however, than capillaries in a state of unusual con- traction, as just now mentioned. And it may be safely affirmed, that the supposition of their existence is not required. For any one who observes the smallest capillary vessels under the microscope, may perceive, that the current of blood which passes through them is almost free from colour,—as the red corpuscles themselves appear to be, when spread out in a single layer. Tissues which are rather scantily permeated by such vessels, therefore, may still be white ; and it is only where the network is very close, and the quantity of blood which passes through it is consequently great, that a perceptible colour will be communicated by the red corpuscles. And we have seen, that the idea that Nutrition can only be carried on by direct communication with vessels, is entirely unfounded; the tissues into which no blood-vessels can be traced, being adapted to nourish them- selves, like cellular Plants, by the imbibition of fluid at their surfaces, on which vessels are (for the most part) copiously distributed. 594. That the blood can only minister to the operations of Nutri- tion, Secretion, &c, whilst it is circulating through the Capillaries, is evident from several considerations. The thickness of the walls of the larger vessels interposes an effectual barrier to its transudation ; and so completely is the blood cut off, even from penetrating these, that they do not derive their own nourishment from the blood which flows in their own tubes, but from a capillary network in their own substance, which is supplied by vessels from collateral branches,— these being termed the vdsa vasorum. Moreover it is by the inoscu- lation of the capillaries alone, that the minute network is formed, which serves to bring the blood into proximity with the minute parts of the tissues to be nourished ; thus let it be supposed that the minute arteries of Muscle were to terminate in veins, without undergoing further subdivision, the islets left between their anastomosing branches would be far too large, and the nutritive materials would consequently not be supplied with sufficient readiness, even supposing that it could freely permeate the walls of these vessels.—The Capillaries, then, must not be regarded as altogether distinct in their endowments, from the vessels with which they are connected on either side ; but merely as intended, by their minute subdivision and inosculation, to bring the blood into sufficiently close relation with the tissues they are to nourish, and to allow a greater degree of transudation of its elements by the comparative thinness of their walls. 595. When the flow of blood through the capillaries of a transpa- rent part, such as the web of a Frog's foot, is observed with the microscope, it appears at first to take place with great evenness and regularity. The influence of the contractions of the heart may be seen to extend itself into the smaller arteries; the blood moving on- wards in them with a somewhat jerking motion. But this influence altogether disappears in the Capillary network; the flow of blood VARIATIONS IN FLOW OF BLOOD IN CAPILLARIES. 345 through this being even and continuous, except when the action of the heart is becoming weak and irregular, or when its influence is impeded by obstruction in the vessels leading to the part,—the blood being then impelled by a succession of jerks, with intervals of com- plete repose.—But on watching the movement for some time, various changes may be observed, which cannot be attributed to the heart's influence, and which show that a certain regulating or distributive power exists in the walls of the capillaries, or in the tissues which they traverse. Not only do we occasionally perceive some of the tubes enlarging, so as to admit several files of blood-discs instead of one, whilst others that previously received several, now only admit one; ■—but we also see vessels coming into view, which were not previously noticed, whilst other vessels seem to become obliterated. This appa- rently new formation and obliteration of vessels, however, do not really take place; for a more close examination shows, that the former of these appearances is due to the entrance of red corpuscles into passages, which existed before, but which were in such a state of contraction as enabled them only to admit the fluid portion of the blood; whilst, by a converse change in certain capillaries, from the dilated to the contracted state, the appearance of obliteration is pro- duced, the red corpuscles being excluded, and the transparent fluid of the blood being alone transmitted by them. 596. But these are by no means all the irregularities, which may be detected by a close scrutiny of the Capillary circulation. The velocity of the current is liable to great and sudden variations, which cannot be accounted for by any change in the heart's action, or in the supply of blood afforded by the arteries; and this change may mani- fest itself, either in the whole capillary network of a part, or in a por- tion of it,—the circulation taking place with diminished rapidity in one part, and with increased energy in another, though both are sup- plied by the same trunk. These variations are sometimes manifested by the complete change in the direction of the movement, in certain of the transverse or communicating branches; this movement taking place, of course, from the stronger towards the weaker current. Not unfrequently an entire stagnation, of longer or shorter duration, pre- cedes the reversal of the direction. Irregularities of this kind are most frequent, when the heart's action is enfeebled or partially inter- rupted ; and it would thus appear, that the local influences by which they are produced, are overcome by the propelling power of the cen- tral organ, when this is acting with its full vigour. When the whole current has nearly stagnated, and a fresh impulse from the heart re- news it, the movement is seldom uniform through the entire plexus supplied by one trunk ; but is much greater in some of the tubes than in others,—the variation being in no degree connected with their size, and being very different in its amount at short intervals. 597. All these circumstances indicate that the movement of blood through the Capillaries is very much influenced by local forces ; although these forces are not sufficiently powerful, in the higher ani- 346 MOVEMENT OF BLOOD IN CAPILLARIES. mals, to maintain it alone. And from other facts it appears, that the conditions necessary for the energetic flow of blood through these vessels, are nothing else than the active performance of the nutritive and other operations, to which they are subservient. The examina- tion of a single one of these processes, will afford us the requisite proof. The blood when circulating through the systemic capillaries, yields a portion of its oxygen to the tissues it permeates, and receives from them carbonic acid. On the other hand, when passing through the pulmonary capillaries, it gives up its carbonic acid to the atmo- sphere, and imbibes a fresh supply of oxygen. Now if either of these changes be prevented from taking place, a retardation and even a complete stagnation, of the blood will take place,—the flow through the capillaries being now resisted, instead of accelerated, by the rela- tion which the blood bears to the tissues. Thus it has been shown, that if an animal be partially deprived of oxygen, so that the arterial blood is not duly aerated (rather resembling the ordinary venous blood), and cannot exert its proper action on the tissues, the pressure upon the walls of the systemic arteries is increased, although the supply of blood propelled by the heart, and the propulsive power of the heart itself, are diminished; and this plainly indicates a retardation in the systemic capillaries, producing an undue accumulation in the arteries.—On the other hand, the suspension of the supply of oxygen to the lungs, either by an obstruction in the air-passages, or by caus- ing the animal to breathe some other gas, brings the pulmonary cir- culation to a stand in a very short time, the blood not being able to undergo its usual changes in the capillaries of those organs; and by this stagnation, the whole movement of blood is speedily checked. The readmission of oxygen, if the suspension of the circulation have not been too long continued, occasions the renewal of the movement in the capillaries, and thence in the whole circle of vessels; and this even aftar the heart has ceased to propel blood towards the lungs. 598. The principles already noticed (§ 547), as put forth by Prof. Draper, seem fully adequate to explain these phenomena. The arte- rial blood,—containing oxygen with which it is ready to part, and being prepared to receive in exchange the carbonic acid which the tissues set free,—must obviously have a greater affinity for the tissues, than venous blood, in which both these changes have already been effected. Consequently, upon mere physical principles, the arterial blood, which enters the systemic capillaries on one side, must drive before it, and expel on the other side of the network, the blood which has become venous whilst traversing it. But if the blood which enters the capillaries have no such affinity, no such motor power can be developed. On the other hand, in the capillaries of the lungs, the opposite affinities prevail. The venous blood and the air in the pulmonary cells have a mutual attraction, which is satisfied by the exchange of oxygen and carbonic acid that takes place through the walls of the capillaries ; and when the blood has become arterialized, it no longer has any attraction for the air. Upon the very same prin- VARIOUS CONDITIONS OF CAPILLARY CIRCULATION. 347 ciple, therefore, the venous blood will drive the arterial before it, in the pulmonary capillaries, whilst respiration is properly going on : but if the supply of oxygen be interrupted, so that the blood is no longer aerated, no change in the affinities takes place whilst it traverses the capillary network ; the blood, continuing venous, still retains its need of a change and its attraction for the walls of the capillaries; and its egress into the pulmonary veins is thus resisted, rather than aided, by the force generated in the lungs. 599. The change in the condition of the blood, in regard to the relative proportions of its oxygen and carbonic acid, is the only one to which the Pulmonary circulation is subservient; but in the Sys- temic circulation, the changes are of a much more complex nature,— every distinct organ attracting to itself the peculiar substances, which it requires as the materials of its own nutrition, and the nature of the affinities thus generated being consequently different in each case. But the same law holds good in all instances. Thus the blood con- veyed to the liver by the portal vein, contains the materials at the expense of which the bile-secreting cells are developed ; consequently the tissue of the liver, which is principally made up of these cells, possesses a certain degree of affinity or attraction for blood contain- ing these materials; and this is diminished, so soon as they have been drawn from it into the cells around. Consequently the blood of the portal vein will drive before it, into the hepatic vein, the blood which has traversed the capillaries of the portal system, and which has given up, in doing so, the elements of bile to the solid tissues of the liver. —The same principle holds good in every other case. 600. We are now prepared, therefore, to understand the general principle, that the rapidity of the circulation of a part will depend in great measure upon the activity of the functional changes taking place in it,—the heart's action, and the state of the general circulation, re- * maining the same. When, by the heightened vitality, or the unusual exercise, of a part, the changes which the blood naturally undergoes in it are increased in amount, the affinities which draw the arterial blood into the capillaries are stronger, and are more speedily satisfied, and the venous blood is therefore driven out with increased energy. Thus a larger quantity of blood will pass through the capillaries of the part in a given time, without any enlargement of their calibre, and even though it be somewhat diminished; but the size of the arteries by which it is supplied, soon undergoes an increase, which adapts it to supply the increased demand. Any circumstance, then, which in- creases the functional energy of a part, or stimulates it to increased nutrition, will occasion an increase in the supply of blood, altogether irrespectively of any change in the heart's action. This principle has long been known, and has been expressed in the concise adage " Ubi stimulus, ibi fluxus;" which those Physiologists, who maintain that the Circulation is maintained and governed by the heart alone, cast into unmerited neglect. , . 601. An undue acceleration of the local circulation, arising from an 348 CIRCUMSTANCES INFLUENCING CAPILLARY CIRCULATION. excess of functional activity in the part, and unaccompanied by any other change, constitutes the state known as active congestion, or deter- mination of blood. This may be artificially produced by the applica- tion of gentle stimulants; and it is usually the first change that occurs, when their action proves sufficiently violent to produce inflammation. From that state, however, it is distinguished by this important charac- ter,—that there is merely an exaltation of the natural function, but no change. Moreover we shall presently see that, in Inflammation, there is a stagnation of blood, not an acceleration. We frequently meet with cases, in which this active congestion becomes very manifest; especially in persons of active minds, who exert their mental powers too violently, and who thereby induce an habitually increased flow of blood towards the head, manifested in the increased pulsation of the carotids, the suffusion of the face and eyes, and the heat of the surface. The balance of the circulation being thus disturbed, there is almost invariably a diminished energy of the movement of blood in other organs, especially the extremities; as indicated by their habitual cold- ness and lividity. In the treatment of such a state (which is often the precursor of serious disease), it should be our object to restore the cir- culation in the extremities, by friction, exercise, &c. ; and to abate the flow of blood towards the head, by restraining the functional activity of the brain, by the application of cold to the surface, by keeping the head high during sleep, and other means of similar tendency. 602. There is another condition of the capillary circulation, also known under the name of Congestion, which is precisely the opposite of the preceding. In this state, there is deficient functional energy in the part and the circulation through it is consequently retarded,—as in the lungs, when there is a partial obstruction to the aeration of the blood. The same cause produces a deficient tonicity of the Arteries, and allows their walls to be unduly distended by the vis a tergo of the blood ; and consequently there is a great accumulation of blood in the part with a retarded movement. This condition, like the preceding, predisposes to Inflammation, although in a different mode, as will be explained hereafter (§ 631). It is relieved by causes which promote the action of the part; thus congestion of the lungs, occasioned by the effusion of fluid into the air-cells, which creates an obstacle to the aera- tion of the blood disappears, when that effusion is absorbed. And congestion of the liver, the result of deficient secreting power in the organ, is relieved by mercurial and other medicines, which promote the flow of bile by stimulating the growth of the hepatic cells. 603. The Capillaries, like the Arteries, possess a power of contrac- tion and dilatation which seems to be very much under the influence of the Nervous System, and particularly of that part of it which con- veys the influence of the Emotions. We have a visible example of this influence, in the act of Blushing; which consists in a sudden enlargement of the capillaries and small vessels of the surface ; whilst the converse state of pallor, which often alternates with it under the MOVEMENT OF. BLOOD IN CAPILLARIES AND VEINS. 349 influence of strong emotion, is evidently due to an unusual contraction of the same vessels. But the effects of this influence are no less sen- sible in other cases; and particularly in the regulation of the quantity of certain secretions, in accordance with the mental state, or the con- dition of the system generally. To the mode in which this regulation is effected, the act of blushing seems to afford us the key ; for it indi- cates that the supply of blood afforded to the glands, may be entirely governed by the influence of the nervous system upon the calibre of the arteries. Thus, the nursing mother, at the sight, or even at the thought of her child, when the usual time for suckling approaches, feels a rush of blood to the breast, exactly resembling that which takes place to the cheeks in blushing, and popularly termed " the draught;" this rush occasions an almost immediate increase in the secretion. In like manner we may explain the influence of the mental state upon the amount of the secretions of the lachrymal, the salivary and many other glands; its influence upon their quality, must probably be effected through changes in the condition of the blood itself. 604. The supply of Nervous agency from the Cerebro-spinal system has been clearly proved to exert no direct influence in maintaining the capillary circulation; since the latter continues as usual, after all the nerves of a part have been divided. This is obviously due to the fact, that the operations of nutrition, secretion, &c, are essentially inde- pendent of this agency. But as they are in some degree influenced by it, so will the capillary circulation be affected through its connection with them. In this manner wTe are to explain the effect of violent impressions upon the nervous centres in bringing to a stand, not merely the action of the heart (§ 581), but the Capillary circulation all over the body. The general vitality of the system appears to be at once destroyed ; so that the capillary circulation, which may usually be seen to continue in the web of a frog's foot for some time after the interruption of the heart's action, is immediately suspended by crush- ing the brain with a hammer. 6. Of the movement of Blood in the Veins. 605. The Venous system is formed by the reunion of the small trunks which originate in the Capillary network; and it carries back to the heart the blood which has been transmitted through this. This blood is dark or carbonated in the systemic veins; whilst it is bright or oxygenated in the pulmonary veins.—The structure of the veins is essentially the same with that of the arteries; but the fibrous tissue of their middle coat less decidedly exhibits the characters, either of the yellow elastic tissue, or of non-striated muscle. Still it possesses no inconsiderable amount of Elasticity; and a certain degree of mus- cular Contractility also. The whole capacity of the Venous system is at least twice, and perhaps more nearly three times, that of the Arte- rial ; and the rate of motion of the blood in them must be propor- tionably slower. 350 RESPIRATORY PULSE. 606. The movement of the Blood through the Veins is, without doubt, chiefly effected by the vis a tergo, or propulsive force, which results from the contractile power of the heart and arteries, aided by the power generated in the capillary vessels. The intermittent flow, which is caused by the interrupted action of the former is usually so far equalized during the passage of the blood through the capillary network, that no pulsation can be shown to exist in the veins ; but in- stances occasionally present themselves, in which a venous pulse may be clearly perceived.—The Venous Circulation is affected, however, by certain other causes which exert little influence on the movement of blood in the Arteries. One of these is the frequently recurring action of Muscles, to which the Veins are peculiarly subject, on account of their position. In every instance in which Muscular movement takes place, a portion of the Veins of the part will undergo compression; and as the blood is prevented by the valves in the veins, from being driven back into the small vessels, it is necessarily forced onwards towards the heart. As each set of muscles is relaxed, the veins that were compressed by it fill out again, to be again compressed on the renewal of the force. Thus, wre see how the general Muscular move- ments of the body have an important influence, in maintaining the Venous Circulation,—how continued exercise, involving the alternate contraction and relaxation of several groups of Muscles, must send the blood more rapidly towards the heart, and thus increase the rapidity of its pulsations,—and how the sudden and simultaneous action of a large number of muscles after repose (as when we rise up from the sitting or lying to the standing posture), may drive the blood to the heart with so violent an impetus as to produce even fatal results, if, by any diseased condition of that organ, it should be rendered unable to dispose with sufficient rapidity, of the quantity of blood thus driven to it. 607. The Respiratory movements exert a slight influence upon the flow of blood through the large veins near the heart; for as the chest is a closed cavity, in which a partial vacuum is produced by the act of Inspiration, whilst its contents are compressed by the act of Expi- ration, the former state will favour the movement of blood from the large veins on the exterior of the cavity, towards the heart, whilst the latter condition will retard it. This produces the phenomenon termed the respiratory pulse; which may be seen in the veins of the neck and shoulders in thin persons, and especially in those who are suffering from pulmonary diseases. The influence of the Respiratory move- ments is made evident by introducing a tube into the Jugular vein, the lower end of which dips into water ; for an alternate elevation and depression of the water in the tube are then witnessed, showing the suction power of the Inspiratory movement, and the expellent force of the Expiratory act. On the other hand, the Expiratory movement, while it directly tends to cause accumulation in the veins, will assist the heart in propelling the blood in the Arteries; and by the com- bined action of these two causes are produced, among other effects, VENOUS CONGESTION.—INFLUENCE OF GRAVITY. 351 the rising and sinking of the Brain, synchronously with expiration and inspiration, which are observed when a portion of the cranium is removed. 608. A pulsatory movement may be occasioned in the great veins near the heart, by another cause entirely distinct from the preceding ; —namely, the regurgitation of blood from the ventricle into the auri- cle, and thence into the venae cavae, during the ventricular systole; and the pulsation thus occasioned is synchronous, therefore, with that in the arteries (proceeding backwards, however, from the heart), in- stead of corresponding with the respiratory movement. This regurg- itation may take place, not from any disease in the valves on the right side of the heart, but simply from over-distension of its cavities, re- sulting from any obstruction to the circulation of blood through the lungs, for when this occurs, the tricuspid valve does not completely close, and allows a portion of the blood to escape from the ventricle backwards into the auricle and vena cava. This want of complete closure, effecting what has been termed the " safety-valve function" of the tricuspid valve, has been particularly noticed in diving animals, in which the circulation through the lungs is liable to be temporarily suspended. The venous pulsation which is thus produced may be noticed in almost every case of long-standing dyspnoea; especially when this is accompanied (as it usually is) by hypertrophy and dilata- tion of the right ventricle of the heart. 609. The Venous circulation is much more liable than the Arterial, to be influenced by the force of Gravity; and this influence is par- ticularly noticeable, when the tonicity of the vessels is deficient. The following experiments performed by Dr. Williams, to elucidate the influence of deficient firmness in the walls of the vessels, and of gravitation, over the movement of fluids through tubes, throw great light on the causes of Venous Congestion. A tube with two equal arms having been fitted to a syringe, a brass tube two feet long, hav- ing several right angles in its course, was adapted to one of them, whilst to the other was tied a portion of a rabbit's intestine four feet long, and of calibre double that of the brass tube, this being arranged in curves and coils, but without angles and crossings. When the two ends were raised to the same height, the small metal tube discharged from two to five times the quantity of water discharged in a given time by the larger but membranous tube; the difference being greatest, when the strokes of the piston were most forcible and sudden, by which the intestine was much dilated at its syringe end, but conveyed very little more wrater. When the discharging ends were raised a few inches higher, the difference increased considerably, the amount of fluid discharged by the gut being much diminished; and when the ends wTere raised to the height of eight or ten inches, the gut ceased to discharge, each stroke only moving the column of water in it, and this subsiding again, without rising high enough to overflow. When the force of the stroke was increased, the part of the intestine nearest the syringe burst. » 352 CIRCULATION WITHIN THE CRANIUM. 610. From these experiments it is easy to understand, how any deficiency of tone in the Venous system will tend to prevent the ascent of the blood from the depending parts of the body, and will conse- quently occasion an increased pressure on the walls of the vessels, and an augmentation in the quantity of blood they contain. All these conditions are peculiarly favourable to the escape of the watery part of the blood from the small vessels ; and this may either infiltrate into the areolar tissue, or it may be poured into some neighbouring serous cavity, producing dropsy. Thus it happens, that such effusions may often be traced to that state of deficient vigour of the system, which peculiarly manifests itself in want of tone of the blood-vessels; and that it is relieved by remedies which tend to restore this. In many young females of leucophlegmatic temperament, for example, there is a tendency to swelling of the feet, by cedematous effusion into the areolar tissue, in consequence of the depending position of the limbs; the oedema disappears during the night, but returns during the day, and is at its maximum in the evening. And the congestion which frequently manifests itself in the posterior parts of the body, towards the close of exhausting diseases, in which the patient has lain much upon his back, is attributable to a similar cause ; of such congestion, effusions into the various serous cavities are frequent results; and such effusions taking place during the last hours of life, are often erroneously regarded as the cause of death. To the same cause we are to attribute the varicose state of the veins of the leg, which is so common amongst persons of relaxed fibre, and especially in those whose habits require them to be much in the erect posture; and this distension occasionally proceeds to complete rupture, the causes of which are fully elucidated by the experiments just cited. 611. It has been thought that the circulation within the Cranium takes place under different conditions from that of other parts of the body. For as the cranium is a closed cavity,—a certain part of which is occupied by the cerebral substance and its membranes, the remain- der being filled up with blood,—it has been argued that the amount of blood in the vessels of the brain must be always the same; and that any disturbance of its circulation must be due to a difference in the relative quantity of blood in the arteries and the veins. This idea appeared to derive support from the results of experiments; which showed that the blood is retained in the vessels within the cranium of animals bled to death, unless an opening be made in the skull so as to allow the air to exert the same pressure upon these vessels as upon those of other parts. But such experiments do not at all sanction the assertion, that the quantity of blood within the cranium is constant; on the contrary, we have reason to believe that it undergoes as much change as in other parts. For although the cerebral substance be incompressible, yet its bulk is subject to con- stant variation, according to the quantity of fluid it contains; and the presence of the cerebrospinal fluid in the sub-arachnoid cavity in the brain and spinal cord, appears to be peculiarly destined to favour this MATERIALS OF THE NUTRITIVE PROCESS. 353 continual change,—the proportions of it contained in the spinal and cerebral cavities, respectively, being governed by the bulk of the other contents of the cranium. Thus if the vessels of the cerebrum be in their ordinary state of fullness, a certain amount of fluid is pre- sent in the sub-arachnoid cavity of the brain ; this will be pressed out into the spinal portion of the cavity, if the cerebral vessels be unusu- ally distended with blood ; whilst it will be increased from the latter source, so as to fill up the vacant space within the cranium, if the cerebral vessels be unusually empty. CHAPTER VII. OF NUTRITION. 1. Selecting power of the Individual Parts. 612. The Blood, which is carried into the different parts of the system, by the Circulating apparatus, is the source from which all the organs and tissues of the body derive the materials of their growth and development; and, as we have seen, it is distributed by the Ca- pillaries of the several tissues, with a degree of minuteness, which varies according to the activity of the nutrient operations taking place in the individual parts. Thus, in Nerve and Muscle, Mucous Mem- brane, and Skin, a constant decay of the old, and development of new tissue, are taking place, when these organs are in a state of functional activity ; and a copious supply of blood is carried through every part of their substance : whilst in Cartilage and Bone, Tendon and Liga- ment, the amount of interchange is very small, and is effected by a much less minute reticulation of capillary blood-vessels. 613. The materials of the nutritive process being prepared in the blood, the process of Nutrition is the act of each individual part; which grows and develops itself, in virtue of its own inherent powers, as long as the requisite conditions are supplied. The mode in which this takes place, in each individual tissue, has been already explained in the former part of this Treatise. We have seen that, in the great majority of cases, the act of Nutrition is, in fact, a process of cell-growth ; and that it takes place under the same conditions, as the production of the simple isolated cell, which constitutes the whole of the humblest forms of Cryptogamic Vegetation,—namely, that it grows from a germ, which draws to itself the materials of its nutrition, and gives to some of them a new arrangement, whereby they form the cell-wall, whilst others are introduced into the cell- cavity,—and that, when it has passed through its regular series of changes, it dies, and sets free its contents. We have seen that, in 23 354 SYMMETRY OF THE NUTRITIVE PROCESSES. some cases, the germs are prepared by previously existing cells of the same kind ; whilst in others they are furnished by certain " nutri- tive centres," which seem to be constantly engaged in the preparation of them, deriving their materials from the blood. Frequently it would seem as if the original or parent-cell was able to continue the pro- duction of secondary cells to an unlimited extent, even though it has itself undergone a considerable change of form. Thus the ultimate follicles of Glands seem to be at first closed cells, which subsequently open at the part nearest to the duct, and establish a connection with it; and having thus changed their condition, they go on deve- loping new generations of secreting-cells in their interior, from their own nuclei or germinal centres, to an unlimited extent. In like manner, the parent-cells of Muscular Fibre, which have coalesced to form the tubular Myolemma, seem to continue to develop new fibrillae from their nuclei, notwithstanding their change of form. 614. The selecting power, which is possessed by the germs of each kind of tissue, and which enables them to draw from the blood the materials which they severally require for their development, mani- fests itself also in the mode in which substances, that ^re abnormally present in the blood, affect the condition and development of the solid tissues. Thus we find that the presence of a certain quantity of Arsenic in the blood, will produce a state of irritation in all the Mucous membranes of the body. The continued introduction of Lead into the circulating system, occasions a modification in the nutrition of the extensor muscles of the fore-arm, producing the form of partial paralysis commonly termed wrist-drop; and the existence of this modification is shown, by the presence of lead in the palsied muscles. Here we have to remark the symmetrical nature of the affection, con- sequent upon the occurrence of the same disorder in the corresponding parts of the two sides of the body; for these muscles appear to have the same kind of tendency to attract lead from the circulating current, in a degree that is equal on the two sides, as they have to draw from the blood the materials of their regular growth, and to develop them- selves in an exactly similar manner. In like manner, the cutaneous eruptions, which are occasionally produced by the internal exhibition of iodide of potassium, are found to be almost precisely symmetrical; the presence of the medicine in the blood being the occasion of a disordered nutrition of certain parts of the skin; and the selecting power of particular spots being evinced, by the exact correspondence of the parts affected on the two sides. 615. The same appears to be the case with regard to substances, whose presence in the blood is rather the result of a disordered con- dition of the digestive and assimilating processes, than of their direct introduction from without. Thus in Lepra and Psoriasis,—chronic diseases of the Skin, which seem to have their origin in a disordered state of the blood, rather than in the solid tissues affected,—we find a remarkable tendency to the repetition of the patches, on the two sides of the body, or on the corresponding parts of the limbs; and this we UNUSUAL ENERGY OF THE NUTRITIVE PROCESSES. 355 0 must attribute to the peculiar attraction, existing between the solid tissues of those parts, and the morbid matter circulating through them. So in those chronic forms of Gout and Rheumatism, which modify the nutrition of the joints, producing a deposit of " chalk-stones," or permanent distortion and stiffening, we almost invariably find the corresponding joints of the two sides affected. The chief exceptions to the general principle, that the presence of morbid or extraneous matters in the blood affects corresponding parts alike, are found to exist where there is much febrile disturbance, or where local causes produce a peculiar tendency to disorder of a single part. The nearer the character of the morbid process is to that of the ordinary nutritive operations, the more nearly does it approach these, in the symmetry with which it develops itself.* 2. Varying Activity of the Nutritive Processes. 616. The nutritive operations go on with very great variations in their relative activity, under different circumstances. As a general rule it may be stated that, the greater the demand for the functional activity of the organ or tissue, the more energetic is its nutrition ; and vice versa. Now this is readily understood, when it is considered, that the active state of many structures essentially consists in an act of nutrition; thus the energy of the secreting processes is really de- pendent, as we have seen, upon the growth of the secreting cells, which make up the essential part of the gland ; and the energy of the absorbing and assimilating processes is dependent upon the develop- ment of the cells, which select and elaborate the nutrient matter. This growth is regulated mainly by the supply of blood; being increased by the afflux of blood towards the part, in consequence of the influence of the nerves upon the vessels, or through any other change in the current of the circulation. Thus the secretions are increased in amount, by emotions of the mind, that act (probably through the sympathetic nerve), in regulating the calibre of the arteries supplying their respective glands; or the interruption of the function of one gland shall occasion an increased nutrition, and consequently an augmented secretion, in its fellow,—as when one of the kidneys is hypertrophied, through a disease in the other, that renders it inca- pable of performing its office. Still it would appear, that there may be variations in the activity of these organs, resulting from causes inherent in themselves (of the nature of which we know little or nothing); and that here, as elsewhere, active nutritive operations will promote the circulation of blood through the parts, whilst a languid state of the function will retard it. 617. In certain other tissues, however, the functional activity would seem to be dependent rather upon a waste or decay of structures pre- viously developed ; this is the case especially in Nerve and Muscle, • See Dr. W. Budd's valuable paper on the "Symmetry of Disease," in vol.xxv. of the Medico-Chirurgical Transactions. 356 VARIOUS CAUSES OF HYPERTROPHY. which are found to undergo disintegration, in exact proportion to the degree in which they are exercised; whilst the degree in which this waste is repaired, depends upon the supply of nutritive material, the quiescent state of the part, and other circumstances. But even here we find, that functional activity occasions increased nutrition; in the same manner as burning a lamp with a high flame increases the amount of fluid drawn up by the wick. For neither the nerves nor the muscles can act with energy, without a large supply of arterial blood; and this is drawn to them on the principles already mentioned, as increasing the energy of the local circulation (§ 600). The deter- mination of blood to the parts, thus established, favours their increased nutrition; and thus we find that, under favourable circumstances, any set of Muscles, which is habitually exercised, undergoes a great increase of development; whilst, in like manner the Nervous centres, if too great a demand be made upon their activity, are liable to become hypertrophied (especially in young persons), and may thus become subject to disorders, which temporarily or permanently de- stroy their powers. In these cases, then, the functional activity determines the increased supply of blood, and occasions the aug- mented growth; and increased nutrition will rarely take place in these tissues, without an especial stimulus of this kind. Thus we find that, when a larger supply of nutritive matter is introduced into the circulation, than is required to repair the waste of these tissues, they do not undergo an increased development in consequence; but an augmented nutrition is produced, either in the adipose tissue, or in the glandular structures by which the superfluous matter is eliminated from the system. 618. Augmented nutrition, or Hypertrophy, then, may result, in certain organs, from an excessive supply of their nutrient materials ; as in the case of the kidney just mentioned ; or as in the enlargement which we not unfrequently meet with in the livers of those who have resided long in warm climates, and who have not sufficiently restricted their supply of non-azotized food to the small amount required for respiration at an elevated temperature, thereby sending an over-sup- ply of that particular class of bodies, to be separated from the blood by the liver. Or, in other cases, the increase of functional activity may be the immediate cause of the increased nutrition ; and this we see, not only in the nervous centres and voluntary muscles, which are put in action by the will, but in parts over which the mind has no control. Thus the heart becomes hypertrophied, when an obstruc- tion exists in the pulmonary or systemic circulation, to overcome which, increased energy of contraction is required ; and in the same manner, the muscular coats of the urinary and gall-bladders acquire an extraordinary increase of thickness, when long-continued obstruc- tion, by calculi or stricture in the canals issuing from them, impedes the free exit of their contents. Sometimes, however, a local hyper- trophy takes place, which cannot be accounted for in either of these modes; as when a single finger is enlarged out of all proportion to VARIOUS CAUSES OF ATROPHY. 357 the rest, or the whole of one hand increases to a much greater size than the other, by the existence (as it would seem) in the individual part, of that tendency to unusual development, which, when it affects the whole body uniformly, produces a gigantic stature. 619. Now a precisely reversed series of conditions diminishes the activity of the nutrient processes, and induces a state of Atrophy. If there be a deficiency in the general amount of nutriment introduced into the system by absorption, a general atrophy results; and the waste being more rapid than the supply, there is a diminution in the volume of all the tissues excepting the nervous, which seems to have its nu- trition kept up even to the last, at the expense of all the rest. Such a condition results not merely from the want of food, but also from the want of power to assimilate it; and thus emaciation may take place to an excessive degree, when food of the most nutritive character is copiously applied, and when the appetite for it is vehement; in con- sequence of disorder in the mesenteric glands, or in some other part of the apparatus particularly concerned in the elaboration of fibrin. A partial atrophy may result, in like manner, from a deficiency of the materials required for the formation of an individual tissue or organ; thus the adipose tissue, throughout the body, may be atrophied, in consequence of an absence of those materials in the food, which are capable of being converted into fatty matter. Or a particular organ may be atrophied, by a diminution of the circulating current that should flow to it, either in consequence of obstruction in the trunk, or by the partial diversion of the stream of blood in another direc- tion ; thus the liver, which is much more developed in the fcetus, relatively to the rest of the body, than it is in the adult, undergoes a partial atrophy immediately after birth, in consequence of the change in the whole course of the circulation, which takes place as soon as the lungs are expanded. 620. But partial atrophy may also take place from causes inherent in a particular organ. Thus we occasionally meet with limbs, which are "blighted ;" never attaining their due size relatively to the re- mainder of the body; yet not exhibiting any defect of organization. Here there would seem to be, from some unknown cause, a deficient power of growth; analogous to that which, when acting on the body in general, confines it within dwarfish dimensions.—One of the most frequent causes of partial atrophy, however, is want of functional ac- tivity in the organ; and this is particularly the case in regard to the Muscular and Nervous systems. Thus, as already remarked (§ 348), any set of Muscles that is long disused, becomes partially atrophied ; which is probably due to the feebleness and languor of the circula- tion, consequent upon the absence of the demand for arterial blood. As soon as the parts are called into use again, their nutrition improves. So also, in regard to the Nerves; the nutrition of both the fibrous and vesicular structures appears to be entirely dependent upon the activity of their function; and as the former are inert without the lat- ter, we may say that the due nutrition of the nervous system entirely 358 VARYING ACTIVITY OF THE NUTRITIVE PROCESSES. depends upon the functional activity of the vesicular matter. Of this we have a well-marked illustration in the fact, that, when the Cornea has been rendered so opaque by disease or accident, as to prevent the penetration of any light to the interior of the eye, the retina and the optic nerve lose after a time their characteristic structure; so that scarcely a trace of the peculiar globules of the former, or of the nerve- tubes of the latter, can be found in them. 621. In the healthy condition of the organism, however, the nutri- tion of every part of the body goes on in a degree sufficient to keep it constantly ready for the performance of its appropriate function ; a regular supply of the requisite materials being furnished in the aliment, and being.prepared by the assimilating processes; and the products of the waste or decay of the tissues, together with such alimentary materials as maybe superfluous, being carried off" by the excreting operations. When the nutrition and the waste are equal, the wreight of the body remains the same ; and this is commonly the case in adult age. But during the earlier periods of life, the powers of growth are greater; the demand for food is very large, in proportion to the bulk of the body; and though the waste is rapid and the ex- creting processes very active (as evinced by the large amount of urea and of carbonic acid set free), the growth predominates over the decay, and the development of the whole structure proceeds at a gra- dually decreasing rate, until the full stature and bulk are attained. The energy of the nutritive process is particularly manifested in the rapidity and completeness with which severe injuries, occasioned by disease or accident, are repaired. In advanced life, on the contrary, although the waste is comparatively small, the renewing processes are enfeebled in a still greater degree; and there is a gradual diminution in the stature and bulk of the body, and in its physical powers. All the functions are performed with diminished energy; and the compa- rative inertness of the nutritive processes is seen in the difficulty with which the effects of severe injuries are repaired, in the length of time requisite for the purpose, and frequently in the imperfection of the result. 622. During the successive periods of life, there are many remark- able changes in the relative nutrition of different organs ; which we can attribute to nothing else than to inherent differences in their own powers of development. Thus, during the early stages of fcetal exist- ence, the greatest energy of growth is seen in certain parts, which are to answer but a temporary purpose, and which are afterwards com- pletely atrophied. This is the case, for example, with the Corpora Wolffiana, which seem to answer the purpose of temporary kidneys, and in connection with which the permanent kidneys and the genital organs are developed ; and of these bodies, though of large size in the early embryo, and evidently of great importance, no trace whatever is afterwards to be discovered. So in regard to the Supra-Renal cap- sules, the Thymus and Thyroid glands, and other organs, we find their proportional size the greatest, and their function evidently the most VARIATIONS OF NUTRITION WITH AGE. 359 active, during fcetal existence and in early infancy; after which their bulk diminishes in proportion to the rest of the body, and their func- tional activity seems almost at an end. 623. Even in the relative development of the organs, which form essential parts of the permanent structure, we find considerable varia- tions at different periods of life. Thus the evolution of the generative system does not usually take place, until the rest of the system is ap- proaching its maturity; but cases sometimes occur, in which this appa- ratus attains its full development, both in the male and the female, at a very early period of childhood, and seems capable of performing its functions. In the Human species, these organs, when once evolved, remain always in a state of preparation for the performance of their function, unless they are atrophied through complete disuse, or have lost their vigour by age, or through excessive demands upon their activity ; but in most of the lower animals, the development of these organs is periodical through the whole of life, taking place at a cer- tain season of the year, and being greatly influenced, it would ap- pear, by the external temperature, and by the supply of food. Thus in the Sparrow, the testes are no larger than mustard-seeds, during the greatest part of the year; but in the spring, they acquire the size of large peas, and it is then only that they possess any procreative power. 624. We are not always to judge of the degree of development of organs, however, by their size alone ; for the completeness of their structure, and their aptitude for the performance of their functions, must also be taken into the account. Thus in the new-born infant, the organs of digestion and assimilation, though of small size, are so completely formed, as to be able at once to take on the duty of re- ceiving and preparing the nutritive materials, provided these are sup- plied in a form adapted to their powers; the circulating apparatus is fully adequate to transmit the products of the action of those organs to the body in general, and to bring back the results of its continual decay; and the respiratory organs, together with other parts of the excretory apparatus, are so completely evolved, as to be able to sepa- rate the effete matter, and cast it out of the system, with an energy equivalent to that of the organs, by which new matter is introduced and appropriated. On the other hand, the Brain, although of larger comparative size at birth, than at any subsequent period of life, is but very imperfectly developed; for its structure is not yet so far com- pleted, as to prepare it for a state of high functional activity. In fact, it would seem as if the use of the organ, as called forth by the new circumstances in which the infant is placed as soon as it comes into the world, is essential to its complete development; and the same may be said of the Muscular system. 625. During the whole period of infancy and childhood, the current of nutrition seems peculiarly directed towards the brain ; for though its size does not continue to increase in proportion with that of the remainder of the body, its structure is evidently being rendered more 360 VARIATIONS OF NUTRITION WITH AGE. perfect, and its functional activity is excited with remarkable facility. Hence it is peculiarly liable to be acted on by various causes, which may produce disease ; and the operation of remedies, which spe- cially affect that organ, is far more powerful than at any other period of life. Thus, whilst a child will bear a fourth, or even a third, of the dose of a purgative adequate for an adult, it is strongly affected by an eighth, or even a twelfth, of the dose of a narcotic or a stimulant, that would be required to produce a corresponding effect in middle life. This peculiar impressibility of the nervous system, resulting from the activity of the nutrient processes which are taking place in it, mani- fests itself also in other ways; thus children are peculiarly liable to have its powers depressed by any sudden shock, such as a blow, or an extensive burn or laceration; whilst, on the other hand, if the depres- sion be not fatal, they recover from its effects much more speedily than an adult would do from a similar condition. 626. During the periods of youth and adolescence, the chief energy of development (except in regard to the generative system, already noticed), appears to be directed towards the Muscular apparatus; which then increases in vigour, in a degree which surpasses its increase of size; and the circulating and respiratory organs, upon whose energetic action there is then a corresponding demand, are peculiarly liable to disturbance of function, inducing disease in them- selves or in other parts. The maladies of this period are for the most part of a sthenic or inflammatory character; resulting, as we shall presently see, from the excessive activity of the assimilating pro- cesses, which are disposed to produce more fibrin than the wants of the body require. Or if, on the other hand, there be an imperfect elaboration of the nutrient materials, as happens in the tubercular diathesis, its effects are peculiarly liable to manifest themselves at this period, when the demand for nutritive matter is greatly aug- mented by the activity of the muscular system. 627. In adult age, there should be such a balance of all the func- tions, arising from the due development and proper use of each organ, as may preserve the body in the state of health and vigour, without any marked change in the relative dimensions of its different parts, through a long series of years. The digestive, assimilating, and excreting organs, as they were the first to come to maturity, are com- monly the first to fail in their activity; but this is very generally the result of over-exertion of their powers, the amount of food introduced into the stomach being rarely (among the higher and middle classes of society at least) kept down to the real wants of the system. The muscular apparatus usually experiences the effects of this diminished nutrition sooner than the nervous system; the vigour of the latter being often sustained in a remarkable degree (as shown by the energy of the mental operations) through a protracted life, when it has not been over-tasked at an earlier period. The very slight impairment of the nutrition of the nervous system, during the general emaciation which results from a wasting disease, or during that more gradual VARIOUS CAUSES OF DEATH. 361 decline of the bodily vigour which is consequent upon advancing age, is a phenomenon which strongly marks it out as the part of the body, to the maintenance of whose integrity everything else is sub- servient; and this is still more remarkably shown in the phenomena of starvation ; in which, notwithstanding the disappearance of the whole of the fat, and the reduction of the weight of the body in general by about 40 per cent., the nervous system appears to lose little or none of its substance (§ 117). 3. Of Death, or Cessation of Nutrition. 628. The general cessation of the Nutritive operations, in Death, usually depends, as formerly explained (§ 65), upon the cessation of the supply of Nutriment, in consequence of the stagnation of the Cir- culating current; and this stagnation may result from the direct ope- ration of three causes; namely,—failure in the propulsive power of the Heart, or Syncope,—obstruction to the flow of blood through the pulmonary capillaries, consequent upon a deficient supply of air, or Asphyxia,—and a disordered state of the blood itself (§ 534), which at the same time weakens the power of the heart, and prevents the performance of those changes in the systemic capillaries, which afford a powerful auxiliary to the circulation; a mode of death, for which the term Necrcemia has been proposed. Each of these conditions may be dependent upon a variety of remote causes, which need not here be particularized. But it is evident that, when either one of them has been established, the nutritive processes must speedily cease, although they may continue for a short time at the expense of the blood in the capillaries of the part. The cooling of the body is another cause of their cessation; and this is one reason why molecular death (or the death of the individual organs and tissues) follows so much more closely on somatic death (or the cessation of the circula- ting and respiratory functions), in warm-blooded than in cold-blooded animals. In either case, however, the solid tissues may preserve for a time their independent vitality; and changes may take place in them, which indicate the continuance of their nutritive actions to a certain extent, even when they have been disconnected from the body. 629. Although the death of the several parts composing the fabric is thus due, in the great majority of cases, to the supensions of the supply of the nutritive materials, and to the lowering of the tempera- ture of the body, yet there are undoubtedly cases, in which the loss of vital power is as complete in the solids as in the fluids; the want of ability to avail themselves of nutriment, being as decided in the former, as the deficiency of supply is in the latter. This is seen, for example, when death results from a sudden and violent shock, which destroys the vitality of the whole system alike (§ 604). But it can scarcely be doubted, that we are to attribute the gradual decay and death, which take place in extreme old age without any symptom of local disease, to a similar cause. We have seen that every individual 362 NATURAL DEATH.—INFLAMMATION. part of the fabric has its own allotted period of life and activity ; each cell passing through a certain series of changes, which are proper to it, and then ceasing to exist; and many organs having only a limited period of activity, and disappearing more or less completely, when this has been accomplished. In both cases, the usual duration of the organ is usually diminished by any previous excess of activity, and increased by moderation in the exercise of its vigour. Thus, the life of the individual cells of the simple cellular plants (on which our observations as to some of the conditions of cell-growth may be best made), is shortened by such external influences as are most favourable to rapidity in their growth and development. And in Man, we con- stantly notice that the duration of the powers of the Brain and the Generative system is the longest, when these organs have been mode- rately exercised ; and that it is much curtailed by the excessive use of either. The duration of their activity, however, is not increased by partial or entire disuse of the organs ; for this induces a state of atrophy, on the principles already mentioned. Now we have every reason to believe, that what is true of individual parts and organs, is true also of the whole structure ; and that the existence of the entire bodily fabric may thus come to an end, without any special disease, in consequence of the limit originally set to its powers of self-renova- tion. It is but rarely, however, that this occurs; the various acci- dents of life, the neglect of ordinary precautions for the preservation of health, and hereditary tendencies to various kinds of morbid action, being too frequently the means of cutting off the term of Human existence, long before its natural expiration. 4. Disordered Conditions of the Nutritive Processes. 630. Having thus passed in review the general conditions, under which the ordinary Nutritive processes take place, it may be well to add a few words in relation to two of their abnormal states ; one or other of which is concerned in a very large proportion of the diseases that afflict the huraan race. In one of these, there is a tendency to the excessive production of Fibrin in the blood ; whilst in the other, there is a want of the proper nutritive power in the tissues, which is apparently due to an imperfect elaboration of that important material. The one of these conditions is termed Inflammation; whilst the other, which is less active, but more insidious, is known as the Tubercular Diathesis. 631. The extraordinary tendency to the production of Fibrin in the blood, which has been already noticed (§ 531) as the most import- ant character of Inflammation, seems to be always conjoined with a depressed vitality of the tissues of some part of the body, which indis- poses them to the performance of their regular nutritive operations ; and this part may undergo a variety of changes, according to the degree in which it is affected. The depressed condition of its nutri- tive operations involves, on the principles explained in the preceding INFLAMMATION; SUPPURATION; GANGRENE. 363 chapter, a languor in the movement of blood through it, together with a distensible state of the capillaries, which causes them to contain a far greater amount of that fluid than under ordinary circumstances. There appears to be, in the vessels of an inflamed part, a peculiar attraction for the white corpuscles of the blood, by which they are drawn together from the circulating current; and there is also a tend- ency to an increased production of them, which is probably the cause of the increase in the total amount of fibrin. This increase of fibrin in the blood, coupled with a diminished power of appropriating it on the part of the tissues, appears to constitute the essential phenomena of the Inflammatory condition, and to be the cause of the other changes which are characteristic of it. 632. The simplest result of this condition, is the effusion of fibri- nous matter, or organizable lymph, into the substance of the part inflamed, or upon the nearest free surface; and thus is produced a condensation of the tissue, or a new growth upon the membrane. But when the depression of vitality is more complete, the tissue at that spot gradually dies and disintegrates ; and whilst itself undergo- ing such changes, it gives origin to similar changes in the effused fibrin, which it converts from aplastic or organizable deposit, into an aplastic or unorganizable one, namely, pus. Thus is produced the Suppurating process; which may either take place in a cavity thus excavated in the substance of a tissue or organ ; or on a free surface. In either case, the surrounding tissues, which are less inflamed, and in which the vitality is impaired but not destroyed, become consoli- dated by a deposition of organizable fibrin, which prevents the infil- tration of pus through their substance. If this should not occur, through a want of power to generate well-elaborated fibrin, the sup- purating process extends itself rapidly, with the most calamitous results; the properties of pus being such as to produce a tendency to decomposition, both in the blood and in the solid tissues into the substance of which it may be carried. 633. The process termed Gangrene, which is the entire loss of vi- tality in the part, with a complete cessation of the circulation through it, is commonly ranked as one of the results of Inflammation; but it can hardly, in strictness, be so regarded. We have a well-marked illustration of the mode in which this local death takes place, in the case of frost-bites produced by Cold; for this agent at the same time produces contraction of the blood-vessels, and depression of the vital powers of the solid tissues, proceeding to the complete destruction of them; whilst in the parts adjoining those which are actually killed, the inflammatory state is developed ; producing an effusion of fibrin, which serves to plug up the mouths of the vessels, and thus to prevent hemorrhage, when the mortified part drops off. Here we see, that the violent action of cold completely destroys the vitality of the part most exposed to it; and this by its direct influence on the properties of the organized structure. No inflammation can take place in the part thus killed, because the vital processes are altogether brought to 364 GANGRENE; ULCERATION.—REPARATIVE PROCESS. an end. But inflammation takes place in the adjoining parts, which are less seriously affected ; for the depression of their vital powers occasions the result already adverted to,—namely, the production of an increased amount of Fibrin in the blood, and an infiltration of this substance into their tissues. The same is the case with regard to the operation of other powerful agents ; such as those which (like Caustic potass, or Sulphuric Acid) destroy the vitality of the parts to which they are applied, by the chemical decomposition of their tis- sues. The Inflammatory process is set up, not in the parts which are killed by the application; but in the surrounding tissues, whose vitality has been simply depressed; and thus, when the slough, or dead part, is cast off, there is a preparation for the development of new tissue to supply its place, from the superabundant plastic mate- rials of the surrounding parts. 634. If, then, we limit the term Inflammation, as there seems reason to do, to that state, in which there is a tendency to stagnated circulation, with increased production of Fibrin, in the vessels of the part, we see that Gangrene cannot be a result of that process, which is one rather of reparation than of destruction. But Gangrene pro- ceeds, where we can distinctly trace its causes, from the violent ope- ration of the same agents, as those which, in a less degree, produce Inflammation. And where this last process is not set up, at the line of demarkation between the living and the dead parts, Gangrene, like Suppuration, has a tendency to spread ; the influence of the decay, which is taking place in one part, having a tendency to propagate itself to the adjoining tissues; and a constantly extending destruction being thus produced. 635. In like manner, certain forms of the Ulcerating process may spread, by the action of a peculiar layer of cells, that is found on the surface of the excavation; these cells appear to possess the power of drawing into themselves the materials of the solid tissues on which they lie, and thus of causing their destruction; and this destructive action may take place to an unlimited degree, if no measures betaken to check it. The application of powerful escharotics (such as nitric acid, lunar caustic, or the actual cautery), which is well known to be one of the most efficient methods of treatment in this kind of diseased action, has the effect of destroying these peculiar cells, together with the adjacent tissue which has been partly affected by them; and of exciting an inflammatory action beneath, by which fibrin may be effused, and preparation made for filling up the breach of substance. 636. Now when the reparation of lost parts takes place, it may be effected in either of two modes;—by a process of growth analogous to the natural one;—or by the formation of a new kind of tissue, termed granulation-structure, from the surface of which a formation of pus takes place, until the cavity is completely filled up by it, and closed over by skin, after which the granulation-structure is absorbed, and a contracted cicatrice is left. The former mode of reparation, which takes place in cold-blooded animals, is the slowest, but it is REPARATIVE PROCESSES.—TUBERCULAR DIATHESIS. 365 the most complete ; for as the breach of substance is filled up by tissue of a permanent kind, there is no subsequent contraction nor cicatrix; nor is there that waste of plastic matter, nor that constitu- tional irritation, which is attendant on the suppurating process. It is, consequently, that which the Surgeon should aim to produce; and the means of accomplishing this consists in keeping down the Inflam- matory process, and in preventing irritation of the exposed surface. 637. In all cases of injury, there is an increased determination of blood to the neighbouring tissues; and an increased production of Fibrin, to serve as the material for repair. Now if this be moderate in its amount (as it usually is in cold-blooded animals), it will be all consumed in the formation of the new tissue, which is to fill up the vacuity. But if it be excessive, it forms an inflammatory effusion; part of which undergoes a low degree of organization, and becomes granulation-structure; whilst another part is poured forth from the copious but imperfectly-formed vessels of that structure, in an unor- ganizable state, forming Pus. This change in its character is mainly due to the irritating influence of cold air; which also tends to keep up the excessive production of Fibrin by the Inflammatory process; and the more carefully the raw surface is kept from contact with it, the more healthy will be its action. The low temperature of cold- blooded animals prevents the air from having a like injurious effect upon their wounded surfaces ; no exclusion of it seems necessary. In warm-blooded animals the desired end may be attained, either by the application of hot dry air, which causes a scab to form, beneath which the reparative process may take place in complete seclusion from external irritation; or by the formation of an artificial covering, equally closely applied, by means of a waxy or resinous ointment, spread in a liquid state (a measure which has proved peculiarly effi- cacious in the treatment of burns); or by the application of steam to the wounded surface, which seems to have a remarkably soothing effect upon it; or, where there is a tendency to violent inflammation (as in wounds of the large joints), by keeping the dressings moist by a continual supply of cool (but not cold) water. All these modes of treatment act in the same manner; tending to exclude irritation, to keep down inflammation, to prevent the over-production of fibrin, and to promote the natural process of slow reparative growth. 638. If the Fibrin of the Blood, however, be not well elaborated, it does not possess its due organizability; and thus, instead of being converted by the Nutritive process into solid tissue, proper to the part in which it is deposited, it is liberated from the vessels in a state, which prevents any but a very imperfect structure from being deve- loped by it. This is the condition of the Tubercular substance, which is so often found to replace the proper tissue, especially in the lungs; being slowly deposited there, by a sort of degradation of the regular nutritive operations; and being effused in larger quantity, when the inflammatory process is set up. There is every degree of gradation between the plastic or organizable deposit of well-elaborated Fibrin, 366 TUBERCULAR DIATHESIS.—MALIGNANT GROWTHS. the caco-plastic or imperfectly-organizable matter of Tubercle, and the aplastic or non-organizable matter of Pus. The Microscopic exami- nation of tubercular deposit shows, that they sometimes contain fully developed cells and fibres, analogous to those of fibrinous exudations ; but that more frequently, the cells and fibres are imperfectly formed, and are accompanied by a large quantity of a granular substance, which strongly resembles coagulated Albumen; and that in many cases, there is scarcely any trace of organization in the mass. The greatest degree of organization is found in the semi-transparent, mili- ary, gray, and tough yellow forms of Tubercle ; the least in the opaque, crude, or yellow Tubercle. 639. The constitutional state, which predisposes to this perversion of the ordinary nutritive operations, and which is known as the Tu- bercular Diathesis, is the result of the continued operation of any causes, that tend to depress the vital powers; such as insufficient nutrition, habitual exposure to cold and damp, protracted mental depression, &c.; or it may be derived from the operation of the same or other causes on the ancestors of the individual, being one of those disorders which has a peculiar tendency to become hereditary. The treatment must be directed to the invigoration of the system by good food, active exercise, pure air, warm clothing, and cheerful occupa- tions ; and by the due employment of those means, at a sufficiently early period, many valuable lives may be saved, which would other- wise fall a sacrifice to Tubercular disease in the lungs, or other import- ant organs. 640. There is another remarkable class of diseases, resulting from a disordered condition of the nutritive processes;—those, namely, of a malignant nature. We not unfrequently meet with abnormal growths of a fatty, cartilaginous, fibrous, or bony structure; which appear to originate in some perverted action of the part itself, and which have little tendency to reappear in the same part, w7hen they have been removed,—still less, to reappear in distant parts. But the various forms of Malignant or Cancerous disease are peculiar in this,—that they are composed of cells, sometimes of a globular form, (see Fig. 30), sometimes elongated or spindle-shaped, having a power of rapid multiplication, and not capable of changing into any other kind of tissue. When a truly cancerous growth has once established itself in any part of the body, it may increase to an unlimited extent, obtain- ing its nourishment from the blood-vessels in its neighbourhood, and destroying the surrounding parts by its pressure, as well as by draw- ing off their supply of aliment. When it has developed itself to a considerable degree in one part, it is very liable to make its appear- ance in others; probably in consequence of the germs of the cells being conveyed in the circulating current to distant portions of the body: and hence the judicious surgeon is disinclined to remove a Cancerous growth of any but the most limited kind; knowing that the disease is almost certain to reappear. There is a strong analogy between such Cancerous growths, and the low forms of Fungoid NEED OF RESPIRATION IN ANIMALS. 367 Vegetation, which develop themselves in the interior of the higher Plants, and even in Animal bodies; and in both cases, the disease may be propagated by inoculation from one individual to another, the transplantation of a few cell-germs being all that is required. CHAPTER VIII. OF RESPIRATION. 1. Essential Nature and Conditions of the Respiratory Process. 641. The function of Respiration essentially consists of an inter- change of oxygen and carbonic acid betwTeen the blood of the Animal and the surrounding medium ; carbonic acid being given out by the blood, and oxygen entering in its stead. It has been already noticed (§ 84) that this function is performed likewise by Plants; although, in consequence of their deriving a large part of their food from the atmosphere by a converse process—the absorption of carbon and the liberation of oxygen,—their true respiration is commonly overlooked. It may, therefore, be regarded as common to all Organized beings. Every one is conscious, in his own person, of the imperative demand for the due performance of this operation. If the breath be purposely held for even a few seconds, a feeling of distress is experienced, which increases every moment, and at last prompts irresistibly to the respiratory movement. And if the admission of air to the lungs be in any way prevented, the respiratory movements are at first increased in energy, violent efforts are made to obtain the needed supply; these are succeeded by irregular convulsive actions, and at the same time insensibility comes on; and within a short time all movement ceases, the circulation of the blood is suspended, and a stop is put to all the vital operations of the body. This state, which is termed Asphyxia, usually comes on, in a wrarm-blooded animal, within ten minutes of the time when the respiration is completely suspended; thus affording the most convincing proof of the importance of that function in the Animal economy. In many cold-blooded tribes, a much longer sus- pension may be borne with impunity; as also by warm-blooded ani- mals, when the general activity of their functions is lowered in the state of hybernation (§ 121). We shall now inquire into the sources of the necessity for this interchange of oxygen and carbonic acid; and the mode in which the suspension of it acts upon the system at large. 642. All Organized bodies, as already explained, are liable to con- tinual decay, even whilst they are most actively engaged in perform- ing the actions of Life; and one of the chief products of that decay is 368 SOURCES OF EXCRETION OF CARBONIC ACID. carbonic acid. A large quantity of this gas is set free, durino- the decomposition of almost every kind of organized matter; the carbon of the substance being united with oxygen supplied by the air. Hence we find, that the formation and liberation of carbonic acid go on with great rapidity after death, both in the Plant and in the Animal; and that they take place also, to a very great extent, in the period that often precedes the death of the body, during which a general decom- position of the tissues is taking place. Thus in Plants, as soon as they become unhealthy, the extrication of carbon in the form of car- bonic acid takes place in greater amount than its fixation from the carbonic acid of the atmosphere; and the same change normally takes place during the period that immediately precedes the annual fall of the leaves, their tissue being no longer able to perform its proper functions, and giving rise, by its incipient decay, to a large increase in the quantity of carbonic acid set free. The same thing happens in the Animal body, during the progress of many diseases which are attended with an unusual tendency to decomposition in the solids and fluids,—such as eruptive fevers :—the quantity of carbonic acid set free in Respiration is greatly increased, although the body remains completely at rest; and notwithstanding this, the blood frequently ex- hibits an unusually dark hue, indicating that it has not been properly freed from the unusual amount of that substance, which it has received from the tissues. 643. Hence, the first object of the Respiratory process, which is common to all forms of Organized being, is to extricate from the body the carbonic acid, which is one of the products of the continual decom- position of its tissues. The softness of many of the tissues of Animals, and the large quantity of fluid contained in their bodies, render them more prone than plants to this kind of decomposition; and, in warm- blooded animals, the higher temperature at which the fabric is usually maintained, adds considerably to the degree of this tendency; so that the waste of their tissues, from this cause alone, is as much greater than that of cold-blooded animals as the latter is than that of Plants. But when the temperature of the Reptile is raised by external heat to the level of that of the Mammal, its need for respiration increases, owing to the augmented waste of its tissues. When, on the other hand, the warm-blooded Mammal is reduced, in the state of hyberna- tion, to the level of the cold-blooded Reptile, the waste of its tissues diminishes to such an extent, as to require but a very small exertion of the respiratory process to get rid of the carbonic acid, which is one of its chief products. And in those animals which are capable of retain- ing their vitality when frozen (§ 136), or when their tissues are com- pletely dried up (§ 159), the decomposition is, for the time, entirely suspended, and consequently there is no carbonic acid to be set free. 644. But another source of Carbonic acid to be set free by the Re- spiratory.process, and one which is peculiar to Animals, consists in the rapid changes which take place in the Muscular and Nervous tissues, during the period of their activity. It has been already shown (§ 361), SOURCES OF EXCRETION OF CARBONIC ACID. 369 that there is strong reason to believe the waste or decomposition of the Muscular tissue to be in exact proportion to the degree in which it is exerted; every development of muscular force being accompanied by a change in the condition of a certain amount of tissue. In order that this change may take place, the presence of Oxygen is essential; and one of the products of the union of oxygen with the elements of muscular fibre is carbonic acid. The same may probably be said of the Nervous tissue (§ 384). Hence it may be stated as a general principle, that the peculiar waste of the Muscular and Nervous sub- stances, which is a condition of their functional activity, and which is altogether distinct from the general slow decay that is common to these tissues with others, is another source of the carbonic acid which is set free from the animal body; and that the amount thus generated will consequently depend upon the degree in which these tissues are exer- cised. In animals which are chiefly made up of the organs of vege- tative life, in whose bodies the nervous and muscular tissues form but a very small part, and in whose tranquil plant-like existence there is but very little demand upon the exercise of these structures, the quan- tity of carbonic acid thus liberated will be extremely small. On the other hand, in animals whose bodies are chiefly composed of muscle, and wThose life is an almost ceaseless round of exertion, the quantity of carbonic acid thus liberated is very considerable. 645. We are enabled to trace the connection between the amount of muscular exertion, and the quantity of carbonic acid set free in the act of respiration, in the class of Insects, better than in any other. They have no fixed temperature to maintain: and they are, conse- quently, not in the condition of warm-blooded animals, in which the quantity of carbonic acid set free is kept up to a more regular standard by the provision to be presently noticed. On the other hand, they are pre-eminent among all Animals, in regard to the energy of their mus- cular power in relation to the bulk of their bodies; and the waste of muscular tissue during their state of activity must therefore be very great. Thus, a Humble Bee has been found to produce one-third of a cubic inch of carbonic acid in the course of a single hour, during which its whole body was in a state of constant movement, from the ex- citement consequent upon its capture; and yet during the whole twenty-four hours of the succeeding day, which it passed in a state of comparative rest, the quantity of carbonic acid generated by it was absolutely less. 646. Besides these sources of Carbonic acid, which are common to all Animals, there is another, which appears to be peculiar to the two highest classes, Birds and Mammals. These are capable of maintain- ing a constantly elevated temperature, as long as they are supplied with a proper amount of appropriate food; and their power of doing so appears to depend upon the direct combination of certain elements of the food, with the oxygen of the air, by a process analogous to com- bustion ; these elements having been introduced into the blood for that purpose, but not having formed a part of any of the solid tissues of the 24 370 SOURCES OF EXCRETION OF CARBONIC ACID. body, unless they have been deposited in the form of fat. The nature of these substances has been already noticed (§ 430). It is quite clear that they cannot be applied in their original form, to the nutri- tion of the tissues that originate in proteine-compounds; and it is tole- rably certain that in the ordinary condition of the body, they undergo no such conversion as would adapt them to that purpose. The Liver seems to afford a channel, by which some of the fatty matters are drawn off from the blood; but even these seem, in part at least, to be re-absorbed (§ 725), and to be thrown off by the respiratory process. 647. The quantity of carbonic acid that is generated directly from the elements of the food, seems to vary considerably in different ani- mals, and in different states of the same individual. In the Carnivo- rous tribes, which spend the greater part of their time in a state of activity, it is probable that the quantity which is generated by the waste or metamorphosis of the tissues is sufficient for the maintenance of the required temperature,—and that little or none of the carbonic acid set free in respiration is derived from the direct combustion of the materials of the food. But in Herbivorous animals of compara- tively inert habits, the amount of metamorphosis of the tissues is far from being sufficient; and a large part of the food, consisting as it does of substances that cannot be applied to the nutrition of the tis- sues, is made to enter into direct combination with the oxygen of the air, and thus to compensate for the deficiency. In Man and other animals, which can sustain considerable variations of climate, and can adapt themselves to a great diversity of habits, the quantity of carbonic acid formed by the direct combination of the elements of the food with the oxygen of the air, will differ extremely under different circumstances. It will serve as the complement of that which is formed in other ways; so that it will diminish with the increase, and will increase with the diminution, of muscular activity. On the other hand, it will vary in accordance with the external temperature ; in- creasing with its diminution, as more heat must then be generated ; and diminishing with its increase.—In all cases, if a sufficient supply of food be not furnished, the store of fat is drawn upon ; and if this be exhausted, the animal dies of cold (§ 117). 648. To recapitulate, then ; the sources of carbonic acid in the Ani- mal body are threefold.—1. The continual decay of the tissues; which is common to all organized bodies; which is diminished by cold and dryness, and increased by warmth and moisture ; which takes place with increased rapidity at the approach of death, whether this affect the body at large, or only an individual part; and which goes on un- checked, when the actions of nutrition have ceased altogether.—2. The metamorphosis, which is peculiar to the Nervous and Muscular tissues; which is the very condition of their activity; and which therefore bears a direct relation to the degree in which they are ex- erted.—3. The direct conversion of the carbon of the food into car- bonic acid ; which is peculiar to warm-blooded animals ; and which NATURE OF THE RESPIRATORY PROCESS. 371 seems to vary in quantity, in accordance with the amount of heat to be generated. 649. Now the function of Respiration has for its object, not merely to extricate the carbonic acid, which is generated in the system ; but likewise to introduce the oxygen, which is required to form that car- bonic acid ;—the proportion of oxygen in the tissues, and in the com- bustible materials of the blood, not being sufficient for this purpose. Hence it is not enough, that the carbonic acid should be removed; for this may be accomplished by causing an a'nimal to breathe an atmosphere which contains no oxygen. Any cold-blooded animal, such as a Frog or a Snail, may be kept in hydrogen or nitrogen for several hours or even days; and will give out, during that time, an amount of carbonic acid nearly as great as if it had been respiring atmospheric air. But the continued production of carbonic acid must have a limit, occasioned by the want of oxygen, and death will then supervene.—On the other hand, a supply of oxygen may be freely afforded ; and yet the presence of even a small amount of carbonic acid in the surrounding atmosphere (in addition to that which is nor- mally present in it, § 81) will impede the extrication of that substance from the blood; and if the excess be considerable, the carbonic acid will not be set free at all; so that the same injurious results follow, as if respiration were altogether prevented from taking place. 650. These two actions are accomplished by the very same act; advantage being taken of the property of" mutual diffusion," which is common to all gaseous substances that do not unite chemically with one another. In virtue of this property, Hydrogen, the lightest of gases, and Carbonic acid, one of the heaviest, when introduced into the same vessel, will be found in a short time to have uniformly mixed, notwithstanding the difference of their specific gravities, which are as 1 to 22. Now this intermixture will take place, when the two gases are separated by a porous septum; each gas passing towards the other, by an action resembling the Endosmose and Exosmose of liquids (§ 491). And it may also take place, when one of the gases is dif- fused through a liquid ; provided that the other gas is likewise capable of being absorbed by the liquid. In this manner, as already mentioned, the surface of venous blood, inclosed in a bladder, may be made to exhibit the arterial hue, by suspending the bladder in an atmosphere of oxygen ; for the carbonic acid of the blood, and the surrounding oxygen, will overcome by their mutual attraction the ob- stacle interposed by the bladder; and the former will be lifted out, so to speak, and will be replaced by the latter. It has been found by experiment, that the free carbonic acid diffused through blood, may be more completely extricated from the liquid, by exposingit to hydrogen, than by placing it under the vacuum of an air-pump; for in the latter case there is nothing to replace it, and the attraction between the gas and the liquid tends to resist the exhausting influence of the vacuum; whilst in the former, the blood receives one gas in exchange for the 372 ESSENTIAL STRUCTURE OF RESPIRATORY ORGANS. other, so that the wThole force of the tendency to mutual diffusion is exercised in lifting out the carbonic acid. 651. The immediate purpose of the organs of Respiration, then,— whatever may be the variety in their form,—is this: to expose the blood to the air, in a state of such minute division as to present a very extended surface, a thin membrane only being interposed between them. For this purpose we find a certain organ, or set of organs, specially set apart in all the higher animals; and this is formed by a prolongation of the general surface, either externally or internally, according to the mode in which the respiration is accomplished. Thus in Fishes and aquatic Mollusks, the blood is aerated by exposure, not directly to the atmosphere, but to the air which is dissolved in the water they inhabit; and the respiratory apparatus is formed in them of an extension of the external surface, at a particular part, into innu- merable delicate fringerlike processes, the gills (Fig. 96); every division of which con- tains a network of blood- vessels (Fig. 100): so that the amount of blood, which is exposed to the surrounding medium at any one time, is collectively very great, al- Doris Joimstoni, showing the tuft uf .xiemai giiis. though the quantity contained in each gill-filament is very minute. On the other hand, in all the air-breathing Vertebrata, the blood is exposed to the atmosphere, through the medium of an internal membranous prolongation, which is continuous with the mucous mem- brane lining the mouth and nostrils; this forms a pair of sacs, termed lungs, communicating wTith the back of the mouth by means of a tube called the trachea or windpipe, through which air is freely admitted to the cavities thus formed (Fig. 10L). The blood is minutely dis- tributed on the walls of these sacs by a close network of capillary vessels (Fig. 102); and not only on the external wralls, but also on numerous partitions, by which the cavities are subdivided with more or less minuteness, so as greatly to extend the vascular surface. 652. Such is the essential nature of the Respiratory apparatus; but in order that it may be carried into the vigorous operation, which is required in the higher classes of animals, various supplementary arrangements are made, for the purpose of promoting the due influence of the air upon the blood. In the first place, the capillary vessels of the respiratory surface are connected with arterial trunks, which issue immediately from the heart, and which thus convey a constant stream of blood from that organ ; whilst they give origin to venous trunks, which terminate directly in the heart, and which are ready to con- vey back to it the blood that has undergone aeration. Thus by the energetic action of the heart, and by the force generated in the capil- laries of the lungs (§ 598), a constant renewal is effected in the blood RESPIRATORY ORGANS IN MOLLUSKS. 373 which is exposed to the air through the medium of these organs. On the other hand, the renewal of the blood would be useless, unless a fresh supply of air were continually being introduced, and that which had been vitiated, by the loss of its oxygen and the admixture of carbonic acid, were removed ; and this is effected by a series of mus- cular movements, which are adapted for the alternate expulsion of the vitiated air from the lungs, and for the introduction of a fresh supply of pure air from the atmosphere. These movements are kept up by a certain part of the nervous system ; but they are not depend- ent upon any exertion of the will, for they continue during profound sleep, and in other states in which even consciousness is altogether suspended. 2. Different forms of the Respiratory Apparatus in the lower Animals. 653. Before proceeding to consider, in more detail, the structure and actions of the respiratory apparatus in Man, we may advan- tageously glance at the mode in which this function is effected in the lower animals.—In the lowest and simplest, which are inhabitants of the water, we do not find any special apparatus for the aeration of the fluids of the body; this being accomplished by the exposure of them to the surrounding medium, through the thin integument; and the interchange of the layer of water (holding air in solution) in contact with the aerating surface, is effected either by the general movement of the body, or by the action of cilia (§ 234), which produce the cur- rents necessary for this purpose. Not unfrequently, the internal sur- faces—such as the walls of the stomach and of other cavities,—seem as much concerned in this function as the external, or even more so; these cavities being distended with water taken in through the mouth, and this water being frequently renewed, by the ejection of that which has been vitiated, and by the introduction of a fresh supply. This is the case in the Sea Anemone, for example, and in many other Polypes ; and there are certain higher forms of the same class, in which there is a great dilatation of the pharynx, which seems peculiarly destined for the aeration of the fluids,—being filled with water, and then suddenly emptied, at tolerably regular intervals. 654. In the various classes of the Molluscous sub-kingdom, we find the respiration provided for, by the adaptation of distinct organs for the purpose. As most of the animals of this group are inhabi- tants of the water, the respiration is usually carried on by means of gills, rather than by any organs resembling lungs. The latter is found, however, in a few species; such as the Snail, Slug, and other terres- trial air-breathing Mollusks ; and usually consists of a simple cavity, situated in the back, communicating directly with the air through an aperture in the skin, and having its walls covered with a minute net- work of blood-vessels. The form and position of the gills differ extremely in the several classes of Molluscous animals. In the lowest, the respiratory surface is formed, as in the higher Polypes, by a dila- 374 RESPIRATORY ORGANS IN MOLLUSKS. tation of the Pharynx ; but sometimes, instead of surrounding a large cavity, it forms a special ribbon-like fold of membrane, passing from one end of it to the other, on which the blood is minutely distributed. In this group of animals, there is a regular system of canals for the conveyance of the blood; but these, in many parts of the system, and especially on the respiratory membrane, do not seem to be fur- nished with distinct walls, and are rather mere channels excavated in the tissues. And the circulation is liable to a continual change in its direction ; the blood being sometimes transmitted to the respiratory surface before it proceeds to the body, and sometimes after it has traversed the other tissues (§ 557). The water in contact with the respiratory surface is continually renewed by the action of the cilia, with which it is thickly covered. 655. In certain other Mollusks inhabiting bivalve shells, we find that the internal surface of the mantle or skin that lines the valves, is the special organ of respiration ; the external water having free access to these, by the separation of the skin along the edges of the valves, so that it enters the cavity in which the viscera are lodged, and bathes their exterior. But in most bivalve Mollusks, the internal surface of the mantle is doubled (as it were) into four ribbon-like folds, which are delicately fringed at their edges, and which have, in fact, the same essential structure as the gills of higher animals (§ 663). To these the blood is transmitted, when it has been rendered venous by tra- versing the vessels of the body generally ; and in these it is exposed, through a surface which is greatly extended by the rainute division of the fringes, to the action of water introduced from without, and constantly renewed by ciliary action. In many of these animals, as in the common Oyster, the two lobes of the mantle are so completely separated, that the water can still enter freely between the valves; but in general, they are more or less united, so that the cavity in which the gills lie is partially closed. There is always a provision, however, for the free access of water from without, by means of two apertures, one for its entrance and the other for its ejection; and in certain species which burrow deeply in sand or mud, these apertures are furnished with long tubes, or siphons, which convey the water from nearer the entrance of the burrow, and carry it thither again. In these, also, a continued flow of water over the respiratory surface is maintained by the vibration of the cilia, with which they are clothed. 656. The position of the gills, in the Mollusca of higher organiza- tion, is extremely variable. Sometimes they are disposed upon the external surface of the body, and form delicate leaf-like or arbores- cent appendages (Fig. 97); whilst in other cases they are enclosed in a special cavity or gill-chamber, to which water is freely admitted from without; a continual interchange being provided for, either by ciliary action, or by muscular movements specially adapted for the purpose. The blood is conveyed to them, after having become venous in traversing the capillaries of the general system, by means RESPIRATORY ORGANS IN WORMS AND CRUSTACEA. 375 Fig. 97. One of the arborescent pro- cesses, forming the gills of Doris Johnsloni separated and enlarged. of large channels and sinuses excavated in the several parts of the body (§ 556); and after being aerated in the gills, it returns to the heart, to be again conveyed to the system. In the Cuttle-fish tribe, there are supplementary hearts at the origin of the branchial arteries,—or vessels that distri- bute blood to the gills; and these have evi- dently for their purpose, to render the respira- tory circulation more energetic, and thus to increase the aeration of the blood, in the de- gree required for the vigorous habits of these animals, which present a remarkable contrast to the sluggish inert character of the Mollusca in general.—In these classes, taken as a whole, the respiration is low in its amount. The blood contains no red corpuscles, excepting perhaps in the highest class ; and the change in its composition, which is effected by the air, is confined, therefore, to the fluid plasma, or liquor sanguinis. And as it is not exposed directly to the air, except in a few species, but to the air contained in the water inhabited by the animals, this change cannot be very energetically performed. But as the life of these animals is chiefly vegetative,—as their movements, except in the highest classes, are few and feeble,—and as they maintain no independent heat,—there is but little need of the interchange, which it is the object of the re- spiratory process to effect; and these animals can sustain the complete suspension of it for a long time. 657. Among many of the Articulated tribes, the respiration is car- ried on upon a similar plan. In some of the lowest, such as the Tape- worm of the intestinal canal, there is no special provision for the aeration of the fluids; the soft integument permitting the extrication of carbonic acid, and imbibition of oxygen, in the required degree. This is but very small, however; the life of these animals being almost purely vegetative. In the Marine Worms, which constitute a numerous and interesting group, endowed with considerable loco- motive powers, and leading a life of almost constant activity, there is, on the other hand, a special provision for this function; the blood being transmitted, in the course of its circulation, to a series of gill- tufts, which are composed of a delicate membrane prolonged from the external surface of the body, and which sometimes have the form of branching trees, and sometimes of delicate brushes made up of a bun- dle of distinct filaments. In either case, the filaments are traversed by blood-vessels, and are adapted to bring the blood into close rela- tion with the surrounding water; and the continual interchange of the latter is provided for by the restless movements of the body. The tufts are sometimes arranged along every segment of the body; and their multiplication prevents them from individually attaining any considerable size. In other cases, they are disposed at intervals; 376 RESPIRATORY ORGANS IN CRUSTACEA AND INSECTS. and they are then larger, being less numerous. Their most beautiful development is where they are present on the head only, the rest of the body being enclosed in a shelly or sandy tube, as in the Serpula and Terebellce. The gill-tufts then frequently present the appearance of a flower, endowed, when alive, with the most brilliant and delicate hues. In many animals of this group, there is a small supplementary heart at the base of every one of the vessels, that distribute the blood to the gills; and this is obviously designed to aid in the respiratory circulation, for which the feeble action of the dorsal vessel would not furnish sufficient power (§ 552). 658. The higher Articulated classes are, for the most part, adapted to atmospheric respiration, on the plan to be presently explained ; but there is one class, that of Crustacea, whose respiration is still carried on through the medium of water. In the lowest forms of this group, there is no special respiratory apparatus; the general surface being soft enough to admit of the required aeration of the fluids through its own substance, and the animal functions being performed with so little activity, that a very small amount of interchange is required. In the higher orders, however, whose bodies are encased within a hard envelop, we find external gills, like those of many Mollusks; and these are attached to the most movable parts of the body,—one or more pairs of legs being in some instances kept in con- stant agitation, for the purpose of producing currents in the surround- ing fluid, that may serve for the aeration of the blood. In the Crab- tribe, which constitutes the highest family of this class, the gills are themselves enclosed within a cavity, formed by a sort of doubling of t the hard integument of the under side of the body ; and a constant stream of water is maintained through this, by means of a peculiar valve, situated in the exit-pipe ; the continual movement of the valve causing a regular stream of water to issue from the gill-chamber, and thus occasioning the entrance of a constantly fresh supply. In these, also, we find a dilatation, the walls of which seem to have contractile powers, at the commencement of each artery that distributes the blood to the gills ; and this collects the venous blood from the various channels, in which it has meandered through the body. It is by the enclosure of the gills within a cavity, and by the consequent protec- tion of them from the drying influence of the air (which would prevent their function from being duly performed), that Crabs and other allied species are enabled to live for a considerable time out of water; and the Land-Crabs, as they are termed, are adapted to spend the greater part of their lives at a distance from the sea, by means of a special glandular apparatus within the gill-cavity, which secretes a fluid that preserves the surface of the gills in the moist condition, requisite for the aeration of the blood through its membrane. Thus the Land- Crabs are air-breathing animals (except at certain seasons, when they frequent the sea-shores), although they breathe by gills. 659. In Insects and other proper air-breathing Articulata, however the character of the respiratory apparatus is very different. The transi- RESPIRATORY ORGANS IN INSECTS. 377 tion from one form to the other is effected through such animals as the Leech and the Earthworm, which seem able to breathe either air or water. These are furnished with a series of small sacs, disposed at regular intervals along each side of the body, and opening by a row of pores, which are termed spiracles or stigmata. The blood- vessels are distributed upon the walls of these sacs; and either air or wrater may be introduced into their interior, by the movements of the body, which are adapted to compress their walls, and then to allow them to dilate. In the Centipedes and their allies, these air- sacs send out prolongations; which have not, however, any very ready communication with each other. But in insects, the spiracles, instead of forming the en- trances to so many distinct sacs, open into a pair of large, tubes, one of which traverses the body on either side, along its whole length. These tubes, termed trachece, have many communications with each other across the body; and they branch out into innumerable prolonga- tions, the ultimate ramifica- tions of which are distributed to every portion of the sys- tem. They occasionally present dilatations of con- siderable size (Fig. 98, a.); especially in the thoracic region of the body, in those insects, which are endowed with great powers of flight. These dilatations or air-sacs appear destined to serve as reservoirs of air, during the time that the insect is upon the wing, its spiracles being then partially closed ; and they may also be useful in diminish- ing the specific gravity of the body. The air-tubes are prevented from having their cavity obliterated through the pressure of the sur- rounding parts, by means of an elastic spiral fibre; which winds round them, between their outer and inner membrane, from one ex- tremity to the other (Fig. 98, b.) ; and which answers the purpose of the cartilaginous rings and plates, in the trachea and bronchi of air- breathing Vertebrata. 660. In this manner, the air that is introduced through the spiracles is carried into every part of the body, and is brought into immediate relation with the tissues to be aerated; so that the carbonic acid which they set free is communicated at once to the atmosphere, in- stead of being taken up by the blood ; and the oxygen they require is imbibed in the same manner. And thus we see how the respiration of this interesting class, which is unequaled for its energy when the body is in a state of activity, is provided for without an active circu- lation of blood, and without the presence of red corpuscles,—which Respiratory apparatus of insects:—a, air vesicles and part of tracheal system of Scolia hortorum. b, portion of one of the great longitudinal tracheae of Carabtcs auratus, with one of its spiracles. 378 COMPARATIVE FORMS OF RESPIRATORY APPARATUS. elsewhere seem to be essential conditions of the interchange of oxygen and carbonic acid between the air and the tissues, wherever this takes place to any great extent. 661. In the Spider tribe, we return to a more concentrated form of the respiratory apparatus ; but, notwithstanding that it is limited within much narrower dimensions externally, it exposes a very large amount of surface on its interior. It consists of a series of sacs, much less numerous than in the lower Articulata, and not communicating with each other. Their lining membrane, however, is doubled into a series of folds, which lie in proximity with each other, like the leaves of a book, and which thus present a very extensive surface within a very small space. Over this surface, the blood is distributed in a minute capillary network; and thus it comes into immediate re- lation with the air, which is received into the cavity through its aper- ture or spiracle. The alternate admission and expulsion of air seem to be provided for, as in Insects, by movements of the body, which first empty the cavities or air-tubes by compression, and then allow them to be re-filled by their own elasticity, the pressure being relaxed. The respiratory cavities in the Spider-tribe have received the name of pulmonary branchice; from their analogy, on the one hand, with the lungs of higher animals ; and, on the other, with the branchial or gill- cavity of the higher Crustacea, the gills in which are formed by pro- longations of the lining membrane, like the leaf-like folds in the air- cavities of the Spider-tribe. 662. The accompanying diagram will give an idea of the relations of these different forms of the respiratory apparatus, amongst themselves, and to that of Vertebrata. Let the line a b represent the general Diagram illustrating different forms of the Respiratory apparatus:—o, simple leaf-like eill; 6, simple respiratory sac; c, divided gill; d, divided sac; e, pulmonary branchia of Spider. surface of the animal; the continuations of that line on its upper side being its external prolongations; and those below, its internal pro- longations or reflexions. Now at a is seen the character of the simple foliaceous or leaf-like gill, such as is found in the lower aquatic ani- mals ; presenting merely a flat expanded surface in contact with the water, over which the blood may be distributed. At b is shown a correspondingly simple inversion ; such as that which forms the re- spiratory sac of the leech, having the blood-vessels distributed upon its walls. A higher form of the gill, such as is found in fishes and RESPIRATORY ORGANS OF FISHES. 379 in the higher aquatic Invertebrata, is seen at c; the surface being greatly extended, by subdivision into minute filaments. A more complex form of the pulmonary apparatus, such as is found in the higher Vertebrata, is shown at d; the blood being distributed, not merely to its outer walls, but to the minute partitions which subdivide its cavity into cells. And at e is represented the respiratory organ of the Spider-tribe ; which bears an obvious resemblance to the lung of the Vertebrated animal, shown at d ; whilst it is evidently as nearly allied to the gill shown at c, provided this be imagined to be sunk within a cavity formed by a depression of the external surface, instead of projecting beyond it.—Thus we see how very close is the real resemblance between all the forms of the respiratory apparatus ; how- ever unlike each other they may at first sight appear to be. 663. The gills of Fishes correspond with those of the higher Mol- lusca in all essential particulars; but they are more largely developed in proportion to the size of the body; and they are placed in a situa- tion that enables them to receive a more regular and constantly- changed supply, both of blood and water. The gills are suspended to bony or cartilaginous arches, of which three, four, or more, are fixed on either side of the neck; and the fringes hang loosely within a cavity, which communicates on the one hand with the mouth, and on the other with the exterior of the body. The mechan- ism of respiration is very complex in these animals; and is evidently adapted to produce the most effectual aeration possible. The mouth is first distended with water; and its muscles are then thrown into contraction, in such a manner as to expel the water, through the aperture on either side of the pharynx, into the gill-cavity. At the same time, the bony arches are lifted and separated from each other, by the action of muscles espe- cially adapted to this purpose; so that the gill-fringes may hang freely, and may present no obstacle to the flow of the water between them. When they have been thus bathed with the aerating liquid, and their blood has under- gone the necessary change, the water is expelled through the out- ward aperture on each side of the back of the neck; which is fur- nished with a large flap or valvular cover termed the operculum. In some of the cartilaginous Fishes, each branchial arch is inclosed in a separate cavity; which communicates on the inner side with the pha- rynx by an orifice peculiar to itself, and by another orifice with the external surface. Thus there is a series of external openings, instead of a single one, on each side of the neck; and these sometimes amount to six or seven, as in the Lamprey, reminding us of the spiracles of Articulated animals ; whilst there is a corresponding series of internal Fig. 100. Capillary network of a pair of leaflets of the gills of the Eel:—a, a, branches of the branchial artery conveying venous blood; b, b, branches of branchial vein, returning aerated blood. The disappear- ance of the dark shading in the network, as it traverses the gill, is designed to indicate the change in the character of the blood, as it passes from one side to the other. 380 RESPIRATORY ORGANS OF FISHES. openings into the pharynx on either side, or into a tube that commu- nicates with it. 664. It is well known, that most Fishes speedily die when removed from the water ; and it can be easily shown, that the deficient aeration of the blood is the immediate cause of their death. But as it might have been expected, that the atmosphere would exert a much more energetic influence upon the blood contained in the gills, than that which is exercised by the air contained in the water, the question naturally arises, how this deficient aeration comes to pass. It is chiefly due to the two following causes; the drying up of the mem- brane of the gills themselves, where it is exposed to the air, so that the aeration of the blood is impeded;—and the flapping together of the filaments of the gills, which no longer hang loosely and apart, but adhere in such a manner as to prevent the exposure of the greater portion of their surface to the air. Those fishes live longest out of water, in which the external gill-openings are very small, so that the gill-cavity may be kept full of fluid; and there are certain species which are provided like the Land-crab, with a particular apparatus for keeping the gills moist, and which perform long migrations over land in search of food, even (it is said) ascending trees. These are exceptions to the general rule. 665. The respiration of Fishes is much more energetic than that of any of the lower aquatic animals; and this is partly due to the great extension of the surface of the gills, partly to the provision just explained for maintaining a constant flow of fresh water over their surface, and partly to the position of the heart at the base of the main trunk that conveys the blood to the gills (§ 558), by which the regu- lar propulsion of that fluid through these organs is secured. Their blood, too, is furnished with red corpuscles; which give important aid in conveying oxygen from the gills to the remote tissues of the body, and in returning the carbonic acid to be excreted. The pro- portion of these varies considerably, in the different species of the class, being very small in those that approach most nearly to the Invertebrata; and there is even an entire absence of them in one remarkable fish, the Amphioxus or Lancelot; whilst they are pre- sent in large numbers in the blood of certain Fishes, which have great muscular activity, and can maintain a high independent tem- perature. 666. It would seem, however, that not even this high amount of respiration is always sufficient for Fishes, which live in small col- lections of water, where their temperature is liable to be greatly augmented by the heat of summer; under .which condition, there is an increased proneness to disintegration in their tissues, and a cor- responding necessity for the extrication of carbonic acid and for the absorption of oxygen. Many fresh-water fishes under such circum- stances, may be seen to come to the surface and to swallow air; and it would seem as if the interior of the intestinal canal then served the purpose of a respiratory surface, the air being expelled from the anus RESPIRATORY ORGANS OF FISHES AND REPTILES. 381 deprived of a large part of its oxygen, and highly charged with car- bonic acid. 667. In addition to their apparatus for aquatic respiration, many Fishes are provided, in their air-bladder, with the rudiments of the air-breathing apparatus of higher animals ; although it is only in cer- tain species, which approach Reptiles in their general organization, that this really affords any aid in the aeration of the blood. The air-bladder in its simplest condition is entirely closed ; and it is then obviously incapable of taking any share in the respiratory function, although it seems to be an organ of some importance to the animal, in regulating its specific gravity, and altering its position in the water. In other cases, it communicates with the intestinal tube by a short wide canal, termed the ductus pneumaticus; and this may serve to admit air, which is taken into the alimentary tube by the process of swallowing just mentioned. In the Sauroid Fishes, just adverted to, the air-bladder forms a double sac, which is evidently the repre- sentative of the double-lung of the air-breathing Vertebrata; and it communicates with the back of the mouth by a regular trachea or windpipe, which has a muscular valve at its commencement, serving to open or close its orifice. Some of these fishes are able to live for a considerable time out of water, their respiration being maintained by these rudimentary lungs; and they can also make a hissing sound, by the expulsion of the air-sacs through the narrow glottis, or entrance to the trachea. 668. The condition of the Respiratory apparatus, and the mode in which the function is performed, in the class of Reptiles, are peculiarly interesting; as it is in this class, that we first meet with the complete adaptation of the Vertebrated structure to the aeration of the blood by the direct influence of the atmosphere. Their general habits of life require but a very feeble amount of aeration, especially at moderate temperatures; their muscular and nervous systems being usually exer- cised in a very low degree; their movements being sluggish, and their perceptions obtuse. In fact, they may be considered, on the whole, as the most vegetative of all Vertebrated animals. In accordance with this character, the lungs are so constructed, as not to expose any very large amount of blood to the air at any one time; and, as we have already seen (§ 563), only a portion of the stream of the circu- lation is diverted to the lungs; the main current being sent to the system with only that amount of aeration, which it has derived from the admixture of the portion of blood that has been aerated in the lungs, with the venous current that has last been returned from the system. 669. The lungs of Reptiles are, for the most part, capacious sacs, occupying a considerable part of the cavity of the trunk; but they are very slightly subdivided, so that the amount of surface they can expose is really small. Where any subdivision exists, it is usually at the upper extremity of the lung, near the point of entrance of the bronchial tube; and where there is no actual subdivision of the cavity, 382 RESPIRATORY ORGANS OF REPTILES. we usually find that its surface is extended in this situation, by the formation of a number of little depressions or pouches in its walls, upon which the blood-vessels are minutely distributed. The greatest amount of subdivision is seen in the lungs of the Turtle tribe; but even in these, the partitions scarcely form a complete division at any part of the lungs; and the ultimate air-cells are of very large size. The air-sacs of Reptiles are not filled, like those of Mammalia, by an act of inspi- ration, but by a process of swallowing, which is comparatively tedious ; and, from the small amount of aerating surface, in proportion to the amount of air thus re- ceived into the cavity, one inflation of the air-sacs lasts for a considerable.time. When the replacement of oxygen by carbonic acid has proceeded to an extent that renders the air no longer fit to remain in the lungs, these cavities are emptied by pressure exercised upon them by the muscles of the trunk; and the slow exit of the air through the narrow glottis is accompanied by a prolonged hiss- sectionofthe Lung of the Turtle. ™g sound, which is the only sort of voice that is possessed by the greater part of the Reptile class. The lungs are again filled by the swallowing-process; and all goes on as before. 670. Now in the Frog tribe, which forms the lowest order of Rep- tiles (and which is sometimes ranked as a distinct class, under the title of Amphibia), the respiration, during the early or Tadpole state, is aquatic ; being carried on by means of gills, and conducted exactly upon the plan of that of Fishes. The lungs are not developed, until a period long subsequent to the animal's emersion from the egg; and as soon as they are ready to come into play, an alteration begins to take place in the circulating system, by which the current of blood is diverted towards them, and away from the gills (§ 562). This change takes place to its full extent in the Frog, Toad, Newt, and their allies ; which henceforth have a respiration and a circulation exactly analogous to that of Reptiles in general; but it is checked in the Proteus, Siren, and other species, which form the perenni- branchiate group,—so called from the persistent character of their gills, which still remain in action, the lungs never being sufficiently developed to maintain the respiration by themselves. The curious influence which Light possesses on this metamorphosis, has been already referred to (§ 95). 671. This order Batrachia is further distinguished from other Rep- tiles, even when the metamorphosis is complete, by the softness and nakedness of the skin; which is destitute of the scales and horny plates, that cover it in the Lizards, Serpents, and Tortoises. The RESPIRATION IN REPTILES AND BIRDS. 383 skin of the Frog tribe is a very important organ of respiration; being richly supplied with blood-vessels; and exposing their contents to the influence of the air, under circumstances nearly as favourable as those afforded by the imperfectly-developed lungs of these animals. Thus a Frog, from which the lungs have been removed, will live a consider- able time at a moderate temperature, if its skin be freely exposed to a moist air; for in consequence of the peculiar mode in which the circulation is carried on in these animals (§ 562), the interruption to the flow of blood through the lungs does not (as in the higher classes) produce a stagnation of the general current through the body; and the blood receives, in its course through the skin, a sufficient amount of aeration for the support of life. Indeed at a low temperature, the influence of water on the skin is sufficient (by means of the air included in the liquid) to remove the small araount of carbonic acid then ready for excretion, and to supply the requisite amount of oxygen; and Frogs may thus live beneath the water for any length of time, without coming to the surface to breathe. But with the rise of the temperature of their bodies, their blood requires a higher degree of aeration; and they then come to the surface to take in air by the mouth, which aerates the blood through the lungs. It appears that, during the heat of summer, the pulmonary respiration, and the influ- ence of the water on the skin, are not sufficient; as it is found that Frogs die, if they are confined to the water under such circumstances, —their natural habit being to quit the water at such times, so that the air may exert its full influence on their skin as well as on their lungs. They do not, however, quit the neighbourhood of water, and soon die if exposed to a dry atmosphere; for if the skin become dry, its aerating function can be no longer performed. The same result happens, if the passage of gases through the skin be impeded by smearing it over with any unctuous substance. We shall presently find reason to believe, that this cutaneous respiration is a very im- portant part of the function, even in Man and the Mammalia. 672. The class of Birds presents a most striking contrast to that of Reptiles, in regard to the energy of the respiratory function, and the extent of the apparatus destined to its performance. The lungs in these animals undergo a minute subdivision; so that the extent of surface, over which they expose the blood to the air, is greatly in- creased. But this subdivision is not carried to the same degree of minuteness as it is in Mammalia; and the required extent of surface would not be afforded by the lungs alone. In addition to these organs, we find large air-sacs, communicating with them, disposed in different parts of the body,—such as the abdominal cavity, the interspaces among the muscles, the spaces between the muscles and the skin, &c. These very greatly increase the respiratory surface; their lining mem- brane being extremely vascular, and adapted to expose the blood to the influence of the air. In most Birds, the bones themselves are hollow; and the lining membrane of their cavities serves as an addi- tional aerating surface, the air being introduced into the interior of 3S4 RESPIRATION IN BIRDS. the bones, by canals that communicate directly with the lungs. So free is this communication, that the respiration has been known to be maintained through the fractured humerus of an Albatross, when an attempt was made to destroy the bird by compressing its trachea. Thus the respiratory surface is extended into the remoter parts of the system, very much as in Insects; and the hollowness of the bones, together with the presence of numerous air-sacs in different parts of the body, contribute to diminish its specific gravity. The large quan- tity of air thus included in different portions of the frame, also serves, like that contained in the air-sacs of Insects, as a reservoir for the supply of the principal aerating organs during active flight, when the respiratory movements are less free. 673. The mechanism of Respiration in Birds is very different from that which produces the respiratory movements in Mammalia. The cavities of the chest and thorax are not yet separated by a diaphragm ; except in a very small number of species, that approach most nearly to the next class. But, on the other hand, the wThole cavity of the trunk is more completely enclosed in a bony casing; the ribs being connected with the sternum by osseous prolongations from the latter, instead of by cartilages; and the sternum itself being so largely de- veloped, as to cover almost the entire front of the body. Now the natural condition of this bony framework is such, that when no pres- sure is made upon it, the cavity it encloses is in a state of distension ; and the state of emptiness can only be produced by a forcible com- pression of the framework, through an exertion of muscular power. The lungs, instead of being freely suspended in the cavity of the chest, as in Mammalia, are attached to the ribs; and their own tissue is endowed with a degree of elasticity, which causes them to dilate when they are permitted to do so. In the state of distension, there- fore, which is natural to the cavity of the trunk, the lungs are ex- panded, and fill themselves with air, which they draw in through the trachea; and this condition they retain, until, by the action of the external muscles upon the bony framework, the cavity of the trunk is diminished, and the air is expelled from the lungs and air-sacs, which are again filled as soon as the pressure is taken off".—As the air-sacs chiefly communicate with the part of the lungs that is most distant from the trachea, the air has to traverse the whole extent of these last organs, both when it is being drawn into the air-sacs, and when it is being expelled from them ; so that it is made to serve for the aeration of the blood in the most effectual manner. 674. Thus, although the respiratory apparatus of Birds does not possess the highly-concentrated development, which we shall find it to present in Mammals, it serves, by the extension of the aerating surface through the body, to bring the air and the blood into most intimate relation; and the energy of the function is further provided for, by the mode in which the pulmonary circulation is carried on (a distinct heart, as it were, being provided for it, § 564), as well as by the arrangement of the blood-vessels, which transmit to the respira- RESPIRATION IN BIRDS AND MAMMALS. 385 tory organs the whole of the blood, that has been returned in a car- bonated state by the great veins of the system. The very large pro- portion of red corpuscles contained in the blood, gives additional effect to these provisions. The very high amount of respiration which is natural to Birds, and which cannot be suspended even for a short time without fatal consequences, has a direct relation (as already explained) with their extraordinary muscular activity; and with the high bodily temperature, which they are fitted to maintain, and which cannot be lowered in any great degree without the suspension of their other functions. Birds are peculiarly susceptible of impurities in the atmosphere; and it has been shown by experiment, that if a Bird, a Mammal, and a Reptile, be placed together in a limited quantity of air, which gradually becomes vitiated by their respiration, the Bird will die first, the Mammal next, and the Reptile last. Or if the Bird be placed alone in a limited quantity of air, and be left until the atmosphere is so vitiated as to be no longer capable of supporting its life, a Mammal will still live for a time in that atmosphere ; and when it is no longer fit to sustain the life of the Mammal, the Reptile may still breathe it without injury for a considerable period. There is strong reason to believe, indeed, that in former epochs of the Earth's history, when the Reptile class was predominant, supplying the place of Mammals on land, and of Birds in the air, the atmosphere was so highly charged with carbonic acid, as not to be capable of sustaining the life of the higher air-breathing Vertebrata. 3. Mechanism of Respiration in Mammalia and in Man. 675. It is in the class of Mammalia, that we find the Respiratory apparatus presenting its highest degree of concentration; and the arrangements for its action the most complete. The lungs are divided into cavities of extreme minuteness; so that the extent of surface which they expose is enormously increased. These cavities, or air- cells are all connected with the trachea, by means of the bronchial tubes and their minute subdivisions; but on account of the minute- ness of these passages, a considerable force would be required to inflate the air-cells with air, if their distension were to be accom- plished by the propulsion of air through the trachea, as we have seen to be the normal mode of inspiration in Reptiles. Moreover, if the air were introduced in this manner, the air-cells would be the last por- tions of the pulmonary structure that would be distended by it; as well as the first to be emptied, when the air is forced out again by external pressure. The mechanism of Respiration in Mammalia, however, is so arranged, that the air is most effectually drawn into the lungs; in- stead of being forced into them; and the distension of the air-cells is far more complete than it could be rendered in the latter method, besides being accomplished in a much shorter time. 676. The general principle of the operation is this. The lungs are suspended in a cavity that is completely closed; being bounded above 25 386 MECHANISM OF RESPIRATION IN MAMMALS. Fig. 102. and around by the bony framework of the thorax, the interspaces of which are filled up by the muscles and membranes; and being entirely cut off frora the abdomen below, by the diaphragm. Under ordinary circumstances, the lungs completely fill the cavity ; their external sur- face, covered by the pleura, being everywhere in contact with the pleural lining of the thorax. But the capacity of the thoracic cavity is susceptible of being greatly altered by the movements of the ribs, and by the actions of the diaphragm and abdominal muscles ; as will presently be explained in more detail. When it is diminished, the lungs are compressed, and a portion of the air contained in them is expelled through the trachea. On the other hand, when it is increased the elasticity of the air within the lungs causes them immediately to dilate, so as to fill the vacuum that would otherwise exist in the tho- racic cavity; and a rush of air takes place down the air-tubes, and into the remotest air-cells, to equalize the density of the air they in- clude (which has been rarefied by the dilatation of the containing cavities) with that of the surrounding atmosphere. 677. The diameter of the ultimate air-cells of the Human lung varies from about the l-200th to the l-70th of an inch. Their shape is irregular; and their walls are, for the most part, flattened against each other. Each of the ultimate ramifications of the bronchial tubes communicates with a cluster of these air-cells grouped around it; those which are in immediate proximity with the tube open into it by well-defined circular apertures ; and the others com- municate with it by opening into these and into each other. Each air-cell is lined by an extension of the mucous mem- brane from the bronchial tubes, and this is covered by a deli- cate pavement-epithelium. Be- tween the adjacent air-cells, is a network of fibrous tissue, that forms the connecting me- dium by which they are held together ; this tissue appears to be of the elastic kind. The pulmonary arteries subdivide into branches, whose ultimate ramifica- tions form an extremely minute capillary plexus; and this is disposed between the walls of the adjacent air-cells, so that each portion of this plexus comes into relation with the air (through the lining mem- brane of the contiguous air-cells) on both sides,—an arrangement which is obviously the most favourable that can be to the aeration of the contained blood. It has been calculated by M. Rochoux, that the number of air-cells grouped around each terminal bronchus is little less than 18,000; and that the total number in the lungs amounts Arrangement of the Capillaries of the air-cells of the Human Lung. STRUCTURE OF THE LUNGS IN MAN. 387 to six hundred millions. If this estimate be even a remote approxi- mation to the truth, it is evident that the amount of surface exposed by the walls of these minute cavities, must be very many times greater than that of the exterior of the body. 678. The larger bronchial tubes are more or less cartilaginous; but the smaller branches do not possess any such deposit in their walls, though still retaining their circular form. We find in the latter a fibrous structure, which seems to possess the properties of non-striated muscle; and by this, the diameter of these tubes appears to be gov- erned. The contractility of the walls of the smaller bronchi may be excited by chemical, electrical, or mechanical stimuli applied to themselves; though it is not readily caused to manifest itself by sti- mulating the nerves. By the continued influence of galvanism, bronchial tubes of a line in diameter have been made to contract, until their cavity was nearly obliterated. What purpose this contractility may serve, during the ordinary actions of the lungs, it is not easy to say; but it manifests itself strongly in certain diseased conditions, especially in Spasmodic Asthma, which appears essentially to consist in a contracted state of the smaller bronchial passages, occasioning an interruption to the passage of air through them. It is interesting to observe, that the contractility of the muscular walls of these tubes has been experimentally found to be greatly diminished by the appli- cation of vegetable narcotics, especially stramonium and belladonna, —substances which are well known to have a powerful remedial in- fluence in spasmodic Asthma. 679. The Lungs themselves appear to be, almost entirely, passive instruments of the Respiratory function. Their contraction when over-distended, and their dilatation after extreme pressure, may be partly due to the elasticity of their structure ; which seems to produce, when acting by itself, a moderately distended state of the air-cavities. This, too, is the condition that seems most natural to the cavity of the chest; the fullest dilatation, or the most complete contraction, of which it is capable, being only accomplished by a forcible effort. 680. The dilatation of the cavity of the chest, which constitutes Inspiration, is accomplished by two sets of movements;—the elevation of the ribs,—and the depression of the diaphragm. From the peculiar mode in which the ribs are articulated with the spinal column at one extremity, and from the angle which they make with the cartilages that connect them to the sternum at the other, the act of elevation tends to bring the ribs and the cartilages more into a straight line, and to carry the former to a greater distance from the median plane of the body, whilst the sternum is also thrown forwards. Consequently the elevation of the ribs increases the capacity of the thorax, upwards, forwards, and laterally. The movement is chiefly accomplished by the Scaleni muscles, which draw up the first rib ; and by the Inter- costals, which draw the other ribs into nearer proximity with each other, so that the total amount of movement in each rib increases as we pass from above downwards,—every one being drawn up by its 388 MECHANISM OF RESPIRATION IN MAN. connection with the one above it, and being drawn nearer to it by the action of its own intercostals. The elevation of the ribs is further assisted by the serratus magnus, and by other muscles connected with the spine and the scapula ; and when the respiratory movement is very forcibly performed, the scapula is itself drawn upwards, by the mus- cles that descend to it from the neck, thus producing an increased elevation of the ribs, and an unusual enlargement of the upper part of the thoracic cavity.—When the respiratory action is to be per- formed, the descent of the ribs is occasioned by the muscles of the spine and abdomen, which proceed upwards from the lower part of the trunk; and this action is aided by the elasticity of the costal car- tilages. 681. In the ordinary act of inspiration, however, the diaphragm performs the most important part. The contraction of this muscle changes its upper surface, from the high arch that it forms when re- laxed and pushed upwards by the viscera below, to a much more level state; though it never approaches very closely to a plane ; being somewhat convex, even when the fullest inspiration has been taken. When thus drawn down, it presses upon the abdominal viscera, and causes them to project forwards, which they are allowed to do, by the relaxation of the abdominal muscles. In tranquil breathing, this action is alone nearly sufficient to produce the requisite enlargement of the thoracic cavity; the position of the ribs being very little altered. In the expiratory movement, the diaphragm is altogether passive ; for, being in a state of relaxation, it is forced upwards by the ab- dominal viscera, which are pressed inwards by the contraction of the abdominal muscles. These last, therefore, are the main instru- ments of the expiratory movement; diminishing the cavity of the chest by elevating its floor, at the same time that they draw its bony frame- work into a narrower compass. 682. In this manner, by the regularly alternating dilatation and contraction of the thoracic cavity, the air within the lungs is alter- nately increased and diminished in amount; and thus a regular ex- change is secured. This exchange, however, can only affect at any one time a certain proportion of the air in the lungs; thus it is pro- bable, that the quantity remaining in these organs after ordinary expi- ration is above 100 cubic inches, whilst the amount usually expired is not above 20 cubic inches. Indeed if it were not for the tendency of gases to mutual diffusion, the air in the remote air-cells might never be renewed.—If any aperture exist, by which air could obtain direct access to the pleural cavity, the lungs would not be dilated by its enlargement; for the vacuum would be supplied much more rea- dily, by the direct ingress of the air (provided the aperture be large enough), than by the distension of the lung. Thus a large penetrat- ing wound of the thoracic cavity may completely throw out of use the lung of that side; and the same result will follow, when an aper- ture forms by ulceration in the substance of the lung itself, establish- ing a free communication between the pleural cavity and one of the MECHANISM OF RESPIRATION IN MAN. 389 bronchial tubes ; so that, of the air which rushes in by the trachea, to compensate for the enlargement of the thoracic cavity, a great part goes at once into that cavity, without contributing to the distension of the lungs, and therefore without serving for the aeration of the blood. 683. The number of the respiratory movements (that is, of the acts of inspiration and expiration taken together) may be probably esti- mated at from 14 to 18 per minute, in a state of health and of repose of body and mind. Of these, the greater part are moderate in amount, involving little movement except in the diaphragm ; but a greater exertion, attended with a decided elevation of the ribs, is usually made at every fifth recurrence. The frequency of the respiratory movements, however, is liable to be greatly increased by various causes, such as violent muscular exertion, mental emotion, or quick- ened circulation; whilst it may be diminished by torpidity of the nervous centres, on which the movement depends,—as we see in apoplexy, narcotic poisoning, &c. An acceleration seems very con- stantly to take place in diseases, which unfit a part of the lung for the performance of its function ; and the rate bears a proportion to the amount thus thrown out of use. Thus, the usual proportion between the respiratory movements and the pulse being as 1 to 4| or 5, it may become in Pneumonia as 1 to 3, or even in severe cases as 1 to 2; the increase in the number of respiratory movements being much greater in proportion, than the augmentation of the rate of the pulse. But it must be remembered by the practitioner, that a simply hysterical state may produce, in young females, an extraordinary acceleration of the respiration; the number of movements being sometimes no less than 100 per minute. There will be a great increase, also, in the number of inspirations, when the regular movements are prevented from being fully performed, by any cause that affects their mechanism, even whilst the lungs themselves are quite sound. Thus in inflammation of the pleura or pericardium, or in rheumatic affections of the intercostal muscles, the full action of the ribs is prevented by the pain which the movements produce; and the same is the case in regard to the dia- phragm, when the peritoneum or the abdominal viscera are affected with inflammation. Under such circumstances, there is an involuntary tendency to make up for the deficiency in the amount of the respira- tory movements, by an increase in their number. 684. The combined actions of the respiratory muscles, which have been now explained, belong to the group termed reflex; being the result of the operation of a certain part of the nervous centres, which does not involve the will, or even sensation, and which may continue when all the other parts of the nervous centres have been removed. In the Invertebrated Animals, we commonly find a distinct ganglionic centre set apart for the performance of the respiratory movements; and the division of the nervous centres in Vertebrated animals, which is the seat of the same function, may be clearly marked out, although it is not so isolated from the rest. It is, in fact, that segment of the medulla oblongata and upper part of the spinal cord, which is con- 390 MECHANISM OF RESPIRATION IN MAN. nected with the 5th, 7th, and 8th pairs of cephalic nerves, and with the phrenic. The entire brain may be removed from above (by suc- cessive slicing), and the whole spinal cord may be destroyed below ; and yet the respiratory movements of the diaphragm will still con- tinue,—those of the intercostal and other muscles being of course suspended, by the destruction of a portion of the cord, from which their nerves arise. But if the spinal cord be divided, between the point at which it receives the 5th and 8th pairs of nerves, and that at which it gives origin to the phrenic, the movements of the dia- phragm immediately cease ; and this is the reason why death is so instantaneous, in cases of laxation or fracture of the higher cervical vertebras, causing pressure upon the spinal cord just below its exit from the cranium; whilst if the injury take place belowT the origin of the phrenic nerve, life may be prolonged for some time. 685. The Respiratory movements, like other reflex actions (§ 394), depend upon a stimulus of some kind, originating at the extremities of the nerves, propagated towards the centre by the afferent trunks, and giving rise to a motor impulse, which is transmitted along the efferent or motor nerves to the muscles, and which occasions their contraction. Now the importance of the respiratory function to the maintenance of life, which has already been sufficiently pointed out, necessitates an ample provision for its due performance; and thus we find that the stimulus for the excitement of the movements may be trans- mitted through several channels. Its chief source, no doubt, is in the lungs; and arises from the presence of venous blood in the capillaries and of carbonic acid in the air-cells. Under ordinary circumstances, —that is, when the blood is being duly aerated, and the air being properly renewed,—the impression thus made upon the nerves of the lungs, is so faint, that we cannot perceive it, even when we specially direct our attention to it. But if we suspend the movements for a moment or two, we immediately experience a sensible uneasiness. The ParVagum is obviously the channel, through which this impres- sion is conveyed to the nervous centres; and it is found that, if the trunk of this nerve be divided on both sides, the respiratory move- ments are greatly diminished in frequency. Hence it is undoubtedly one of the principal excitors of the respiratory movements. 686. But the sensory nerves of the general surface, and more par- ticularly the sensory portion of the Fifth pair, which supplies the face, are most important auxiliaries, as excitor nerves; the inspiratory movement being peculiarly and forcibly excited by impressions made upon them, especially by the contact of cold air or water with the face. The power of the impression made by the air upon the gene- ral surface, and particularly upon the face, in exciting the inspiratory movement, is w-ell seen in the case of the first inspiration of the new- born infant, which appears to be excited solely in this manner. An inspiratory effort is often made, as soon as the face has emerged from the Vagina of the mother; whilst, on the other hand, if the face be prevented from coming into contact with cool air, the inspiratory REFLEX CHARACTERS OF RESPIRATORY MOVEMENTS. 391 effort may be wanting. When it does not duly take place, it may often be excited by a slap with the flat of the hand upon the nates or abdomen ; a fact which shows the special influence of impres- sions upon the general surface, in rousing the motor impulse in the medulla oblongata, and in causing its transmission to the muscles. The deep inspirations which follow a dash of cold water upon the face, or the descent of the cold douche or of the divided streams of the shower-bath upon the body, or the shock of imraersion in the cold plunge-bath, all testify to the powerful influence of such impressions in the adult; and the efficacy of other kinds of irritation of the skin, such as beating with holly twigs, in maintaining the respiratory move- ments in cases of narcotic poisoning, shows that the required impres- sions are not restricted to the contact of cold air or water. It seems probable, from various facts, that the presence of venous blood in the arterial capillaries of the system, and the consequent stagnation in the current through them (§ 597), may exert an influence through the Sympathetic system: which may be transmitted, by the copious inos- culations of that system with the Par Vagum, to the Medulla Ob- longata ; and which may there serve as a valuable auxiliary in excit- ing the respiratory movements. 687. Of the mode in which the impressions, thus transmitted to the Medulla Oblongata, act in exciting the motor impulses which issue from it, nothing is known; but these impulses, directed along the phrenic, intercostal, and other nerves, produce the requisite move- ments. When the stimulus is unusually strong, various nerves and muscles are put in action, which do not co-operate in the ordinary movements of inspiration; and it may sometimes be observed, that movements are thus excited in parts, which will not act in obedience to the will, being to all appearance completely paralyzed. This fact shows how completely the class of actions in question is independent of the influence of the mind ; but we must not lose sight of the con- trol which the mind, especially in the higher classes of animals, pos- sesses over them. Various actions of the respiratory muscles, par- ticularly those of weeping and laughing, are the most direct means of expressing the passions and emotions of the mind ; and are invo- luntarily excited by these. And, again, the respiratory actions are placed to a certain degree under the control of the will ; in order that they may be subservient to the production of vocal sounds, and to the actions of speech, singing, &c. The will cannot long suspend the respiratory movements; for the stimulus to their involuntary per- formance soon becomes too powerful to be any longer resisted. And it is well that it should be so ; for if the performance of this most im- portant function were left to our own choice, a few moments of for- getfulness would be productive of fatal results. But it is to the power which the will possesses, of directing and controlling the respiratory movements, that we owe the faculty of producing articulate sounds, and thus of holding the most direct and intimate converse with each other. 392 INTERCHANGE OF OXYGEN AND CARBONIC ACID. 6S8. It is essential for the due performance of the respiratory move- ments, that the portion of the nervous centres, on which they depend, should be in a state of activity. This is the case, under ordinary circumstances, throughout life. The state of perfect quiescence, to which the brain is liable, never affects the medulla oblongata ; and the respiratory movements are consequently kept up with as much regularity and energy (in proportion to the requirements of the sys- tem), during our sleeping, as during our waking hours. But if any cause induce torpidity of the medulla oblongata, the respiratory move- ments are then retarded, or even suspended altogether; and all the consequences of the cessation of the [aeration of the blood speedily develop themselves (§ 706). This is seen in apoplexy; when the pressure, or other cause of suspended activity, which at first affected the brain alone, gradually propagates its influence downwards. The same is the case in narcotic poisoning; in which also the brain is the first to be affected, and may suffer alone; but if the noxious influence be propagated to the medulla oblongata, it manifests itself in the re- tardation of the respiratory movements, and, when sufficiently power- ful, in their complete suspension. Under such circumstances, it is requisite to resort to all possible means of keeping up the respiratory movements ; and when these fail, artificial respiration may be suc- cessfully employed. For if, by such means, the circulation can be prevented from failing for a sufficient length of time, the ordinary pro- cesses of nutrition go on, the poisonous matter is gradually decom- posed or eliminated by the secreting organs ; and the nervous centres resume their usual functions. A torpid condition of the medulla oblongata, inducing a retardation of the respiratory movements, seems to be one of the morbid conditions attendant upon typhoid fever ; and probably depends in the first instance upon a disordered state of the blood, which does not exert its usual vivifying influence. In such cases, the proportion of the respiratory movements to the pulse sinks as low as 1 to 6, or even as 1 to 8; and thus the due aeration of the blood is not performed, and its stimulating properties are still further diminished. 4. Chemical Phenomena of Respiration. 689. Having now fully considered the means, by which the Atmo- sphere and the blood are brought into relation in the lungs, we have to examine into the results of their mutual action. It will be remem- bered that the Atmosphere contains about 21 per cent, of Oxygen to 79 of Nitrogen, by measure ; or 23 parts of Oxygen to 77 of Nitro- gen, by weight. The Nitrogen seems to perform no other part than that of diluting the oxygen; at least the results of the most recent and exact experiments render it very doubtful, whether (in the respiration of Man at least) any change is effected in the nitrogen of the inspired air. The leading phenomenon of respiration, is the removal of a cer- tain quantity of Oxygen from the air, and its replacement by Carbonic AMOUNT OF CARBONIC ACID EXHALED. 393 acid. The relative proportions, which the oxygen absorbed, and the carbonic acid exhaled, bear to one another, have been variously stated. The most recent and trustworthy experiments on this subject (those of Brunner and Valentin) lead to a very interesting result. According to the law of the u mutual diffusion" of gases, the volumes of any two gases, that pass through a porous medium to mingle with each other, will be respectively in the inverse proportion to the square roots of their specific gravities. Now when oxygen is on the outer side, and carbonic acid on the inner, the volume of oxygen that passes inwards will exceed that of the carbonic acid that passes out- wards ; and this in the proportion of 1174 to 1000. This calculation, deduced from the relative densities of the two gases, corresponds so closely with the actual result of experiments upon the respiration of Man, that it seems next to certain, that the interchange of oxygen and carbonic acid, which occurs between the air and the blood in the lungs, takes place in exact accordance with this law of mutual dif- fusion. 690. Now Carbonic Acid contains precisely its own volume of oxygen; consequently, of the 1174 parts of oxygen absorbed, 1000 are excreted again by the lungs in the form of carbonic acid; and there remain 174, or nearly 15 per cent., to be accounted for in other ways. It is certain that some of this enters into combination with the sulphur and phosphorus of the original components of the body; and converts these into sulphuric and phosphoric acids ; and the remainder must enter into other chemical combinations, very probably uniting with the hydrogen of the fatty matter, to form part of the water which is exhaled from the lungs. 691. It is difficult to form an exact estimate of the actual quantity of Carbon, thrown off from the lungs in the form of Carbonic Acid during any lengthened period ; since the amount disengaged during experiments carried on for a limited time, cannot, for many reasons, be taken as affording a fair average. Thus the quantity will vary with the external temperature, with the state of previous rest or activity, with the length of time that has elapsed since a meal, and particularly with the general development of the body. The amount of carbonic acid exhaled is greatly increased by external cold ; as is shown in the results of such experiments as the following. Small Birds and Mammals having been enclosed in a limited quantity of air, for the space of an hour, at ordinary temperatures, the quantity of carbonic acid they produced was noted. The experiment was then repeated at a temperature nearly approaching that of the body ; and was performed a third time at a temperature of about 32°. The following are the comparative amounts. Temp. 590—680. Temp 860—106O. Temp, about 32°. Grammes. Grammes. Grammes. A Canary 0-250 0-129 0-325 A Turtle-dove 0-684 0-366 0-974 Two Mice 0-498 0-268 0-531 A Guinea-pig 2-080 1-453 3-006 394 AMOUNT OF CARBONIC ACID EXHALED. Thus it would appear that the quantity of carbonic acid exhaled be- tween 86° and 106° is not much more than half of that which is exhaled between 59° and 68°; and is only about two-fifths of that which is given off at 32°. 692. The quantity of carbonic acid exhaled during exercise, and for a certain time after it, and also after a full meal, is considerably increased ; whilst on the other hand, it is greatly diminished during sleep. Thus a person who was excreting 145 grains of carbon per hour, whilst fasting and at rest, excreted 165 after dinner, and 190 after breakfast and a walk; whilst he only excreted 100 during sleep. The variation with the general development of the body, and also with the sex and age, is considerable. Thus, the exhalation is almost always greater in males than in females of the same age, at every period of life except childhood. In males, the quantity increases regularly from eight to thirty years of age, remaining nearly stationary until forty;—thus it averages 77*5 grains of carbon per hour at eight years ; 135 grains at fifteen ; 176-7 grains at twenty ; and 189 grains between thirty and forty. Between forty and fifty, there is a well- marked diminution, the average being then 156 grains ; and the dimi- nution continues up to extreme old age, when the amount exhaled scarcely exceeds that which is extricated at ten years of age ; thus, between sixty and eighty, it was 142-5 grains; and in a man of a hundred and two, it was only 91-5 grains. These average results, however, are widely departed from in individual cases ; an extraor- dinary development of the muscular system being always accompanied by a high rate of extrication of carbon ; and vice versa. Thus a man of remarkable muscular vigour, whose age was twenty-six years, exhaled 217 grains of carbon in an hour ; a robust man of sixty ex- haled 209-4 grains; and an old man of ninety-two, who in-his younger days had possessed uncommon muscular powers, and who preserved a remarkable degree of energy, still consumed at the rate of 151 grains per hour. On the other hand, a man of slight muscular development, at the age of forty-five, only exhaled 132 grains; and another at the age of seventy-six, only 92-4 grains. 693. In females, nearly the same proportional increase goes on, up to the time of puberty ; when the quantity abruptly ceases to increase, and remains stationary so long as menstruation continues regular. The average quantity of carbonic acid exhaled by girls nearly approaching puberty, is about 100 grains per hour ; and it remains at this standard until nearly the close of menstrual life. At the period of the cessation of the catamenia, it undergoes a perceptible increase; the average, between forty and fifty years of age, being about 130 grains per hour; and the quantity exhaled in a woman of great muscular development, and of forty-four years of age, rising to 152-4 grains in an hour. After the age of fifty, or thereabouts, the quantity decreases, as in men. It is remarkable that, during pregnancy, there is the same increase in the exhalation of carbon, as there is after the final cessation of the cata- AMOUNT OF CARBONIC ACID EXHALED. 395 menia ; and the same takes place, if the menstrual discharge be tem- porarily suspended, through any other cause. 694. It is obviously difficult, then, to obtain exact estimates, from any experiments conducted for a short time only, of the total amount of carbon thrown off during a lengthened period ; since the condition of the individual varies so greatly at different times ; and the variation amongst different individuals is so great. Moreover, of the total amount of carbon excreted in a gaseous form, a certain part is un- doubtedly set free from the skin; and the proportion of this has not been yet determined. As a means of measuring the whole quantity of carbonic acid set free, without causing the respiratory movements to be performed in any unnatural manner, Prof. Scharing constructed an air-tight chamber, of dimensions sufficient to allow an individual to remain in it for some time without inconvenience; and so arranged, that he could eat and drink, read, or sleep, within it. This was con- nected with an apparatus, by which the air was continually renewed; and the air drawn off was carefully analyzed in order to determine the quantity of carbonic acid contained in it. The average per hour, in different states, having been ascertained, it was calculated that, allow- ing seven hours for sleep in adults, and nine hours for children, the total amount of carbon consumed in the twenty-four hours was as follows:— No. Weighing. Grains. Oz. Troy. 1. A male, aged thirty-five, 2. A male, aged sixteen, 3. A soldier, aged twenty-eight, 164 4. A girl, aged nineteen, 5. A boy, aged nearly ten, 6. A girl, aged ten, 695. .This estimate is perhaps rather too low; as it does not take sufficient account of the great increase, which is produced by exercise. Another method'has been adopted by Prof. Liebig; who endeavoured to ascertain the total amount of carbon excreted from the body in the form of carbonic acid, by comparing the amount of carbon taken in as food, with that contained in the feces and urine; the difference being set down to the account of respiration. His estimate amounts to the very large sum of 13-9 oz. of solid carbon per day, which he consi- ders to be thus set free by the lungs and skin; but this is almost cer- tainly above the truth. The observations were made upon a body of soldiers who were subjected to severe daily exertion; and they were far from being exactly conducted, many of the items being set down by guess only, whilst of others no account whatever was taken. We may perhaps consider 10 or 11 oz. as more nearly representing the araount of carbon consumed by adult men exposed to severe exer- tion ; whilst from Prof. Scharling's experiments it may be inferred, that from 7 to 8 oz. of carbon are thrown off during the twenty-four hours by the lungs and skin of adult men not using much active exer- 131 lbs. 3380 or 7-0 1151 lbs. 3450 or 7-2 164 lbs. 3692 or 7-7 111 lbs. 2555 or 5-3 44 lbs. 2050 or 4-3 46 lbs. 1938 or 4-0 396 AMOUNT OF CARBONIC ACID EXHALED. tion ; to which another ounce or two may be added, as the increased quantity excreted during moderate exercise.—On the other hand, from experiments made upon the quantity of carbonic acid exhaled from the lungs alone during a given time, it would appear that the pul- monary excretion of carbon amounts to between b\ and 8 oz. in the twenty-four hours; and the difference may be partly set down to the account of the cutaneous respiration. 696. If we assume 10 oz. or 4800 grains of solid carbon as the total amount excreted from the lungs and skin of a male adult, using active exercise in the course of twenty-four hours, we find that the volume of carbonic acid thus generated must be nearly 37,000 cubic inches, or more than 21 cubic feet. Of this about 16 cubic feet are probably extricated from the lungs. But it is probable that about 10 cubic feet per day is near the ordinary average. Now it has been ascertained, that the whole quantity of air which passes through the chest during that time under similar circumstances, is about 266 cubic feet; so that the proportion of carbonic acid contained in the expired air seems to average about 4 per cent. It is certain, however, that this proportion may rise much higher; particularly when the respiratory movements are slowly and laboriously performed. Now in order that the blood should be properly aerated, it is requisite that the air should contain no previous impregnation of carbonic acid ; since the diffusion of even a moderate per centage of that gas through the inspired air, seriously impedes the exhalation of more. Thus it was found by Messrs. Allen & Pepys, that when 300 cubic inches of air were respired for three minutes, only 28^ inches of carbonic acid (or somewhat more than 9 per cent.) were present in it; though the rate of its production in a parallel experiment, in which fresh air was taken in at each inspira- tion, was 32 cubic inches per minute, or 96 cubic inches in three minutes. That it is not the deficiency of oxygen, but the presence of carbonic acid in the inspired air, which impedes the free aeration of the blood, is proved by the recent experiments of Dr. D. B. Reid ; who has shown that an animal maybe kept alive in a limited quantity of air, until nearly all its oxygen is exhausted, if an effectual provi- sion be made for drawing off the carbonic acid as fast as it is generated. 697. An animal thus made to breathe an atmosphere, which con- tains less than its normal proportion of oxygen, resembles one which is made to breathe a rarefied atmosphere, such as that which exists on the summits of high mountains. All persons who have made such ascents, have experienced the insufficiency of rarefied air to sustain the degree of respiration required for active exertion. As long as the body remains at rest, no inconvenience is perceived; but as soon as the muscular system is put into action, the insufficiency of the supply of oxygen is manifested by the feeling of distress and languor ; which becomes so severe, that the individual, if unused to such ascents, is obliged to stop and take breath at every few steps. The necessity for doing so will be easily understood, when it is remembered that when the pressure of the atmosphere is reduced to half its usual CHANGES EFFECTED IN THE BLOOD. 397 amount, the bulk of a given weight of air will be twice as great as at the surface of the earth, or the same measure will weigh only half as much. Consequently, when the chest is completely filled with air, the real quantity of oxygen included in it, is only half of that, which is drawn in by a corresponding inspiration at the earth's surface. 698. Although an impregnation of carbonic acid, to the amount of seven or eight per cent., would be required to destroy life in most warm-blooded animals, yet a much smaller proportion is sufficient to produce very injurious results. Thus the discomforts occasioned by the presence of a crowded audience in a church, lecture-room, or theatre, which is not provided with sufficient ventilation, are due in great part to the continued respiration of air, which becomes loaded in the course of an hour or two with carbonic acid gas, to the amount of from one-half to two per cent.,—as has been ascertained both by direct experiment, and by calculation. And there can be little doubt, that the habitual respiration of such air, in the narrow and noisome dwellings of the poor, or in crowded factories and workshops, has a tendency to produce, both directly and indirectly, much loss of phy- sical and mental vigour, and also to bluht the acuteness of the moral feelings. 699. Having thus considered the changes produced by the Respi- ratory function, in the air submitted to it, we have next to inquire into converse series of change effected by it in the blood. The nature of these cannot be well stated with precision; as they have not yet been fully determined. It was formerly supposed, that the venous blood arrives at the lungs charged with carbon; and that this carbon is united with the oxygen of the air, in the lungs themselves. Nu- merous facts, however, go to prove, that the blood comes to the lungs charged with carbonic acid; and that it gives out this ready formed, and receives oxygen in its stead. Thus it has been already shown, that there is a positive disappearance of oxygen ; more of that element being withdrawn from the atmosphere, than is restored to it in the condition of carbonic acid; so that we know that the surplus must be received into the blood. Moreover, the quantity of oxygen ab- sorbed exactly replaces the quantity of carbonic acid set free, accord- ing to the law of " mutual diffusion;" which could scarcely be the case, unless the latter were contained in the blood already formed. Further, cold7blooded animals may be made to breathe nitrogen or hydrogen for a sufficient length of time, to cause a large quantity of carbonic acid to be disengaged; and this must have been brought to the lungs ready formed, since no oxygen was present there to gene- rate it/« Lastly, it can be shown by experiment, that oxygen, carbo- nic acid, and nitrogen exist in a free state in blood, arterial as well as venous; but that the proportion of oxygen is greater in arterial than in venous blood, whilst that of carbonic acid is less. The following table expresses the per centage of each kind of gas in the two sorts of blood respectively; as deduced from the experiments of Magnus. 398 EXHALATION OF WATER FROM THE LUNGS. Arterial Blood. Venous Blood. Carbonic acid . . . 62-3 716 Oxygen .... 23-2 15-3 Nitrogen .... 14-5 13-1 Thus it appears that the quantity of nitrogen is very nearly the same in both, as would be anticipated from what has been already stated in regard to its non-participation in the respiratory process; whilst about one-third of the free oxygen of arterial blood disappears during its cir- culation in the systemic capillaries, to be replaced by an equivalent amount of carbonic acid ; and a converse change takes place in the pulmonary capillaries, this additional portion of free carbonic acid being set free, and replaced by oxygen. 700. Thus it is evident, that a part of the change effected in the blood consists in an alteration in the proportion of the gases which always exist in it, either entirely free, or in a state of such loose com- bination that they can be removed by the air-pump. But it cannot be doubted, that a portion of the effect consists in the oxidation of the proteine of the fibrinous constituent; since the fibrin of arterial blood possesses properties that distinguish it from that of venous. And it seems probable, also, for the reasons formerly stated, that the hemato- sine of the red corpuscles undergoes a change under the influence of oxygen in the lungs, and a converse change in the systemic capillaries, where it is subjected to the influence of carbonic acid. This much appears tolerably certain :—that a part of the oxygen imbibed in the lungs, is appropriated to the Oxidation of the matters set free by the decomposition of the solid tissues;—wThilst another part enters into combination with fatty, saccharine, or farinaceous matters, existing in the blood itself, and destined to be carried off in the form of carbonic acid and water, without ever entering into the composition of the solid fabric. The relative amounts of carbonic acid formed in these two modes, vary in different animals, and in different states of the same individual; for a man in a warm atmosphere, taking a moderate amount of exercise, may thus set free, by the waste of his muscular and other tissues, a sufficient quantity of carbon, for the maintenance of his animal heat by its union with oxygen; but this is far from being sufficient, when a larger amount of heat must be evolved, to sustain the temperature of the body in a colder climate. 701. The blood parts in the lungs with a very large amount of moisture; for the inspired air is always saturated with fluid, as soon as it reaches the air-cells ; and, as it is heated at the same time to about 98°, it thus receives a considerable addition, even if it were previously charged with as much as it could contain at a lowertempe- rature. The total quantity of fluid thus disengaged will vary, there- fore, with the amount previously contained in the atmosphere; being greater as this was less, and vice versa; the expired air being always charged with as much as it can contain at the temperature of 98° or 99°. It cannot be doubted, that a great part of this water is a simple EXHALATION OF WATER FROM THE LUNGS.—ASPHYXIA. 399 exhalation of that which has been absorbed ; but, on the other hand, it seems probable that a portion of it may be actually formed in the system, by the union of a portion of the oxygen absorbed in the lungs, with the hydrogen of the combustible matters of the blood. In the various forms of saccharine and farinaceous aliments, the proportions of hydrogen and oxygen are such as would of themselves form water, when the carbon is withdrawn; but in oily and fatty matters, the proportion of oxygen is far too small thus to neutralize the hydrogen ; and it seems likely that, by their oxidation in the blood, as by their combustion elsewhere, water is actually generated by the union of atmospheric oxygen with their hydrogen, whilst carbonic acid is pro- duced by its union with their carbon. 702. Along with the water thus extricated from the lungs, a certain amount of organic matter is set free. If the fluid be collected in a closed vessel, and be exposed to warmth, a very evident putrid odour is exhaled from it; and if the expired air be made to pass through sulphuric acid, that liquid is coloured red. Every one knows that the breath itself possesses, occasionally in some persons, and constantly in others, a fetid taint; when this does not proceed from carious teeth, ulcerations in the air-passages or lungs, or other similar causes, it must result from the excretion of the odorous matter, in combination with watery vapour, from the pulmonary surface. That this is the true account of it seems evident, from the analogous phenomenon of the exhalation of turpentine, camphor, alcohol, and other odorous sub- stances, which have been introduced into the venous system, either by natural absorption, or by direct injection ; and also from the sud- denness with which the odour manifests itself, when the digestive apparatus is slightly disordered. 5. Effects of Insufficiency, or Suspension, of the Aerating Process. 703. The change which the Blood undergoes, by being brought into relation with atmospheric air in the respiratory organs, is so important to life, that the entire suspension of it inevitably produces a fatal termination, at no remote period; and if it be insufficiently performed, various disorders in the system are nearly sure to manifest themselves. The state which is induced by the entire suspension of the aerating process, is termed Asphyxia; a word which literally means the absence of pulse, and would be applicable, therefore, to the stoppage of the circulation from any cause; though it is usually em- ployed to designate the particular condition, resulting from suspended respiration. Asphyxia may be produced in aquatic animals, as well as in those which breathe air, by cutting them off from the influence of the atmosphere ; for if a Fish be placed in water from which the air has been expelled by boiling, it is precisely in the condition of an air-breathing animal placed in a vacuum, since it has no power of obtaining oxygen by decomposing the water it inhabits, and is en- tirely dependent for the aeration of its blood, upon the air that is 400 ASPHYXIA;—ITS PHENOMENA. absorbed by the liquid. Again, if a fish be placed in water impreg- nated with carbonic acid, its death is nearly as instantaneous as that of an air-breathing animal immersed in an atmosphere of that gas. 704. Asphyxia may result from a great variety of causes. Thus there may be a mechanical obstruction to the entrance of air through the trachea; as in hanging, strangulation, or drowning; or as in occlusion of the aperture of the glottis, by oedema of its lips, or by the presence of a foreign body in the larynx. Or, again, the passage may be perfectly free, and yet no air may enter, in consequence of some obstacle to the performance of the respiratory movements. This obstacle may be mechanical; as when a quantity of earth has fallen round the body, in such a manner as completely to prevent the disten- sion of the chest and abdomen. Or it may result (and this is a most frequent occurrence) from torpidity or complete inactivity of the gan- glionic centre, which is concerned in the respiratory actions; or from interruption to the transmission of its influence along the nervous trunks. Further, when there is no obstacle to the free ingress or egress of air, Asphyxia may be produced by the want of oxygen in the atmosphere that is respired, or by the presence of carbonic acid in too large an amount. And the presence of other gases, which exert a directly poisonous influence on the blood,—such as sulphuretted hydrogen,—produces a state, which may be included under the same general description. 705. Now when, from any of these causes, the free exchange of carbonic acid for oxygen in the pulmonary capillaries is checked, the first effect of the interruption appears to be, the stagnation of the blood in the pulmonary capillaries. This stagnation is evidently due, not to any deficiency of power in the heart; for that organ is not yet affected; but to the insufficiency of the heart's power, acting alone, to drive the blood through the pulmonary capillaries; the force which should be generated by chemical changes in them (§ 598), being deficient. The stagnation is not, however, complete at first; since the quantity of oxygen contained in the lungs is sufficient to produce an imperfect arterialization of the blood; and the blood thus partially changed is transmitted to the left side of the heart, and is thence pro- pelled to the system. Owing to its half-venous condition, it cannot exert its usual stimulating influence on the tissues, especially the muscular and nervous; and their powers are consequently weakened. For the same reason, it does not receive its usual auxiliary force in the systemic capillaries (§ 599); since the changes, which it ought to undergo in them, can only be partially performed. 706. As the air included in the lungs loses more and more of its oxygen, and is more and more charged with carbonic acid, the aera- tion of the blood in the pulmonary capillaries becomes more and more imperfect; the quantity of blood which is allowed to return to the heart is gradually diminished, and its condition becomes more and more venous; and at last, the pulmonary circulation is altogether sus- pended. From the relation which the respiratory circulation bears to ASPHYXIA ;—ITS PHENOMENA AND TREATMENT. 401 the systemic, in all the higher classes of animals, save Reptiles, it follows that the systemic circulation must in like manner be brought to a stand. The venous blood accumulates in the pulmonary artery, in consequence of the obstruction of its capillaries; it distends the right cavities of the heart; and the accumulation extends to the venous system of the body in general, especially affecting those or- gans which naturally receive a large quantity of venous blood, such as the liver and spleen. The arterial system, on the other hand, is emptied in a corresponding degree; nearly all its blood having passed through the systemic capillaries; and no fresh supplies being received from the heart. From this deficiency, and from the venous character of the blood which the systemic arteries do contain, it results that the nervous and muscular systems lose their power; insensibility comes on, at first accompanied with irregular convulsive movements; but in a short time there is a total cessation of all movement except that of the heart; and the pulsations of that organ become feebler and feebler, until they cease altogether. The immediate cause of the cessation of the heart's action appears to be different on the two sides. Both are equally affected by the want of arterial blood in the capillaries of their own substance; but the right side suffers from over-distension, which produces a sort of paralysis of its muscular tissue ; whilst the left side retains its contractility, but is not excited to contraction, for want of the stimulus of arterial blood in its cavi- ties. 707. In those warm-blooded animals, which are not endowed with any special provision for enabling them to sustain life during the pro- longed suspension of the respiratory process, insensibility and loss of voluntary power almost invariably supervene within a minute and a half, after the admission of air to the lungs has been entirely prevent- ed ; though the respiratory efforts and convulsive actions, which are dependent upon the medulla oblongata and spinal cord, may continue for a minute or two longer. The circulation generally comes to a complete stand within ten minutes at farthest.—The chief exceptions are in the case of diving animals, which are provided with large arterial and venous reservoirs, that serve to maintain the circulation during a prolonged suspension of the respiratory process ; for the arte- rial plexuses being ordinarily filled, they afford a supply of aerated blood to the systemic capillaries, when other blood is wanting; and the reservoirs connected with the venous system, which were pre- viously empty, receive this blood, and prevent it from exercising undue pressure on the heart. To such an extent is this provision carried in some animals, that the Whale has been known to remain under water for an hour. Another exception exists in the case of hybernating Mammals, which are reduced for a time to the condition of cold-blooded animals; and which can, like the latter, sustain a prolonged suspension of the aerating process. And there is reason to believe that, in the state of Syncope or fainting,—in which the circulation is already reduced to a very low amount, in consequence 26 402 ASPHYXIA,—ITS PHENOMENA AND TREATMENT. of a partial failure in the heart's power, all the functions of the body being nearly suspended, and the demand for aeration being conse- quently very small,—the respiration may be suspended for a long period, even in the Human subject, without fatal results. Thus more than one case has been credibly recorded, in which recovery has taken place after complete submersion for more than three-quarters of an hour; and it is probable that, in these instances, a state of Syncope came on at the moment of immersion, through the influence of mental emotion, or of concussion of the brain. 708. In the restoration of an animal from the state of Asphyxia, it is above all things of importance to renew the air in the lungs; for in this way the blood in the pulmonary capillaries will be aerated ; the capillary circulation will be re-established ; the right side of the heart will be relieved of its excessive load of venous blood ; and the left side will receive the stimulus of a fresh supply of arterial blood ; so that, if its movements have not ceased altogether, it may be speedily restored to due activity. At the same time, the temperature of the body should be kept up by artificial warmth; and the circulation in the skin should be excited by friction. Where no other means are at hand for introducing pure air into the lungs (of which means the application of galvanism along the course of the phrenic nerve, so as to produce contraction of the diaphragm, will probably be the most effectual), the object may be attained by forcibly compressing the trunk on all sides, so as to empty the lungs as much as possi- ble, and then allowing the chest to dilate again, by the elasticity of its walls. In this manner, a large proportion of the carbonic acid may be expelled, and a considerable proportion of the fresh air introduced, in the course of a few minutes. If air be blown into the lungs by the bellows, great care must be taken to prevent the employment of too much force, which is likely to produce rupture of the air-cells. 709. Now when, from the more prolonged action of various causes, that impede the due performance of the respiratory function, the aeration of the blood in the lungs is insufficient for health, though not such as to produce a complete stagnation of the movement, a variety of results may follow; of which some, or others, will manifest them- selves according to the condition of the general system, and the pecu- liarities of the individual. Thus deficient respiration has an undoubted tendency to produce, in some persons, what is termed " fatty degene- ration" of the liver; the fatty matter, which ought to be eliminated by the respiratory process, being thrown upon the liver to be sepa- rated by it, and distending its cells (§ 723). And there is reason to -believe, that a similar cause may produce fatty degeneration of the kidney, in cases where there is a peculiar determination of blood to that organ. Again, the due elaboration of the fibrin of the blood is undoubtedly prevented by an habitually deficient respiration ; and various diseases, which result from the imperfect performance of this elaboration, consequently manifest themselves. The Scrofulous dia- OF THE SECRETING PROCESS IN GENERAL. 403 thesis is thus frequently connected with an unusually small capacity of the chest.—Further, an habitual deficiency of respiration may im- pede, though it does not check, the circulation in the lungs ; and thus a tendency arises, in various pulmonary diseases, to an overloading of the pulmonary arteries, to a dilatation of the right cavities of the heart, and to a congestion of the venous system in general, as marked by lividity of the surface, by venous pulsation, &c. This state may result, not merely from obstruction in the lungs themselves, but from deficiency of the respiratory movements, consequent upon torpidity of the medulla oblongata (as in apoplexy and narcotic poisoning), or upon partial interruption of the nervous circle requisite for all reflex movements. Thus when the par vagum is divided, the number of respiratory movements is greatly diminished, and a partial stagnation of the blood in the lungs is the result. The same happens in certain forms of typhoid fever, in which the respiratory movements are pre- ternaturally slow, in consequence of torpidity of the medulla oblon- gata. Now in this state, an effusion of the watery part of the blood into the air-cells of the lungs (as in other cases of obstructed circula- tion) is very liable to occur; and when the lungs are thus loaded with fluid, the respiratory process is still more impeded, and the dis- order has thus a tendency to increase itself. CHAPTER IX. OF SECRETION. 1. Of the Secreting process in general; and of the Instruments by which it is effected. 710. We have seen that, in the process of Nutrition, the circulat- ing current not only deposits the materials, which are required for the renovation of the solid tissues; but also takes back the substances, which are produced by the decay of these, and which are destined to be thrown off from the body. We have also seen that it supplies the materials of certain fluids, which are separated from it to effect various purposes in the economy;—such as the Salivary and Gastric fluids, which have for their office to assist in the reduction of the food. Now the process, by which the fluids of the latter kind are separated from the Blood, is precisely the same in character as that, by which the products of decay are eliminated from it; and the structure of the or- gans concerned in the two is essentially the same. Hence both pro- cesses are commonly included under the general term Secretion, which simply denotes separation. Considered in its most general point of view, this designation may be applied to every act, by which sub- 404 NATURE OF THE SECRETING PROCESS. stances of any kind are separated from the blood. Thus the function of the floating cells, which are concerned in the production of Fibrin (§ 213), may be termed one of secretion; because they draw from the blood a supply of Albumen, upon which their converting action is exercised; but as the product of their operation is returned to the blood again, and is employed for higher purposes in the economy, the process is usually termed Assimilation. In the same manner, the elaborating action of the Lymphatic Glands, with the Spleen, Thy- mus Gland, &c, is not usually termed Secretion; since, although it is exercised upon matter drawn from the blood, the product appears to be delivered back into the circulating current, through the medium of the Lymphatic System (Chap. V). With much more justice, how- ever, the process of Respiration may be regarded as one of Secretion; for it consists, as we have seen, in the constant elimination of a sub- stance from the blood, which cannot be retained in it without the most injurious consequences. 711. There is an important difference in the characters of the prin- cipal products of the Secreting process; which is connected with the purposes that are to be answered by their separation. Some of these products are altogether different from the ordinary constituents of the animal fabric, and from the materials which the blood supplies for the nutrition of these. So different are they, that their presence in the circulating current has an injurious effect; and the great object of their separation is the maintenance of the purity of the blood. These poisons, for such they may be considered, are generated in the system by the decay and decomposition to which its several parts are liable; and they are just as noxious to it, as if they were absorbed from without. We have seen that the retention of Carbonic acid in the blood for even a few minutes is fatal; both by putting a stop to the circulation, and by acting unfavourably upon the substance of some of the most important organs in the body. The same fatal result attends the retention of Urea and of Biliary matter, which are among the other products of the decomposition of the tissues ; but, although as certain, it is not so speedy, because the general circulation is not affected by the loss of secreting power on the part of the Kidneys and the Liver, and because the accumulation of the noxious matter is slower.—On the other hand, the ingredients that are met with in those secreted fluids, which are destined to answer some purpose in the economy, almost invariably bear a close correspondence with the ordinary ma- terials of the blood. Thus in the Salivary, Gastric, Pancreatic, and Lachrymal fluids, the principal part of the solid matter consists of the saline and of the albuminous constituents of the blood,—the latter in a more or less altered condition. In Milk, again, we trace the ordi- nary constituents of the blood, with very little change. Thus it appears, that the separation of these fluids is not required so much to maintain the Blood in a state of purity, as to supply what is needed for some subsequent operation in the economy; and hence, if the secreting process be interrupted, in the case of any one of them, the NATURE OF THE SECRETING PROCESS. 405 suspension has usually no further effect, than that of disturbing the process to which the fluid is usually subservient. If the secretion of Gastric fluid be checked, for example, under the influence of strong mental emotion, the Digestive operation is prevented from taking place. 712. The essential character of the true Secreting operation seems to consist,—not in the nature of the action itself, for this is identical with those of Assimilation and Nutrition, being (as we have seen, § 239), a process of cell-growth,—but in the position in which the cells are developed, and the mode in which the products of their action are afterwards disposed of. Thus the cells at the extremities of the intestinal Villi (§ 241), the cells of which the Adipose tissue is made up (§ 259), and the cells of which the greater part of the substance of the Liver is formed (§ 239), all have an attraction for fatty matter ; and draw it from the neighbouring fluids, at the expense of which they are developed, to store it up in their own cavities. But the cells of the first kind, when they have come to maturity, set free their contents, which are delivered to the absorbent vessels, to be introduced into the circulating current:—those of the second kind seem more permanent in their character, and retain their contents, so as to form part of the ordinary tissues of the body, until they are re- quired to give them up for other purposes, when the matters, which they have temporarily separated from the circulating current, are restored to it again without change ;—and the cells of the third class, when they liberate their contents (which they are continually doing), cast them forth into the hepatic ducts, by which they are carried into the intestinal canal, whence a portion of them at least is directly con- veyed out of the body. 713. It is, then, in the position of the Secreting cells,—which causes the product of their action to be delivered upon a free surface, com municating, more or less directly, with an external outlet,—that their distinctive character depends. All the proper Secretions are thus either poured out upon the exterior of the body, or into cavities pro- vided with orifices that lead to it. Thus we shall see that a con- siderable quantity of solid matter, and a very large quantity of fluid, of which it is desirable that the system should be freed, are carried off from the Cutaneous surface. Another most important secretion, containing a large quantity of solid matter, and serving also to regu- late the quantity of fluid in the body,—namely, the Urinary,—is set free by a channel expressly adapted to convey it directly out of the body. The same may be said of the Mammary secretion ; which is separated from the blood, not to preserve its purity, nor to answer any purpose in the economy of theindividual, but to afford nutriment to another being. And of the matters secreted by the very numerous glandulae situated in the walls of the intestinal canal, a great part are obviously poured into it for no other purpose, than that they may be carried out of the body by the readiest channel. 714. The cells covering the simple membranes that form the free surfaces of the body, whether external or internal, are all entitled to 406 CHARACTER OF GLANDULAR STRUCTURE. be regarded as secreting cells ; since they separate from the blood various products which are not again to be returned to it. But the secreting action of some of these seems to have for its object the protection of the surface; thus the epidermic cells secrete a horny matter, by which density and firmness are imparted to the cuticle; whilst by the epithelial cells of the Mucous Membrane of the alimen- tary canal, and of other parts, their protective Mucus seems to be elaborated. But in general we find that special organs, termed Glands, are set apart for the production of the chief secretions; and we have now to consider the essential structure of these organs, and the mode of their operation.—A true Gland may be said to consist of a closely packed collection of follicles, all of which open into a common chan- nel, by which the product of the glandular action is collected and delivered. The follicles contain the secreting cells in their cavities ; whilst their exterior is in contact with a network of blood-vessels from which the cells draw the materials of their growth and deve- lopment (Fig. 90). In any one of the higher animals, we may trace out a series of progressive stages of complexity, in the various glands included within their fabric; and in following any one of the glands, that attain the highest degree of development (such as the Liver or Kidney), through the ascending scale of the Animal series, we should trace a very similar gradation from the simplest to the most complex form. 715. That there is nothing in the form or disposition of the com- ponents of the glandular structure, which can have any influence upon the character of the secretion it elaborates, is evident from the fact, that the very same product,—e. g., the Bile, or the Urine,—is found to issue from nearly every variety of secreting structure, as we trace it through the different groups of the Animal kingdom. The peculiar power, by which one organ separates from the blood the elements of the Bile, and another the elements of the Urine, whilst a third merely seems to draw off a certain amount of its albuminous and saline constituents, is obviously the attribute of the ultimate ?!C«?«*g CeiLS' Which are the real aSents in the secreting process ($ 239). Why one set of cells should secrete bile, another urea and so on, we do not know; but we are equally ignorant of the reason for which one set of cells converts itself into Bone, another into Muscle, and so on. This variety in the endowments of the cells, by whose aggregation and conversion the fabric of the higher Ani- mals is made up, is a fact which we cannot explain, and which must be regarded (for the present, at least), as one of the « ultimate facts" of Physiological Science. 716. Passing by the extended membranous surfaces, and the pro- tective cells with which they are covered, we find that the simplest form of a secreting organ is composed of an inversion of that surface into a series of follicles, which discharge their contents upon it bv separate orifices. Of this we have an example in the gastric follicles even in the higher animals; the apparatus for the secretion of the Gastric fluid never attaining any higher condition than that of a SIMPLEST FORMS OF GLANDULAR STRUCTURE. 407 series of distinct follicles, lodged in the walls of the stomach, and pouring their products into its cavity by separate apertures. In Fig. Fig. 103. Fig. 104. Glandular follicles in ventriculus succenturiatus of Falcon. Origin of the Liver from the intestinal wall, in the embryo of the Fowl, on the fourth day of incubation : —a, heart; 6, intestine $c, everted portion giving ori- gin to liver;. d, liver; e, portion of yolk-bag. Fig. 105. 103 is represented a portion of the Ventriculus succenturiatus of a Falcon ; in which the simplest form of such follicles is seen. A some- what more complex condition is seen in some of the Gastric follicles of the Human stomach (Fig. 75); the surface of each follicle being further extended by a sort of doubling upon itself, so as to form the commencement of secondary follicles, which open out of the cavity of the primary one.— Now a condition of this kind is common to all glands, in the first stage of their evolution; and in this stage we meet with them, either by examining them in the lowest animals in which they present themselves, or by looking to an early period of their embryonic deve- lopment in the highest. Thus, for ex- ample, the Liver consists, in certain Polypes and in the lowest Mollusca, of a series of isolated follicles, lodged in the walls of the stomach, and pouring their product into its cavity by separate orifices ; these follicles being recognized as constituting a biliary apparatus, by the colour of their secretion. And in the Chick, at an early period of incubation, the condition of the Liver is essen- tially the same with the preced- ing ; for it consists of a cluster of isolated follicles, not lodged in the walls of the intestine, but clustered round a sort of bud or diverticulum of the intestinal tube, which is the first condition of the Rudimentary Pancreas from Cod;— a, pyloric extremity of stomach; 6, intes- tine. Fig. 106. Mammary Gland of Ornithorhyncus. 408 DIFFERENT FORMS OF GLANDULAR STRUCTURE. hepatic duct, and into which they discharge themselves (Fig. 104). So, again, the Pancreas first presents itself in the condition of a group of prolonged follicles, or cceca, clustered round the commencement of the intestinal tube (Fig. 105); which is its permanent condition in many Fishes. And the Mammary Gland possesses an equally simple structure in the lowest of all the Mammalia (to which group it is re- stricted ;—namely, in the Ornithorhyncus (Fig. 106). 717. The next grade of complexity is seen, where a cluster of the ultimate follicles open into one common duct, which discharges their product by a single outlet; a single gland often containing a number of such clusters, and having, therefore, several excretory ducts. A good example of such a condition, in which the clusters remains iso- lated from one another, is seen in the Meibomian glands of the eye- lid (Fig. 107); each of which consists of a double row of follicles, set upon a long straight duct, that receives the products of their secreting action, and pours them out upon the edge of the eyelid. And of the more complex form, in which a number of such clusters are bound together in one glandular mass, we have an illustration in the acces- sory glands of the genital apparatus, in several animals, which dis- charge their secretion into the urethra by numerous outlets (Fig. 108); or in the Mammary glands of Mammalia in general, the ulti- mate follicles of which are clustered upon ducts that coalesce to a Fig. 107. Fig. 108. Fig. 109. Meibomian glands of upper Portion of Cowper's gland, Lobule of Lachrymal Gland; lid of new-born infant. from Hedgehog; the follicles from fetal sheep. distended with air. considerable extent, though continuing to form several distinct trunks even to their termination. Such glands may be subdivided, there- fore, into glandulce or lobules, that remain entirely distinct from each other (Fig. 109).—In the highest form of Gland, however, all the ducts unite; so as to form a single canal, which conveys awTay the products of the secreting action of the entire mass. This is the con- dition in which we find the Liver to exist, in most of the higher ani- mals ; also the Pancreas, the Parotid Gland, and many others. In some of these cases, we may still separate the gland into numerous distinct lobules, which are clustered upon the excretory duct and its branches, like grapes upon a stalk; in others, however, the branches DEVELOPMENT OF GLANDS. 409 of the excretory duct do not confine themselves to ramifying, but inosculate so as to form a network, w7hich passes through the whole substance of the gland, and which connects together its different parts, so as to render the division into lobules less distinct. This seems to be the case in regard to the Liver of the higher Vertebrata. 718. Whatever degree of complexity, however, prevails in the general arrangement of the elements of the Glands in higher animals, these elements are themselves everywhere the same, consisting of follicles that enclose the real secreting cells (Figs. 110 and 111). Now from the history of the development of Glands in general, it ap- pears that the follicles may be considered as parent cells; and that the Fig. 110. Fig. 111. Two follicles from the liver of Cartinus Ultimate follicles of Mammary gland, mcznas (Common Crab), with their contaiu- with their secreting cells, a, a;—b, b, the ed secreting cells. nuclei. secreting cells in their interior constitute a second generation, developed from the nuclei or germinal spots on the walls of the first. It has been pointed out by Mr. Goodsir, that the continued development and de- cay of the glandular structure,—in other words, the elaboration of its secretion, may take place in two different modes. In one class of Glands, the parent-cell, having begun to develop new cells in its in- terior, gives way at one point, and bursts into the excretory duct, so as to become an open follicle, instead of a closed cell: its contained or secondary cells, in the progress of their own growth, draw into themselves the matter to be eliminated from the blood, and, having attained their full term of life, burst or liquefy, so as to discharge their contents into the cavity of the follicle, whence they pass by its open orifice into the excretory duct: and a continual new production of secondary cells takes place from the germinal spot or nucleus, at the extremity of the follicle, which is here a permanent structure. In this form of gland, we may frequently observe the secreting cells ex- isting in various stages of development within a single follicle ; their size increasing, and the character of their contents becoming more distinct, in proportion to their distance from the germinal spot (which is at the blind termination of the follicle), and their consequent proxi- mity to the outlet (Fig. 110). In some varieties of such glands, how- ever,—as in the greatly prolonged follicles, or tubuli uriniferi, of the kidney,—the production of new cells does not take place from a single germinal spot at the extremity of the follicle, but from a number of points scattered through its entire length. 719. In the second type of Glandular structures, the parent-cell 410 COMPARATIVE STRUCTURE OF THE LIVER. does not remain as a permanent follicle; but, having come to maturity and formed a connection with the excretory duct, it discharges its entire contents into the latter, it then shrivels up and disappears, to be replaced by newly-developed follicles. In each parent-cell of a gland formed upon this type, we shall find all its secondary or secreting cells at nearly the same grade of development; but the different parent- cells, of which the parenchyma of the gland is composed, are in very different stages of growth, at any one period,—some having discharged their contents and being in progress of disappearance, whilst others are just arriving at maturity and connecting themselves with the excretory duct; others exhibiting an earlier degree of development of the secondary cells; others presenting the latter in their incipient con- dition ; whilst others are themselves just starting into existence, and as yet exhibit no traces of a second generation.—The former seems to be the usual type of the ordinary Glands; the latter is chiefly, if not entirely, to be met with in the Spermatic glands. 2. Of the Liver. 720. The Liver is more rarely absent than any other Gland; being discoverable, under some form or other, in all but the very lowest members of the Animal kingdom. Its simple condition in the higher Polypes has been already noticed (§ 716); and it is met with, under an almost equally simple form, in the Star-fish. As we ascend the scale, however, we find it assuming a much greater importance, and presenting a great increase in size. This is particularly the case in the Molluscous classes ; and also in the Crustacea,—a class which, in mode of respiration and in general habits, bears a great resemblance to the Mollusca. In nearly all such animals, the Liver makes up a large proportion of the mass of the body. It usually consists of a series of large follicles, which branch out into smaller ones (Figs. 112 and 113), Fig. 112. Fig. 113. Lobule of Liver of Squilla Mantis; exterior. Lobule of Liver of Squilla Mantis cut open. and of which several open into one excretory duct; but these ducts remain separate, and discharge their contents into the intestine by several distinct orifices.—In Insects and other air-breathing Articu- STRUCTURE OF THE LIVER. 411 lata, however, the Liver is much less developed ; and its type remains much simpler. We usually find it consisting of a small number of caecal tubuli, which open separately into the intestinal canal, just below the stomach. These tubuli are often so long, as to pass several times from one extremity of the visceral cavity to the other, being doubled upon themselves; in other instances, we find that the prin- cipal tube or canal is beset with rows of short follicles, somewhat in the manner of Fig. 107. But they never cluster together, so as to form a solid glandular mass. The low development of the liver, in these animals, bears an evident relation with the high development of their respiratory apparatus; whilst, the respiration being compara- tively feeble in the aquatic Mollusca and Crustacea, the development of the liver in those classes is enormous. 721. There is much difficulty in ascertaining the mode in which the elementary constituents of the Liver are arranged, in the fully- developed condition of that organ in the higher Vertebrata. At an early period of its development, as already remarked, it may be Fig.114. easily shown to consist, in the Fowl, of a series of distinct caeca, clustered round a projection from the intestinal canal, and opening separately into it (Fig. 104); and it is a peculiarly interesting fact, that this very condition should exist as the permanent form of the Liver, in that curious little fish, the Amphioxus or Lancelot, which retains the embryonic type in so many parts of its conformation. In the Tadpole, again, the distinct caeca are very evident (Fig. 114); but here we see that the projection of the intestinal canal, instead of being a simple wide eaecum, has become extended in length and contracted in diameter, at the same time dividing and subdividing, so as to form an arborescent excretory duct, whose ramifications extend through the entire glandular mass. In this manner, then, is formed the complex system of hepatic ducts, which we find in the liver of the higher Vertebrata, branching out from the main trunk. But the mode in which the ultimate ramifica- tions of these are arranged, and their relations with the secreting cells, which make up the parenchyma of the gland, have not yet been fully elucidated. The following are the principal facts, that have been ascertained on the subject. 722. The entire Liver is made up of a vast number of minute lobules, of irregular form, but about the average size of a millet-seed. Each of these lobules contains the component elements, of which the Liver of Tadpole; showing distinct and free caecal terminations of the biliary ducts. 412 DISTRIBUTION OF BLOOD-VESSELS OF THE LIVER. entire organ is made up;—namely, branches of the hepatic artery and vein, branches of the portal vein, branches of the hepatic ducts, and secreting cells. The lobules are connected together in part by areolar tissue, but in great part by the anastomosis of the blood-vessels and hepatic ducts, which supply the .adjoining lobules; indeed there is frequently no definite division of the glandular substance into lobules, other than that, which is marked out by the arrangement of these canals (Figs. 115 and 117). The branches of the Hepatic Artery are principally distributed upon the walls of the hepatic ducts, and upon the trunks and branches of the portal and hepatic veins, supplying them with their vasa vasorum; also upon Glisson's capsule and its prolongations into the substance of the liver,—which prolonga- tions form the greatest part of the connecting structure, that holds together the several elements. There is strong reason to believe, that the blood which the liver receives from the hepatic artery is not destined to supply the materials for the biliary secretion, until it has become venous by traveling through the network, in which it is sub- servient to the nutrition of the tissues it permeates, as it is in other parts of the systemic capillary system.—The supply of blood, from which the materials of the biliary secretion are chiefly drawn, is afforded by the Vena Portce, which collects it as a Vein from the chy- lopoietic viscera, and which then subdivides as an Artery to distribute it to the different parts of the Liver. Its branches proceed to the cap- sules of the lobules, covering the whole external surface of the latter with their ramifications, and sending capillary twigs inwards, which converge towards the centre of each lobule (Fig. 115). As the prin- Fig. 115. Fig. 116. cipal branches of these veins ramify in the spaces between the lobules, they are termed inter-lobular veins.—On the other hand, the branches HEPATIC DUCTS, AND CELLS OF PARENCHYMA. 413 of the Hepatic Vein pass from the trunks to the centre of each lobule, from which they send out diverging capillary twigs towards the cir- cumference ; and these last, coming into connection with the converg- ing capillaries of the portal vein, establish a free capillary communi- cation between the interior and exterior of each lobule. Thus the portal blood is first distributed to its exterior, then penetrates its sub- stance, and then, after permeating the parenchymatous substance in numerous minutely-divided streams, is collected and carried off by the hepatic vein, of which a twig originates in the centre of each lobule. Owing to the peculiar position of the branches of the hepatic vein in the centre of each lobule, the lobules are appended to its main trunks almost in the manner of leaves upon a stem (Fig. 116).—The precise relation of the capillaries of the hepatic artery with those of the por- tal and venous systems, has not yet been well ascertained; but there seems reason to believe, with Mr. Kiernan, that the arterial capillaries discharge themselves into the ultimate ramifications of the portal vein ; and that thus the blood of the former, having become venous by transmission through the nutritive capillaries of the liver, mingles with the other venous blood collected by the venae portae, to supply the materials of the secretory function, which are eliminated frora it during its passage into the hepatic vein. 723. The Hepatic Ducts also form a plexus, which surrounds the lobules ; connecting them together, and sending branches towards the interior of each. The mode in which they terminate, however, and the precise relation in which they stand to the hepatic cells, which form nearly the entire parenchyma of the Gland, are yet unexplained. Fig. 117. Fig. 118. Horizontal section of two superficial lobules, showing Glandular cells of Liver:—o, nucleus' the interlobular plexus of biliary ducts; 1,1, intralobu- 6, nucleolus; c, adipose particles. lar veins; 2, 2, trunks of biliary ducts, proceeding from the plexus which traverses the lobules; 8, interlobular tissue; 4, parenchyma of the lobules. (After Kiernan.) These cells are of a flattened spheroidal form, and commonly lie in piles, their faces adhering to one another; and these piles seem to be directed especially from the circumference to the centre of each lobule. Every one of them presents a distinct nucleus; and the cavity of the cell is filled with yellow amorphous biliary matter, having one or two large adipose globules, or five or six small ones, intermingled with it. 414 CHEMICAL COMPOSITION OF BILE. Their diameter is usually from l-1500th to l-2000th of an inch ; and they are easily obtained in a separate condition, by scraping a piece of fresh Liver. The biliary matter which they contain, marks them out as the real agents in the secreting process ; this process consisting, it is evident, in the growth of the hepatic cells, w-hich, in the course of their development, eliminate from the blood the biliary matter, for which they have a special affinity. The mode in which the particles thus eliminated, are discharged into the hepatic ducts, to be by them conveyed to the intestine, cannot be understood, until the relation between the secreting cells and the ultimate ramifications of the ducts shall have been more precisely determined. 724. The Bile which has been secreted by the hepatic cells, and which has found its way into the ramifications of the hepatic ducts, may be directly conveyed by the trunk of the latter into the intestine, or it may regurgitate along the cystic duct into the gall-bladder. It is probable that the secreting process is constantly going on; although, as in other cases, it may vary in its degree of activity at different times. When the process of digestion is taking place, and the small intestine is filled with chyme, there is probably an uninterrupted flow of bile into its cavity; but when the intestine is empty, the bile seems not to be admitted into it, but rather to flow back into the gall-bladder, in which it is stored up as in a reservoir, until the time w-hen it may be needed. In this reservoir it undergoes a certain degree of concentration, by the absorption of its watery part; and it also becomes mixed with a large proportion of mucus, which is secreted by the walls of the gall-bladder. —As the analyses of Bile have been chiefly made upon the fluid ob- tained from this receptacle, they probably over-estimate the proportion of solid matter contained in this secretion; which is usually stated at from 8 to 9£ per cent. Of this solid matter about a tenth consists of alkaline and earthy salts, corresponding with those of the blood ; whilst the remainder is made up of organic constituents. These are very readily decomposed, and enter into new combinations with the sub- stances employed to separate them; so that different chemists, by em- ploying different means of analysis, have obtained results which seem far from conformable. All are agreed, however, that the chief part of the solid ingredients of bile are allied to fat in composition; con- sisting of a very large proportion of carbon and hydrogen, and of a comparatively small amount of oxygen and azote. According to Dr. Kemp, the organic portion of ox-bile may be represented by the for- mula 48 Carbon, 42 Hydrogen, 13 Oxygen, and 1 Nitrogen. This substance, essentially corresponding with the bilic acid, choleic acid, bilin, picromel, &c, of different Chemists, seems to be a fatty acid' (§ 261), united with soda, so as to constitute a soap. In healthy bile, the proportion of Cholesterine appears to be very small; and it is held in solution by the preceding ingredient: but in many disordered states, and especially in disease of the Gall-bladder, this element is present in much larger amount; and it usually forms the principal, if not the sole, ingredient in biliary concretions. It is a white crystalizable PURPOSES OF THE BILIARY SECRETION. 415 fatty matter, somewhat resembling spermaceti, free from taste and odour, and composed almost entirely of carbon and hydrogen,—its formula being 36 Carbon, 32 Hydrogen, and 1 Oxygen.—The Colour- ing matter of Bile is a substance distinct from the preceding; that of the Ox and other graminivorous animals appears to be identical, or nearly so, with the Chlorophyll of the leaves on which they feed; but that of human bile seems to possess different properties, and to be derived frora the proper constituents of the blood. 725. Regarding the destination and purposes of this secretion in the Animal economy, the following may be considered as a tolerably com- plete summary; though it is difficult to speak with precision on some points; since the organic constituents of the Bile are liable to be so easily altered by various reagents, that they are with difficulty recog- nized. A portion of the Bile unquestionably passes off, in Man and most other animals, with the feces. This portion, which includes the colouring matter, is probably that which would be most injurious, if retained in the blood, and is most purely excrementitious. But the soapy portion has quite another destination. Just as ox-gall is com- monly used to remove grease spots, by its solvent power for fatty mat- ter, so does the bile seem to act in the living body, by rendering soluble the fatty matters of the food, and thus enabling them to be absorbed by the lacteals (§ 494). Hence, if the passage of bile into the intestine be prevented (as in the recent experiments of Schwann) without any check to its separation from the blood, the animals gradually lose their plumpness, and at last die in a state of emaciation,—the fatty matter of their food not being introduced into their absorbent system, nor ap- plied to the maintenance of their respiration. The fatty matter of the bile, when re-absorbed with that of the newly-ingested food, is proba- bly, like it, carried off by the respiratory process: but it is easily shown, that the biliary matter cannot supply more than one-sixth" or one- eighth of the amount of carbon eliminated from the lungs in the form of carbonic acid ; and that it cannot be (as supposed by Liebig) the chief fuel of the process of combustion, which is kept up through the agency of those organs. 726. The elements of the Bile may be altogether supplied by the disintegration of the tissues; and this must certainly be the case, when the amount of food taken is no more than enough to supply the waste of the system. We may regard it, then, as one office of the Liver to remove from the blood such products of that disintegration, as are rich in carbon and hydrogen. But there can be little doubt, that the Liver has also for its office, to draw off from the blood any superfluity which may exist in the non-azotized compounds derived from the food, be- yond the amount that is requisite for the supply of the respiratory pro- cess, or that can be deposited as fat. For we continually witness the results of habitual excess in the amount of such sbstances, in pro- ducing that state of the system commonly termed bilious; of which all the symptoms are referable to the accumulation of the elements of the bile from the blood, and the consequent deterioration in the purity of 416 COMPARATIVE STRUCTURE OF KIDNEY. the circulating fluid. Where a tendency to such a state exists, proper means should be taken to stimulate the liver to increased activity; but the chief reliance should be placed on the avoidance of those articles of diet, which contain a large proportion of non-azotized matter, and on abstinence from superfluous nutriment of any description. That the less hydro-carbon separated from the blood by Respiration, the more is eliminated from it by the Biliary secretion, seems to be a general prin- ciple throughout the Animal kingdom; the Liver and Respiratory organs bearing, almost everywhere, an inverse ratio to each other in their degree of development. Fig. 119. 3. Of the Kidneys and the Urinary Excretion. 727. The Kidneys are perhaps the most purely excreting organs in the body; their function being to separate from the Blood certain matters that would be injurious to it if retained -, and these matters being destined to immediate and complete re- moval from the system. We have seen that, in the Lungs, the excretion of Carbonic acid is made subservient to the absorption of Oxy- gen ; and the separation of a fatty acid from the blood, which is effected by the Liver, is a means of introducing a new supply of fatty matter into the system. There is no ulterior purpose of this kind in the secreting action of the Kidney; the product of which is invari- ably conveyed directly to an outlet, by which it may be discharged from the body. Some traces of Urinary organs may be detected in most of the higher Invertebrata; but it is in Fishes, that they first present a considerable de- velopment ; and in ascending through the Ver- tebrated series, we find them rapidly increasing in the complexity of their organization, and in their functional importance, although their size and extent are not so great. In Fishes the Kidneys very commonly extend the whole length of the abdomen ; and they consist of tufts of uniform sized tubules, which shoot out transversely at intervals from the long ure- ter, and which are connected together by a loose web of areolar tis- sue, that supports the network of vessels distributed upon their walls. This condition of the urinary organs is very analogous to that of the Corpus Wolffianum or temporary kidney of the embryo of higher ani- mals (Fig. 119,/). A similar condition is found in the true Kidney of higher animals at an early grade of development (as shown in Fig 120); the tubuli uriniferi being short and straight. In their more ad- vanced condition, however, they become long and convoluted; and the ramifications of the capillary vessels come into very close relation with them (Fig. 121). It is in the higher Reptiles, that we first meet Embryo of Green Lizard :— o, heart; b, duplex aorta; c, vena cava; d, intestine; e, liver; /, rudiment of Wolffian body; g, g, rudiments of ex- tremities. STRUCTURE OF HUMAN KIDNEY. 417 h the distinction between the cortical and medullary substance ; former being the part in which the blood-vessels are most copi- Fig. 120. Kidney of foetal Boa:—the urinary tubes as yet short and straight. ously distributed, and in which the tubuli have the most convoluted arrangement; and the latter consisting chiefly of straight tubuli, con- verging towards the points at which they discharge themselves into the ureter (Fig. 122). The bundles of tubuli and their vascular Fig. 121. Fig. 122. Portion of Kidney from Coluber:—a, a, vascular trunk; Pyramidal fasciculi of tubuli 6, 6, ureter; c, c, converging fasciculi of tubuli uriniferi. uriniferi of Bird, terminating in one of the branches of the ureter. plexuses remain distinct, however, in Birds and in the lower Mam- malia, so as to give to the gland a lobulated character; but in the Human Kidney,they come into closer contact; and the vascular con- nection between the plexuses of the different bundles is such, as to prevent any separation into distinct lobules. 728. The act of secretion appears to be effected, as in other Glands, by the epithelial cells lining the tubuli uriniferi; these cells drawing the materials of their development from the vascular plexus upon the exterior of these tubuli; and delivering them up, when they have com- pleted their own term of existence, to be carried off through the open orifices of the tubuli. But the Kidney contains another apparatus, of a very peculiar description ; which appears specially destined for the separation of the superfluous^wid of the system. When a section of the Kidney is slightly magnified (Fig. 124, b), the cut surface is seen to be studded by a number of little dark points ; each one of which, when examined under a higher magnifying power, is found to consist of a knot of minute blood-vessels, formed by the convolutions of thin-walled capillaries (Fig. 125, m). According to the recent inqui- ries of Mr. Bowman, each one of these knots is included in the extremity of one of tubuli uriniferi, which swells into a flask-like capsular dilatation to receive it. Each of these vascular tufts (called 27 418 CIRCULATION IN THE KIDNEY. Malpighian bodies, after their discoverer), is directly supplied by a branch of the renal artery (Fig. 125, af); which, upon piercing the Fig. 123. Fig. 125. Distribution of the Renal ves- sels ; from Kidney of Horse :—a, branch of Renal artery; af, afferent vessel; m, m, Malpigh- ian tufts ; ef, ef, efferent vessels; p, vascular plexus surrounding the tubes; st, straight tube ; ct, convoluted tube. Magnified about 30 diameters. A section of the Kidney, sur- Portion of the Kidney mounted by the supra-renal cap- of a newborn infant, a, sule;the swellings upon the surface natural size; 1, 1. Cor- mark the original constitution of pora Malpighiana, as the organ, as made up of distinct dispersed points in the lobes. 1. The supra-renal capsule. cortical substance; 2, 2. The vascular portion of the kid- papilla. b, a smaller ney. 3,3. Its tubular portion, con- part magnified; 1,1, Cor- sistmg of cones. 4, 4. Two of the pora Malpighiana; 2, 2, papillae projecting into their cor- tubuli uriniferi. responding calices. 5, 5, 5. The three infundibula; trie middle 5 is situated in the mouth of a calyx. 6. The pelvis. 7. The ureter. capsule, subdivides into a group of capillaries; and these, after form- ing the convoluted tuft, coalesce into a single efferent trunk (ef), which may be considered as representing (in a small way) the vena porta-. For the efferent trunks of the Malpighian bodies discharge their blood into the capillary plexus, which surrounds the tubuli uriniferi, and from which the solid matter of the urinary secretion is elaborated ; just as the vena portse supplies the capillary plexus, from which the bihary secretion is elaborated in the liver. The special purpose of the Malpighian bodies appears to be, to allow of the trans- it ? Sf ^ WatZ °f the blood' which is filtered off (so to speak) hrough the thin walls of their capillaries, and thus passes into the tubuli uriniferi. It is well known that the fluid and solid constituents of the urinary secretion bear no constant relation to each other; the amount of fluid depending mainly upon the degree of fullness of the blood-vessels; whilst the amount of solid matter is governed, as we shall presently see, by the previous waste of the tissues. The quan- tity of fluid in the blood-vessels is governed by the relative amount that has been absorbed, and that which has been exhaled from the skin ; so that the quantity to be drawn off by the Kidneys is increased either by augmented absorption, or by diminished exhalation. The CHARACTERS OF THE URINARY EXCRETION. 419 Malpighian bodies seem to act the part of a system of regulating valves ; permitting the transudation of only enough fluid to dissolve the solid matter, when there is no superfluity of water in the vessels; but allowing the escape of an almost unlimited amount of it, when increased imbibition has rendered the vessels unusually turgid. 729. The average amount of Urine excreted in twenty-four hours, by adults who do not drink more than the wants of nature require, is probably from 30 to 40 oz. ; and its average specific gravity may be about 1020. The quantity of fluid is usually less, and the specific gravity of the secretion consequently greater in summer than in win- ter ; on account of the larger proportion of fluid exhaled by the skin during the former season. The quantity of solid matter has been found to vary, within the limits of ordinary health, from 3-6 to 6*7 per cent.; and the extent of variation in disease is doubtless much greater. About one-third of the solid matter is made up of alkaline and earthy salts; and the remainder is made up of organic compounds. The salts are partly those of the blood, which will not be separated during the transudation of the serum through the membranous walls of the Malpighian capillaries, although the albuminous matter is kept back (§ 196). But there is a much larger proportion of the alkaline and earthy phosphates in the urine, than is present in the blood ; and this is liable to a further increase under circumstances to be presently alluded to. 730. The organic compounds present in the Urinary secretion (in its healthy state at least), are undoubtedly the result of the waste or disintegration of the animal fabric; as well as (in certain cases) of the decomposition of constituents of the blood, which have never under- gone conversion into organized tissue. Their unfitness to be retained within the system, is proved by the fatal results which speedily ensue, when their elimination by the secreting process receives a check; and also by the crystaline form, in which the most characteristic of them present themselves,—such a form being altogether incompatible with the possession of plastic or organizable properties. Of these com- pounds, the most important, in Man, is that which is named Urea. It exists in Urine in a state of perfect solution ; and may be readily separated from it in the form of transparent colourless crystals, which have a faint and peculiar but not urinous odour. In its ultimate composition it is identical with Cyanate of Ammonia, being made up of 2 Carbon, 4 Hydrogen, 2 Nitrogen, and 2 Oxygen,—a formula much more simple than that of almost any other organie substance. If we compare its composition with that of Proteine, we shall find that it contains a far larger proportion of Nitrogen, and far less Carbon and Hydrogen. Thus, making Oxygen the standard of comparison, we find that 1 Equivalent of Proteine contains 40 C, 31 H, 5 N, 12 O. 6 Equivalents of Urea contain 12 C, 24 H, 12 N, 12 0. 731. Hence it seems evident, that the great purpose of the Urinary 420 UREA, AND URIC ACID. excretion is to carry off those products of the metamorphosis of the azotized tissues, which can neither be set free in the condition of car- bonic acid and water through the lungs, nor got rid of by the agency of the liver in the form of solid biliary matter. The amount of Urea in the Urine is liable to very great variation, in accordance with the degree in which the disintegrating process has been taking place in the solid fabric; and also in conformity with the amount of azotized matter, which has been taken in as food. Supposing that the latter were so precisely adjusted to the wants of the system, as to supply only that which is required for its maintenance, we might then mea- sure the amount of previous waste, by the quantity of Urea present in the Urine. There can be no doubt as to the fact, that, other things being equal, the amount of Urea is greatly increased by any unusual exertion of the Muscular system ; but such an increase cannot be in- variably, or even usually, attributed to this cause; since it is equally certain, that any superfluity in the amount of azotized matter received into the blood, must be drawn off by the urinary excretion, and thus that an increase in the quantity of urea may be occasioned by an ex- cessive use of proteine-compounds as articles of food. The average proportion of Urea, under ordinary circumstances as to diet and exer- cise, seems to be from 20 to 35 parts in 1000 ; but it may be raised to 45 parts by violent exercise, and to 53 parts by an exclusively ani- mal diet; whilst it may fall as low as from 12 to 15 parts, when the diet is deficient in azotized matter. The total daily excretion of Urea in adult males seems to average about 430 grains, and that of females nearly 300 grains; but these averages may be widely departed from, on the side either of excess or diminution, according to the circum- stances already noticed. It is interesting to observe, that children of eight years old excrete, on the average, half as much Urea as adults; whilst, in very old persons, the quantity sinks to one-third, or even less. In proportion to their relative bulks, therefore, children excrete at least two or three times the quantity of urea, that is set free by adults ; and four or five times that which is excreted by old persons; —a fact which corresponds with other indications of the far greater rapidity of interstitial change in the earlier periods of life, than in adult or advanced age. 732. There is an organic compound, nearly allied to Urea in com- position, but differing from it in its distinctly acid properties, and also in its comparative insolubility. This substance, termed Uric or Lithic Acid, forms but a small proportion of the solid matter of Huraan Urine in the state of health ; but it is the chief element in the Urine of the lower Vertebrata; and its presence in too large a proportion is a frequent source of disease in Man. Its ultimate composition is 10 Carbon, 4 Hydrogen, 4 Nitrogen, 6 Oxygen; it crystalizes in fine scales of a brilliant white colour and silky lustre ; and it is so sparingly soluble in water, that at least 10,000 times its own weight of fluid is required to dissolve it. In healthy Human urine, it is in a state of perfect solution; but it is precipitated immediately on the ad- URIC AND HIPPURIC ACIDS. 421 dition of a small quantity of any acid, even the Carbonic: it is evi- dent, therefore, that it is held in solution by union with some base; and according to Liebig, this base is Soda, obtained from the bibasic Phosphate of Soda, which is present in the urine, and which, by yielding up a part of its base, gives the acid reaction that is characr teristic of the fluid in a healthy state. It is not unfrequently seen, that the Urine, although clear when voided, deposits Lithic acid when it is cooled; and this deposit may be due either to the presence of more Lithic acid than the Phosphate of Soda can take up when cold; or to the presence of some other acids in the Urine, which set free the Lithic acid, when the solvent power of the Phosphate is diminished by the depressed temperature. 733. The amount of Uric acid in healthy Urine does not seem to be much influenced by the diet, or by the waste of the tissues; never varying much, either by excess or diminution, from 1 part in 1000. It is liable, however, to be greatly increased in certain disordered states of the system ; and the surplus, not being kept in solution by the Phosphate of Soda, is deposited as a sediment, which usually has a crystaline character, and is tinged of a reddish hue by the colour- ing matter of the urine. Not unfrequently the Uric acid is deposited in combination with an alkaline base; and the colour of the sediment is then usually of a brick-red. Such depositions may take place directly from the blood ; thus, in attacks of Gout, urate of soda is separated from the circulating blood, and is deposited in the tissues around the affected joints, forming the concretions termed " chalk- stones." There can be no doubt that, when there is a positive ex- cess of Uric acid in the Urine, it may be generally reduced by dimin- ishing the quantity of azotized matter in the food ; but when the deposit is consequent upon the presence of some other acid in the urine, bur treatment should be rather directed to the neutralization of this, or to the prevention of its formation. 734. There seems reason to believe that we are to regard Hippuric acid as a normal element of the Urine of Man, although it has been usually supposed to be restricted to the Herbivorous quadrupeds, where it replaces LTric Acid. Its composition and properties are very different from those of that substance. When pure, it forms long transparent four-sided prisms; it is soluble in 400 parts of cold water, and dissolves readily at a boiling heat; and it has a strong acid reac- tion, with a bitterish taste. It is composed of 18 Carbon, 8 Hydro- gen, 1 Nitrogen, and 5 Oxygen, with 1 equiv. of Water. When exposed to a high temperature, or subjected to the putrefactive pro- cess, it is partly converted into Benzoic acid ; and it is on the presence of the latter in putrefied Human Urine, that the belief in the existence of Hippuric acid in the same fluid when fresh, is chiefly grounded. It is a curious fact, that the administration of Benzoic acid causes the appearance of a large additional quantity of Hippuric acid in the Urine; so that its presence is then sufficiently evident. This fact has been applied to the treatment of disease ; for as the salts of Hippuric 422 OTHER CONSTITUENTS OF URINE. acid are much more soluble than those of Lithic acid, it is obviously advantageous to cause the effete crystaline matters, which are de- stined for elimination from the system, to be discharged as soluble Hippurates, rather than to be deposited as insoluble Lithates; and it is asserted by Mr. A. Ure, that he has succeeded, by the administra- tion of Benzoic acid, in preventing the deposition of Gouty concre- tions, and even in removing them when they had been formed. 735. Another acid has usually been regarded, until recently, as one of the regular constituents of healthy urine, and as liable to un- dergo a considerable increase in disease. This is the Lactic; an acid which is readily formed in Milk, by the metamorphosis of its saccha- rine elements. But it seems doubtful, from the recent inquiries of Liebig, whether we are to admit it as a regular constituent of healthy human Urine; and it appears that a peculiar azotized compound, which is not entitled to the designation of an acid, but which forms a definite combination with Zinc, has been mistaken for it. The amount of this azotized product, and of the compounds it forms (usually designated as lactic acid and the lactates), has been found to undergo considerable increase, when the food ingested was alto- gether destitute of azote, and when the proportion of urea was the smallest. It would seem as if some of the azotized matter, result- ing from the disintegration of the tissues, was then discharged in this form, rather than in that of the more highly azotized compound, urea. 736. Of the substances ranked under the head of Extractive Mat- ters, very little is definitely known. They seem to consist, for the most part, of non-azotized compounds in a state of change ; and their usual proportion, which is about 10 parts in 1000, has been found to increase to 16^ parts when the diet was exclusively vegetable, and to diminish to 5 parts when only animal food was ingested. 737. The Urine also contains a considerable amount of Saline matter, of which the acids as well as the bases are derived from the mineral kingdom ; and the excretion of them, after they have served their purpose in the economy, appears to be one of the chief functions of the Kidney. Of these, a part may find their way directly into the urine from the serum of the blood, when its water is being filtered off (so to speak) through the walls of the Malpighian capillaries ; for although, from the peculiar properties of animal membranes (§ 196), the albuminous constituents of the serum are held back, the saline matter, which is in a state of perfect solution, must pass with the water. This is probably the chief source of the large quantity of the muriates of soda and ammonia contained in the urine. But the Urinary secretion seems to be specially destined to eliminate the saline compounds, which are formed by the acidification of the Sulphur and Phosphorus, taken in with the proteine-compounds as food. These substances are united with Oxygen in the system, and are thus con- verted into Sulphuric and Phosphoric acids; which acids unite with alkaline bases, that were ingested in combination with Citric, Tartaric, SALTS OF THE URINE. 423 Oxalic, and other organic acids; the latter undergoing decomposition within the system, and leaving the bases ready to unite with others. Such weakly combined bases abound in the food of Herbivorous ani- mals ; and their urine is almost invariably alkaline, the quantity of the Sulphuric and Phosphoric acids generated in the system not being sufficient to neutralize it. On the other hand, they are nearly absent in the food of the Carnivora; and their urine is therefore almost inva- riably acid, from the want of neutralization of the Sulphuric and Phosphoric acids. 738. The Alkaline Sulphates, whether taken in as such, or formed in the manner now described, are soluble enough to be always passed off in a fluid form ; but this is not uniformly the case with the Phos- phates, which are frequently deposited as sediments of a dead-white aspect, sometimes crystaline, and sometimes wholly or partly amor- phous. The crystaline sediment consists of the triple phosphate, or phosphate of ammonia and magnesia; the amorphous contains an ad- mixture of the phosphate of lime, The urine, when these are depo- sited, is usually alkaline, sometimes very decidedly so; and there is reason to think that, in many cases, this alkaline character, and the deposit of phosphatic sediments, are due to an alkaline secretion frora the walls of the bladder and urinary passages, which result from an irritable state of their membrane,—the urine, as secreted by the kid- ney, having its usual properties.* That an alkaline condition of the urine, resulting from the presence of an unusual amount of bases, is capable of producing a phosphatic deposit, is shown by the simple experiment of adding ammonia to healthy urine, which will occasion a precipitation of the triple phosphate. 739. But there can be little doubt, that a frequent cause of the deposit is excessive production of the phosphatic salts, arising from the increased waste or disintegration .of Nervous matter, which takes place when it is in a state of unusual activity, either from intense thought, from prolonged exertion, or frora continued anxiety. The general principles already set forth, in regard to the dependence of the functional activity of the Nervous Centres upon a supply of arte- rialized blood (§ 384), show the probability that every act of theirs involves the oxygenation of a certain quantity of nervous matter. In this oxygenation, phosphoric acid will be produced, from the large amount of phosphorus contained in the nervous matter ; and this will unite in part with ammonia, which is perhaps set free by the same metamorphosis, or is derived from other sources; and in part with bases derived from the food. The experience of every studious man must have shown him (if he make any observations on the matter at all) the frequent coincidence between the presence of phosphatic deposits in his urine, and an excess of mental labour; and there are many instances on record, in which the periodical recurrence of the * See Dr. G. O. Rees on the Analysis of the Blood and Urine in Health and Dis- ease, 2d Ed. p. 136. 424 DISORDERED STATES OF THE URINARY SECRETION. latter has been so invariably followed by the recurrence of the former, that no reasonable doubt can exist as to their mutual connection. 740. It is very important, for the successful treatment of those Urinary deposits, which consist of the normal elements of the Urine, —namely, Lithic Acid, and the Phosphates,—that the leading facts already stated should be borne continually in mind. In the first place, these sediments may depend upon the general condition of the fluid, and not upon any excess in the constituents of which they are com- posed ; thus a lithic deposit may result from the presence of an excess of some other acid in the urine; and a phosphatic sediment may be produced by the excess of bases. In such cases, then, our treatment should be directed, not to diminish the quantity of the peculiar con- stituents of the deposits, but to rectify the state of the Urine on which their precipitation depends. But, in the second place, the sediments may be present in such great amount, as to indicate that their consti- tuents are present in the urine to an excessive degree ; and our treat- ment must then be directed towards the diminution of the quantity produced. Thus the tendency to lithic acid deposit may be frequently cured, by simply diminishing the quantity of azotized matter in the food; and the undue formation of the phosphates may be often kept in check by that mental repose, which is peculiarly required after long-continued and severe exercise of the intellectual faculties, or strong excitement of the feelings. 741. There is no doubt whatever, that the total suspension of the Urinary secretion is productive of rapidly fatal results, from the accu- mulation of the elements of the secretion in the blood ; and it would appear that the tissue on which their presence in the circulating fluid exerts the most injurious effects, is the Nervous. It is probable that Urea is the substance, which is most directly concerned in producing the noxious influence ; and we see an effort made by the system (so to speak) to get rid of it, in those cases in which a discharge of urinous fluid takes place by unusual channels, such as from the mucous mem- brane of the stomach, the mamma, the umbilicus, the jiose,&c, when the usual secreting action of the Kidney has been suspended. Al- though the accounts of such cases have been treated with ridicule by some Physiologists, yet there seems no valid reason to discredit them, when it is borne in mind that, in persons who have died from the complete suspension of the secretion, effusions containing urea have been found in the serous cavities of the trunk, and in the ventricles of the brain. The poisonous influence of an accumulation of urea in the blood, when strongly exerted, produces, in the first instance, irregular or convulsive movements, which are dependent upon irritation of the Spinal system of nerves; then loss of consciousness, depending upon the suspension of the powers of the Brain; and, lastly, complete sus- pension of the powers of the spinal system, so that the ordinary reflex actions cease, and life becomes extinct from the stoppage of the re- spiratory movements (§ 688). There is reason to believe that many convulsive motions, for which no obvious cause can be assigned, have CUTANEOUS GLANDULE. 425 their origin in a disordered condition of the blood, resulting from im- perfect elimination of Urea; thus it has been ascertained that, in seve- ral cases of puerperal convulsions, urea was present in the blood; the functional power of the kidney being diminished by chronic disease. It is especially to be noticed, that most of the cases in which the urinary secretion is discharged through some irregular channel, occur in persons who have been subject to those convulsive affections, which are commonly designated as hysterical; and that the discharge of a large quantity of urine through the natural channel, is often the termination of an hysterical paroxysm. It is desirable, therefore, that in all such obscure cases, the state of the urinary secretion should be carefully looked to. 4. Of the Cutaneous and Intestinal Glandulce. 742. The Glandulse which are disposed in the substance of the Skin, and in the walls of the Intestinal canal, although individually minute, make up by their aggregation an excreting apparatus of no mean importance. The Skin is the seat of two processes in parti- cular ; one of which is destined to free the blood of a large quantity of fluid ; and the other to draw off a considerable amount of solid matter. To effect these processes, we meet with two distinct classes of glandulae in its substance ; the Sudoriparous or sweat-glands; and the Sebaceous or oil-glands. They are both formed, however, upon the same simple plan; and can frequently be distinguished only by the nature of their secreted product. 743. The Sudoriparous or per- spiratory glandulae form small oval or globular masses, situated just beneath the cutis, in almost every part °f the surface of the body. Each is formed by the convolu- tion of a single tube; which thence runs towards the surface as the efferent duct, making numerous spiral turns in its passage through the skin, and penetrating the epi- dermis rather obliquely, so that its orifice is covered by a sort of little valve of scarf-skin, which is lifted Fig. 126. The anatomy of the skin. 1. The epidermis, showing the oblique laminae of which it is com- posed, and the imbricated disposition of the ridges upon its surface. 2. The rete mucosum or deep layer of the epidermis 3. Two of the quadrila- teral papillary clumps, such as are seen in the palm of the hand or sole of the foot; they are composed of minute conical papillae. 4. The deep layer of the cutis, the conum. 5. Adipose cells. 6. A sudoriparous gland with its spiral duct, such as is seen in the palm of the hand or sole of the foot. 7. Another sudoriparous gland with a straighter duct, such as are seen in the scalp. 8. Two hairS from the scalp, enclosed in their follicles; their relative depth in the skin is preserved. 9. A pair of sebaceous glands, open- ing by short ducts into the follicle of the hair. 426 VARIATIONS IN CUTANEOUS EXHALATION. up as the fluid issues from it. The convoluted knot, of which the gland consists, is copiously supplied with blood-vessels. On the palm of the hand, the sole of the foot, and the extremities of the fingers, the apertures of the perspiratory ducts are visible to the naked eye, being situated at regular distances along the little ridges of sen- sory papillae, and giving to the latter the appearance of being crossed by transverse lines. According to Mr. Erasmus Wilson, as many as 3528 of these glandulae exist in a square inch of surface on the palm of the hand; and as every tube, when straightened out, is about a quarter of an inch in length, it follows that in a square inch of skin from the palm of the hand there exists a length of tube equal to 882 inches, or 73£ feet. The number of glandulae in other parts of the skin is sometimes greater, but generally less than this ; and according to Mr. Wilson, about 2800 inches may be taken as the average num- ber of pores in each square inch throughout the body. Now the number of square inches of surface, in a man of ordinary stature, is about 2500; the number of pores, therefore, is seven millions; and the number of inches of perspiratory tubing would thus be 1,750,000; or 145,833 feet; or 48,611 yards ; or nearly 28 miles. 714. From this extensive system of glandulae, a secretion of watery fluid is continually taking place; and a considerable amount of solid matter also is drawn off by the epithelium-cells that line the tubuli. Under ordinary circumstances, the fluid is carried off in the state of vapour, forming the insensible perspiration; and it is only when its amount is considerably increased, or when the surrounding air is already so loaded with moisture as to be incapable of receiving more, that the fluid remains in the form of sensible perspiration upon the sur- face of the skin. It is difficult to estimate the proportion of solid matter contained in this secretion ; partly on account of the great variations in the amount of fluid eliminated by the Sudoriparous glands, which are governed by the temperature of the skin; and partly because the secretion can scarcely be collected for analysis, free from the sebaceous and other matters which accumulate on the surface of the skin. According to Anselmino it varies frora \ to \\ per cent.; and consists in part of lactic acid, to which the acid reac- tion and sour smell of the secretion are due; in part of a proteine- compound, which is probably furnished by the epithelium-cells that line the tubes ; and in part of saline matters, directly proceeding from the serum of the blood. 745. The amount of fluid excreted from the skin is almost entirely dependent upon the temperature of the surrounding medium; being increased with its rise, and diminished with its fall. The object of this variation is very evident; being the regulation of the temperature of the body. When the surface is exposed to a high degree of external heat, the increased amount of fluid set free from the perspiratory glands becomes the means of keeping down its own temperature; for this fluid is then carried off in a state of vapour, as fast as it is set free; and in its change of form, it withdraws a large quantity of caloric SUPPRESSED EXHALATION. 427 from the surface. But if the hot atmosphere be already loaded with vapour, this cooling power cannot be exerted ; the temperature of the body is raised ; and death supervenes, if the experiment be long con- tinued. The cause of the increased secretion is probably to be looked for in the increased determination of blood to the skin, which takes place under the stimulus of heat.—The entire loss by Exhalation from the lungs and skin, during the twenty-four hours, seems to average a little above 2 lbs. In a warm dry atmosphere, however, it has been found to rise to as much as 5 lbs.; whilst in a cold damp one, it may be lowered to If lb. Of this quantity, the pulmonary exhalation is usually somewhat less than one^third, and the cutaneous somewhat more than two-thirds ; but when the quantity of fluid lost is unusually great, the increase must be chiefly in the Cutaneous exhalation ; since, as already pointed out (§ 701), the amount of exhalation from the lungs is not influenced by the external temperature, but only by the degree in which the surrounding air is previously saturated with moisture. 746. The variations in the amount of fluid set free by Cutaneous and Pulmonary Exhalation, are counterbalanced by the regulating action of the Kidney; which allows a larger proportion of water to be strained off in a liquid state from the blood-vessels, as the Exhalation is less,—and vice versa. The Cutaneous and Urinary excretions seem to be vicarious, not merely in regard to the amount of fluid which they carry off from the blood; but also in respect to the solid matter which they eliminate from it. It appears that at least 100 grains of effete azotized matter are daily thrown off from the skin; and any cause which checks this excretion, must increase the labour of the Kidneys, or produce an accumulation of noxious matter in the blood. Hence attention to the functions of the skin, at all times a matter of great importance, is peculiarly required in the treatment of Urinary diseases; and it will be often found that no means is so useful in removing the lithic acid deposit, as copious ablution and friction of the skin, combined with exercise. When the Exhalant action of the skin is completely checked, by the application of an impermeable varnish, the effect is not (as might be anticipated) an elevation of the temperature of the body; on the contrary it is lowered, in conse- quence, as it would appear, of the interruption to the aeration of the blood through the skin, which is a function of such importance in the lower animals (§ 671), and of no trifling account in Man; and in a short time, a fatal result ensues. A partial suppression by the same means gives rise to febrile symptoms, and to Albuminuria, or escape of the albuminous part of the liquor sanguinis into the urinary tubes, in consequence (it would appear) of the increased determination which then takes place towards the kidneys. These facts are interesting, as throwing light upon the febrile disturbance, which accompanies those cutaneous diseases, that affect the wThole surface of the skin at once, and interfere with its functions; and as accounting also for the Albu- 428 SEBACEOUS GLANDS. minuria, which frequently manifests itself during their progress, espe- cially in Scarlatina. 747. The Skin is likewise furnished with numerous Sebaceous glands, which are distributed more or less closely over the whole surface of the body; being least abundant where the Perspiratory glandulae are most numerous; and vice versa. They are altogether absent on the palms of the hands and the soles of the feet; and are particularly frequent in the skin of the face, and in the scalp. They differ greatly in size and in degree of complexity; sometimes con- sisting of short straight follicles; sometimes closely resembling the Sudoriparous glandulae, the tubes, however, being usually straighter and wider; and being sometimes much more complex in structure, consisting of a number of distinct sacculi clustered round the extremity of a common duct, into which they open, and forming little arborescent masses, about the size of millet seeds. In some situations they ac- quire still greater complexity. Thus the Meibomian glandulae, which are found at the edge of the eyelids, and which secrete an unctuous matter for their lubrication, are long sacculi branching out at the sides (Fig. 107); and the glandulae of the ear passage, which secrete its cerumen or waxy matter, and which belong to the general Sebaceous system, are formed of long tubes, highly contorted, and copiously sup- plied with blood-vessels. In the hairy parts of the skin, we usually find a pair of Sebaceous follicles opening into the passage through which every hair ascends (Fig. 126, 9). The purpose of the seba- ceous secretion is evidently to prevent the skin from being dried and cracked by the influence of the sun and air. It is much more abundant in the races of mankind, which are formed to exist in warm climates, than in the races that naturally inhabit cold countries; and the former are accustomed to aid its preservative power, by lubri- cating their skin with vegetable oils of various kinds; which process they find to be of use in protecting it from the scorching influence of the solar rays.—The Sebaceous follicles are frequently the residence of a curious parasite, the Demodex folliculorum; which is stated by Mr. Erasmus Wilson to be present in great numbers in the skin of almost all inhabitants of large towns; the activity of their cutaneous glandular system being much checked, by the want of free exposure to pure air, and by inert habits of life. 748. To what extent the Sebaceous secretion can be regarded as destined to free the Blood from deleterious matters, it may not, per- haps, be very easy to say; but with regard to the functions of the skin taken altogether, as a channel for the elimination of morbific matters from the blood, it is probable that they have been much underrated ; and that much more use might be made of it, in the treatment of dis- eases,—especially of such as depend upon the presence of some mor- bific matter in the circulating current,—than is commonly thought advisable. We see that Nature frequently uses it for this purpose; a copious perspiration being often the turning-point or crisis of febrile diseases, removing the cause of the malady from the blood, and ELIMINATING ACTION OF THE SKIN. 429 allowing the restorative powers free play. Again, certain forms of Rheumatism are characterized by copious acid perspirations ; and instead of endeavouring to check these, we should rather encourage them as the best means of freeing the blood from its undue accumu- lation of lactic acid. And it is recorded that in the " sweating sick- ness," which spread throughout Europe in the 16th century, no re- medies seemed of any avail but diaphoretics ; which, aiding the powers of nature, concurred with them to purify the blood of its morbific matter. The hot-air bath, in some cases, and the wet sheet (which, as used by the Hydropathists, is one of the most powerful of all dia- phoretics), will be probably employed more extensively as therapeutic agents, in proportion as the importance of acting on the skin, as an extensive collection of glandulae, comes to be better understood. The absurdity of the "Hydropathic" treatment consists in its indis- criminate application to a great variety of diseases; no person, who has watched its operation, can deny that it is a remedy of a most powerful kind ; and if its agency be fairly tested, there is strong rea- son to believe, that it will be found to be the most valuable curative means we possess for various specific diseases, which depend upon the presence of a definite " materies morbi" in the blood, especially Gout and chronic Rheumatism; as well as for that depressed state of the general system, which results from the "wear and tear" of the bodily and mental powers. 749. The Mucous surface of the Alimentary Canal is furnished, like the skin, with a vast number of glandulae, varying in complexity, from the simple follicle, to a mass consisting of numerous lobules opening into a common excretory duct. The functions of these, as already pointed out, are equally various. The simple follicles ap- pear destined, for the most part, to secrete the protective mucus, which intervenes between the membranous wall and the substances contained in the canal, and which serves to protect the former from the irritat- ing action of the latter. The more complex follicles of the Stomach elaborate the Gastric fluid, which is the prime agent in the digestive process (§ 496). But there is strong reason to believe, that the func- tion of the numerous glandulae, which beset the walls of the small intestine, and which are knowTn as Brunner's and Peyer's glands (after the names of their discoverers), is purely excretory ; and that they are destined to eliminate putrescent matters from the blood, and to convey them, by the readiest channel, completely out of the body. That the putrescent elements of the feces are not immediately de- rived from the food taken in, so much as from the secreting action of the intestinal glandulae, appears from this consideration ;—that fecal matter is still discharged, even in considerable quantities, long after the intestinal tube has been completely emptied of its alimentary contents. We see this in the course of many diseases, when food is not taken for many days, during which time the bowels are com- pletely emptied of their previous contents by repeated evacuations; and whatever then passes, must be derived from the intestinal walls 430 GENERAL RELATIONS OF EXCRETING PROCESSES. themselves. Sometimes a copious flux of putrescent matter con- tinues to take place spontaneously ; whilst it is often produced by the agency of purgative medicine. The " colliquative diarrhoea," which frequently comes on at the close of exhausting diseases, and which usually precedes death by starvation, appears to depend, not so much upon a disordered state of the intestinal glandulae, as upon the gene- ral disintegration of the solids of the body, which calls them into ex- traordinary activity, for the purpose of separating the decomposing matter. 750. Thus we perceive, that we have here, also, to watch for the indications of Nature; and that this extensive system of intestinal glandulae, being the principal channel for the elimination of putres- cent matters from the blood, should be especially attended to, when there is reason to think that such matters are present in too large an amount. Hence, when diarrhoea is already existing, we may often do more good by allowing it to take its course, or even by increasing it by the agency of purgative medicines, than by attempting to check it, and thus causing the retention of the morbid matter in the circu- lating current. But, on the other hand, it is necessary to bear in mind the extreme irritability of the intestinal mucous membrane; and care- fully to avoid exciting it, when it is already in excess, or when there is danger that it will supervene,—as in that form of Fever in which there is a peculiar liability to inflammation and ulceration of the walls of the alimentary canal. 5. General Summary of the Excreting Processes. 751. We have now passed in review the various processes, by which the products of the disintegration of the animal tissues are car- ried off; and we have seen that the necessity for their removal is much more urgent than for their replacement. A cold-blooded animal may subsist for some weeks, or even months, without a fresh supply of food ; the waste of its tissues being so small, if it remain in a state of rest, as to be quite compatible with the continuance of its life ; and a warm-blooded animal may live for many days or even weeks, pro- vided that it has in its body a store of fat sufficient to keep up its heat by the combustive process. But in either case, if the exhalation of carbonic acid by the lungs, the elimination of biliary matter by the liver, the separation of urea or uric acid by the kidneys, or the with- drawal of putrescent matter by the intestinal glandulae", be completely checked, a fatal result speedily ensues;—more speedily in warm- blooded animals than in those which cannot sustain a high inde- pendent temperature, on account of the greater proneness to decom- position in the bodies of the former, than in those of the latter;—and more speedily in the latter, when their bodies are kept at an elevated temperature by the warmth of the surrounding medium, than when the degree of heat is so low, that there is little proneness to sponta- neous change in the substance of their bodies. RELATIONS OF BILIARY AND PULMONARY EXCRETIONS. 431 752. It may be taken as a general principle, in regard to the Excreting processes (including Respiration), that they have a three- fold purpose ;—in the first place, to carry off the normal results of the waste or disintegration of the solid tissues, and of the decomposition of the fluids; in the second place, to draw off the superfluous aliment- ary matter, which, though received into the circulating current, is not converted into solid tissue, in consequence of the want of demand for it;—and in the third place, to carry off the abnormal products, which occasionally result from irregular or morbid changes in the system. Thus by the Lungs are excreted a large amount of carbon, and some hydrogen, resulting from the disintegration of the tissues, especially the nervous and muscular; the same elements in animals that take in a large proportion of farinaceous or oleaginous aliment, may be derived immediately from the food, without any previous conversion into solid tissue ; and there can be little doubt that the respiratory function is also an important means of purifying the blood from various deleterious matters, either introduced from without (such as narcotic poisons), or generated within the body (such as the poison of fever*). And it is important to bear this last circumstance in mind ; since it enables us to understand how, if time be given, the system frees itself from such noxious substances ; and points out the duty of the medical attendant to be rather that of supporting the powers of the body by judiciously devised means, and of aiding the elimination of the morbid matter through the lungs and skin by a copious supply of pure air, than of interfering more actively to promote that which Nature is already effecting in the most advantageous manner. 753. In like manner, the Liver is charged with the separation of hydrocarbon, in a fluid form ; for which a supply of oxygen is not requisite. This product is partly derived from the waste of the sys- tem ; but the arrangement of the biliary vessels leads to the belief, that much of it is at once derived from crude matter, taken up by the mesenteric veins, and eliminated from them by the hepatic cells, without ever passing into the general circulation. And various facts seem to indicate, that the Liver is also destined to remove from the blood extraneous substances, which are noxious to it. Thus, in cases where death has resulted from the prolonged introduction of the salts of Copper into the system, a considerable amount of that metal has been obtained from the substance of the gland.—It has been already pointed out (§ 720) that the Liver and Respiratory organs are deve- loped in an inverse proportion to each other, in the different classes of animals ; the Liver being largest where the respiration is most feeble, and vice versa. Now it is important to bear in mind, that the functional activity of the liver in any individual must be in like manner the greater, as the amount of respiration is less; the hydro- * There is strong reason to believe that, in many instances, a small amount of poisonous matter introduced from without, in the form of a contagion or miasm, may lead, by a process resembling fermentation, to the production of a large quantity of similar noxious substances in the animal fluids. 432 DEPURATING ACTION OF THE KIDNEYS. carbon, which is eliminated by the lungs, when their activity is the greatest, being thrown upon the liver for separation, when the respi- ration is feeble. We have seen that the amount of carbonic acid exhaled at high temperatures, is much less than that set free in a colder atmosphere ; consequently, the liver is called upon to do more in warm climates, and is therefore peculiarly liable to disordered action,—unless the diet be carefully regulated, in accordance with the wants of the system. 754. The effects of diminished respiration, in producing an increase in the fatty constituents of the liver, are peculiarly well marked in the diseased condition produced in the geese, that are Fig. 127. being prepared for celebrated Strasburg pates. The unfortunate bird is closely confined at a high tem- perature ; so that the respiration is reduced to its minimum amount by the combined effects of warmth and muscular inaction ; and it is then crammed with gedepwi'th Fat5;— °a, maize, which contains a large amount of oily matter. adi°poseedgiobuies.s; *' Tfte consequence is, that its liver soon enlarges, and becomes unusually fatty ; its cells being gorged with oil-globules, instead of each containing no more than one or two : and it is then ready for the epicureans who set so high a value on the pate defoie gras. A similar diseased condition of the liver frequently presents itself in Man, as a consequence of chronic disorders of the respiratory organs, which diminish the amount of hydrocarbon elimi- nated through their agency; this "fatty liver" is peculiarly common in the advanced stages of Phthisis. 755. With regard to the Kidneys, it has been already pointed out that they are the special emunctories of the azotized products of the decomposition of the tissues; and that they serve also to convey away the overplus of such earthy and alkaline salts, as are readily soluble. Moreover, it has been shown that the surplus proteine-compounds which are not required for the nutrition of the system, must be ex- creted by their agency, after having been metamorphosed into urea. And we have now to notice, that other matters of an injurious charac- ter, whether introduced from without, or generated within the sys- tem, are drawn off by the same channel. Thus the saline compounds, taken up by the absorbent process (§ 493), are for the most part set free through these organs; especially when their properties are such, as to excite the action of the kidneys in a peculiar degree. Thus, Prussiate of Potash has been detected in the urine, within two mi- nutes after it had been introduced into the stomach. It has been sometimes noticed that Iodide of Potassium, when administered as a medicine, is retained within the body for some days, producing ex- tensive cutaneous eruptions, or some other unusual consequence; and that it then suddenly begins to pass off by the kidneys, and is ex- creted in very large quantities. The effect of the inhalation of the vapour of turpentine, even in a very diluted state, in speedily imparting to the urine the odour of violets, is an evidence that not DEPURATING ACTION OF SKIN, ETC 433 merely the actual substances imbibed, but new and peculiar com- pounds to which they give rise, are thus eliminated by the Kidneys. 756. The most singular variations in the excretory function of the Kidneys are seen, however, when the Urine is charged with sub- stances which are not only foreign to it, but are altogether foreign to the healthy body. The most remarkable instance of this is seen in the disease termed Diabetes, in which a large quantity of Sugar is formed, either directly from the food, or by the disintegration of the solid tissues; and in which this compound is eliminated by the Kid- neys, imparting to the urine a saccharine taste. And another example of the same general fact is seen in the "oxalic diathesis," in which an unusual arrangement of the elements, that usually form urea or uric acid, gives rise to a new and peculiar compound, oxalate of am- monia; this being drawn off by the kidneys, and being decomposed by the calcareous matter present in the urine, gives rise to a deposit of oxalate of lime. In the treatment of such diseases, our attention must be given, not so much to the secreting organ, as to the condition of the system at large, of which the character of the secreted product is the indication or exponent. 757. To what has already been stated in regard to the exhalant functions of the Lungs and Skin, it may be added that many states of disease are marked by an unusual odour emitted from the body; and there can be little doubt that the peculiar odorous matter is pre- formed in the blood,—as we know that the ordinary scent of any species (whether Man, Dog, Horse, Goat, &c.,) may be set free from the blood of that species, by the addition of sulphuric acid. The ex- istence of such odours, therefore, is not to be attributed to disordered function in the excreting organs; but to the formation of morbid pro- ducts in the interior of the body, which these, organs do their best to remove. The fetid breath, which frequently accompanies an attack of indigestion, is another instance of the power of the lungs to eli- minate not merely Carbonic acid, but other products of the changes in composition, which the food undergoes when introduced into the system. 758. The same remarks apply, and with yet greater force, to the intestinal glandulae ; whose function it is, not merely to remove the putrescent matter ordinarily formed by the disintegration of the tis- sues, or by the decomposition of unassimilated food, but also to draw off the still more offensive products of such changes as take place in disease. Thus there are conditions of the system, in which, without any well-marked disorder, the feces emit a peculiarly fetid odour; and with these is almost always associated a depressed state of mind. Now it can scarcely be doubted, that the real fault is here rather in the early part of the nutritive operations, than in the excretory func- tion; and that the fetor of the contents of the intestine depends upon the undue formation of putrescent matter in the system, which, by tainting the blood, causes its action upon the brain to become un- healthy. The object of the physician will be here to eliminate the 28 434 HEAT OF ANIMALS AND PLANTS. morbid product, by the moderate use of purgatives ; and so to regu- late the diet and regimen, as to correct the tendency to its formation. —An excessive fetor in the evacuations, as well as in the exhalations from the skin and'lungs, is peculiarly characteristic of those very se- vere forms of typhus (now, happily, of comparatively rare occurrence), which are termed putrid fevers. Here the whole of the solids and fluids of the body appear to have an unusual tendency to decomposi- tion, in consequence of the introduction of some morbid agent, which acts as a ferment; and the system attempts to free itself from the pro- ducts of that decomposition, by the various organs of excretion, par- ticularly the Skin and intestinal surface. 759. It is of great importance that the Student should form clear conceptions on this subject; and that he should not (as too often happens), by directing his remedies to the mere symptoms or results of a disease, act in precise opposition to the natural tendency of the system to free itself from some unusual noxious matter, through those channels which are ordinarily destined to carry off only the regular products of its disintegration. CHAPTER X. OF THE DEVELOPMENT OF HEAT, LIGHT, AND ELECTRICITY IN THE ANIMAL BODY. 760. It has been shown, in an earlier part of this volume (Chap. II.), that all Vital actions require a certain amount of Heat for their performance ; and that there is a great variety amongst the different classes of Animals, both in regard to the degree of Heat which is most favourable to the 'several processes of their economy, and in regard to their own power of sustaining it, independently of oscilla- tions in the temperature of the surrounding medium. As a general rule, the Invertebrated animals are cold-blooded; that is, they have little or no power of sustaining an independent temperature. The degree of energy of their vital actions entirely depends, therefore, upon the warmth they receive from the air or water they inhabit; they . have no power of resisting the depressing influence of cold ; and they are generally so organized, as to pass into a state of complete inaction or torpidity, when the temperature sinks below a certain point,—after gradually becoming more and more inert with every diminution in the heat of their bodies. The same is true, also, of most Fishes and Reptiles: but the animals of the former class, from the more equable temperature of the medium they inhabit, are not so liable to be re- duced to inaction as the latter ; being usually so organized, as to retain their activity so long as the water around them continues liquid ; and HEAT OF ANIMALS AND PLANTS. 435 being actually imbedded in a frozen state, when the water around them is converted into ice, without the loss of their vitality. There are certain Fishes, however,—such as the Tunny, Sword-fish, and other large species of the Mackarel tribe,—wdiich are able to sustain a temperature considerably above that of the sea they inhabit; thus in the Bonito, the heat of the body has been found to be 99°, when the temperature of the surrounding sea was but 80|°. It is not pro- bable, however, that the temperature of the body would be kept up to the same standard, if that of the sea should be considerably low- ered ; but it would probably remain at from 18° to 20° above the latter. • And in like manner, it has been noticed that many of the more active Reptiles possess the power of sustaining the temperature of their bodies at 10° or 15° above that of the surrounding air. 761. The classes of animals, which are especially endowed with the power of producing and maintaining heat, are Insects, Birds, and Mammalia. The remarkable variations which present themselves in the temperature of the first of these classes, and the connection of these variations with the condition of the animals, in regard to ac- tivity or repose, have already been sufficiently noticed (§ 123).—The temperature of Birds is higher than that of any other class of ani- mals; varying from 100° to 1110 or 112°. The lowest degree is found in some of the aquatic species, as the Gull, and in those which principally live on the ground, as the Fowl tribe ; and the highest in the birds of most active flight, as the Swallow. The temperature of the Mammalia seems to range from about 96° to 104°; that of Man has been observed as low as 96^°, and as high as 102°. The varia- tions are dependent in part upon the temperature of the external air; but are influenced also by the general condition of the body, as to repose or activity, the period of the day, the time that has elapsed since a meal, &c. A somewhat larger amount of caloric is generated during the day, than in the night; and the body is usually warmer, by a degree or two, at noon, than at midnight. There is also a slight increase during the digestion of a meal; and exercise is a powerful means of raising the temperature.—The range of temperature is much greater in disease ; thus the thermometer has been seen to rise to 106° in Scarlatina and Typhus, and to 110f° in Tetanus; whilst it has fallen to 82° in Spasmodic Asthma, and to 77° in Cyanosis and Asiatic Cholera. 762. In searching for the conditions, on which this production of heat within the Animal body is dependent, it is very important to bear in mind, that a similar generation of Caloric may be observed" in the Vegetable kingdom. It appears from the most recent and exact experiments, that all living Plants are somewhat warmer than similar dead plants exposed to the same atmosphere; and that the elevation is the greatest in the leaves and young stems, in which the most active vital changes are taking place. But the most decided production of heat occurs in the flowering of certain Plants, such as the Arum, which have large fleshy receptacles, on which a great 436 CONDITIONS OF DEVELOPMENT OF HEAT. number of blossoms are crowded ; thus a thermometer placed in the centre of five spadixes of the Arum cordifolium has been seen to rise to 111°; and one placed in the midst of twelve spadixes has risen to 121°; whilst the temperature of the surrounding air was only 66°. In the germination of seeds, also, a great elevation of temperature occurs; which is rendered most evident by bringing together a num- ber of seeds, as in the process of malting, so that the caloric is not dissipated as fast as it is generated ; the thermometer, placed in the midst of a mass of seeds in active germination, has been seen to rise to 110°. 763. Thus it is evident that the chemical changes, which are in- • volved in the operations of Nutrition, are capable of setting free a large amount of heat; which, although ordinarily dissipated from the vegetating surface too speedily to manifest itself, becomes sensible enough, when this rapid loss is checked. If we further examine into the nature of the chemical changes, which appear most concerned in this elevation of temperature, we find that they uniformly consist in the combination of the carbon of the plant with the oxygen of the atmosphere; so that a large quantity of carbonic acid is formed and set free, precisely in the manner of the Respiration of Animals. This process is so slowly performed, in the ordinary growth of Plants, that it is concealed (as it were) by the converse change,-—the fixation of carbon from the carbonic acid of the atmosphere, under the influence of light (§ 83). But it takes place with extraordinary energy during flowering and germination; a large quantity of carbon being set free, by union with the oxygen of the air; and the starchy matter of the receptacle, or of the seed, being converted into sugar. Now it has been ascertained by careful experiments, that the amount of heat gene- rated is in close relation with the amount of carbonic acid set free; and that, if the formation of the latter be prevented, by placing the flower or the seed in nitrogen or hydrogen, no elevation of tempera- ture takes place ; whilst if the process be stimulated by pure oxygen, so that a larger proportion of carbonic acid is evolved, the elevation of temperature is more rapid and considerable than usual. 764. Upon examining into the conditions under which Caloric is generated in the Animal body, we find them essentially the same. Wherever the temperature of the body is maintained at a regular standard, so as to be independent of variations in the warmth of the surrounding medium, we find a provision for exposing the blood most freely to the influence of oxygen, and for extricating its carbonic acid ; thus in Birds and Mammals, the blood is distributed, in a minute ca- pillary network, on the walls of the pulmonary air-cells, the gaseous •contents of which are continually renewed; and in Insects, the air is carried into every part of the body, by the ramifying trachea?. We constantly find a proportion between the amount of heat evolved, and that of carbonic acid generated ; this is peculiarly evident in Insects, whose respiration and calorification vary so remarkably (§ 123); but it is also proved by comparing the araount of carbonic acid generated CONDITIONS OF DEVELOPMENT OF HEAT. 437 by warm-blooded animals, when the external temperature is low, and when more heat must be evolved to keep the temperature of their bodies up to its proper standard, with that generated by the same animals in a warmer atmosphere, when the proper animal heat is diminished in amount (§ 691). 765. The sources of the Carbonic Acid thrown off by the lungs, have been already pointed out (Chap. VIII.): it is partly derived from the metamorphosis of the tissues; but partly, in all but purely carnivorous animals, more directly from the non-azotized portion of the food. The precise mode in which the carbon of this is united with the oxygen derived from the atmosphere, is not yet known ; but it is certain that, in whatever manner the combination takes place, a certain measure of caloric must be generated. It appears, however, from various experiments, that the whole quantity of caloric gene- rated by an animal in a given time, is greater than that which would be evolved by the combustion of the carbon, included in the carbonic acid evolved during the same time. Hence it is evident that other chemical processes occurring within the body are concerned in the maintenance of the temperature; and it is not difficult to point to some of these. It is probable, in the first place, that some of the hydrogen of the food may be " burned off" by union with the oxygen of the atmosphere, so as to form part of the water which is exhaled from the lungs. Again, the sulphur and phosphorus of the food are converted, by oxygenation, into sulphuric and phosphoric acids; in which process, heat must be generated. In the composition of urea, moreover, oxygen is present in much larger proportion, than it is in the proteine-compounds by the metamorphosis of which it is formed ; so that in its production, too, caloric will be generated. In fact it may be stated as a general truth, that the whole excess of the oxygen absorbed over that which is contained in the carbonic acid exhaled (§ 690), must be applied to purposes in the laboratory of the system, in which caloric will be disengaged. Still, the amount of carbonic acid exhaled must always be the measure of the chemical processes, by which heat is generated in the body; because it is itself the result of the chief of these processes (the union of carbon and oxygen), and because the surplus amount of oxygen which is absorbed, and which is applied to other purposes, entirely depends upon it. 766. The power of maintaining a high independent temperature is usually much less in young warm-blooded animals, than in adults. There are considerable variations in this respect, however, amongst different species; for where the young animal is born in such an ad- vanced condition, as to be thenceforth almost independent of parental assistance, it is capable of maintaining its own temperature ; but where it is born in such a state as to require to be supplied with food by the parent for some time, it is also more or less dependent upon the warmth imparted to it from the parental body. This is peculiarly the case w7ith the young of the Human species, which is 438 REGULATION OF HEAT IN MAN. longer dependent upon parental aid, than that of any other animal. In the case of children born very prematurely, the careful sustenance of their heat is one of the points most to be attended to in rearing them ; and ev*en the most vigorous infants, born at the full time, are far from being able to keep up their proper standard without assist- ance, if exposed to a cool atmosphere. It has been ascertained that, during the first month of infant life, the mortality in winter is nearly double that of summer,—being 1-39 in January to 0-78 in July ; and this striking difference cannot be attributed to any other cause, than the injurious influence of external cold, which the calorifying powers of the infant do not enable it to resist. As age advances, the power of generating heat increases, and the body becomes much more inde- pendent of external vicissitudes; so that, in adult life, the winter mortality is to that of suraraer, only as 1-05 to 0-91, or less than one- sixth more. In advanced age, the calorifying power again diminishes; and this we should anticipate, from the general torpor of the nutritive operations in old persons. Between 50 and 65 years of age, the re- lative winter and summer mortality are nearly as in the first month of infancy; and at 90 years, the average mortality of winter is much more than twice that of summer, being as 1-58 to 64. 767. It appears that there is not merely a difference in calorifying power at different ages, but at different seasons; the amount of heat generated in summer not being sufficient, in many animals, to prevent the body from being cooled down by prolonged exposure to a tempe- rature, which is natural to them in winter. To what extent this is the case with Man, it is difficult to say. His constitution is distin- guished by its power of adapting itself to circumstances ; and he can live under extremes of temperature more wide than those, which most other animals can endure (§ 113). Whether in the torrid zone, or in the arctic regions, he can maintain his healthy condition under favourable circumstances; in each case his natural appetite leading him to the use of that kind and amount of food, which are best suited to the wants of his system. But the longer he has been habituated to a very warm or a very cold climate, the more difficult he at first finds it to live comfortably in one of an opposite character; as his consti- tution, having become adapted to one particular set of circumstances, requires lime to accommodate itself to an opposite one. 768. The means by which the heat of the body is prevented from rising above its normal standard, even in the midst of a very high temperature in the surrounding air, are of the most simple character. The excreting action of the skin is directly stimulated by the applica- tion of warmth to the surface; and the fluid which is poured forth, being immediately vaporized, converts a large quantity of sensible caloric into latent, and thus keeps down the temperature of the skin. By this provision, the body may be exposed with impunity to dry air of 600° or more, so long as the supply of fluid be maintained. But it cannot long sustain exposure to air saturated with vapour, even ANIMAL LUMINOSITY. 439 though it may not be many degrees hotter than the body; because the cooling act of evaporation from the skin cannot then be carried on. 769. The evolution of Light is a very interesting phenomenon, chiefly witnessed among the lower Animals, and usually supposed not to occur in any class above Fishes. It is particularly remarkable among the Radiata and inferior Mollusca. A large proportion of the Acalephce, or Jelly-fish tribe possess the property of luminousness in a greater or less degree; and it is to small animals of this class, which sometimes multiply to an amazing extent, that the beautiful phenome- non of phosphorescence of the sea is chiefly due. In the midst of the soft diffused light thus occasioned, brilliant stars, ribbons, and globes of fire are frequently seen; these appearances being due to the lumino- sity of the larger species of the same tribe, or to that of other marine animals.—Some of the most remarkable examples of luminosity, in regard to the brilliancy of the light emitted, occur in the class of Insects. Here the emission is confined to one portion of the body, or to two or more isolated spots, instead of being diffused over a larger surface ; and it is proportionably increased in intensity. 770. The phenomenon of Animal Luminousness appears usually attributable to the formation of a peculiar secretion ; which, in many instances, continues to shine after removal from the Animal, so long as it is exposed to the influence of oxygen: and it seems not unrea- sonable to believe, that it depends upon a slow process of combustion, analogous to that which takes place when phosphorus is exposed to the air. There is a special provision, in Insects, for conveying a large supply of air through the peculiar substance, which is deposited beneath the luminous spots; and the power which Glow-worms, Fire- flies, &c, possess, of suddenly extinguishing their light, and as sud- denly renewing it, seems to depend upon their control over the air- aperture or spiracle by which air is admitted,—the stoppage of the supply of air causing the immediate cessation of the luminousness, and its re-admission occasioning a renewal of the process on which it depends.—It is probable, however, that in certain cases, the lumino- sity is rather of an electrical character. There are several of the smaller Annelida or marine Worms, which are brilliantly luminous when irri- tated ; the luminosity having the character, however, of a succession of sparks, rather than of a steady glow. It appears from the recent experiments of M. Quatrefages, that this peculiar luminosity is the especial attribute of the muscular system; and that it is produced with every act of muscular contraction in these animals. 771. Although no such luminosity is commonly manifested in any of the higher vertebrata, or in Man, yet there are well-authenticated cases, in which the phenomenon has presented itself in the living sub- ject,*—luminous emanations from dead animal matter being of no unfrequent occurrence. In most of these cases, however, the indi- * See an account of several cases of the Evolution of Light in the Living Human Subject, by Sir Henry Marsh, M. D., M. R. I. A., &c.t 440 ANIMAL ELECTRICITY.—ELECTRIC FISHES. viduals exhibiting the luminosity had suffered from consumption, or some other wasting disease, and were near the close of their lives at the time; so that it is probable that a decomposition of the tissues was actually in progress, analogous to that which, when it occurs after death, imparts luminosity to the decaying body. One instance is recorded in which a large cancerous sore of the breast emitted light enough, to enable the hands on a watch-dial to be distinctly seen when it was held within a few inches of the ulcer; here, too, decomposition was obviously going on, and the phosphorescent matter produced by it was exposed to the oxygenating action of the atmosphere. 771. Slight manifestations of free Electricity, or, in other words, disturbances of Electric equilibrium, are very frequent in living ani- mals ; and they are readily accounted for, when we bear in mind that nearly all chemical changes are attended with some alteration in the electric state of the bodies concerned; and when we consider the number and variety of such changes in the living animal body. When slight, however, they can only be detected by refined means of observation; and it is only when they are considerable, that they attract notice. The most remarkable examples of the evolution of free Electricity in Animals, are to be found in certain species of the class of Fishes; the best known of which are the Torpedo or Elec- tric Ray, and the Gymnotus or Electric Eel. These possess organs, in which Electricity may be generated and accumulated in large quan- tities, and from which it may be discharged at will. The shock of a large and vigorous Gymnotus is sufficiently powerful to kill small animals, and to paralyze large ones, such as men and horses: that of the Torpedo is less severe, but it is sufficient to benumb the hand that touches it. 772. The electric organs of the Torpedo (which, from being found on European shores, has been the most studied) are of flattened shape, and occupy the front and sides of the body; forming two large masses, which extend backwards and outwards from each side of the head. They are composed of two layers of membrane separated by a con- siderable space; and this space is divided by vertical partitions into hexagonal cells like those of a honeycomb, the ends of which are directed towards the two surfaces of the body. These cells, wThich are filled with a whitish soft pulp, somewhat resembling the substance of the brain, but containing more water, are again subdivided hori- zontally by membranous partitions; and all these partitions are pro- fusely supplied with blood-vessels and nerves.—The electrical organs of the Gymnotus are essentially the same in structure; but they differ in shape, in accordance with the conformation of the animal.—In these, and the other Electrical fishes, the electric organs are supplied with nerves of very great size, larger than any others in the same animals, and larger than any nerves in other animals of similar bulk. These nerves arise from the top of the spinal cord, and seem analo- gous to the pneumogastrics of other animals. 773. The following conditions appear to be essential to the mani- ELECTRIC FISHES. 441 festation of the Electric powers of these animals. Two parts of the body must be touched at the same time; and these two must be in different electrical states. The most energetic discharge is procured from the Torpedo, by touching its back and belly simultaneously; the electricity of the back being positive, and that of the belly negative. When two parts of the same surface, at an equal distance from the electric organ are touched, no effect is produced, as they are equally charged with the same electricity; but if one point be further from it than the other, a discharge occurs, the intensity of wdiich is propor- tioned to the difference in the distance of the points from the electric organ. However much a Torpedo is irritated, no discharge can take place through a single point; but the fish makes an effort to bring the border of the other surface in contact with the offending body, through which a shock is then transmitted. This, indeed, is probably the usual wray in which the discharge is effected.—The identity of animal with common Electricity is proved, not merely by the similarity of the effects upon the feelings produced by the shock of both ; but also by the fact that a spark may be obtained, and chemical decompositions effected by the former, precisely as by the latter. 774. The voluntary power of the animal over its Electric organs, is dependent upon their connection with the nervous centres. If all the nerve-trunks supplying the organ on one side, be divided, the animal's control over that organ will be destroyed ; but the power of the other may remain uninjured. If the nerves be partially divided on either or both sides, the power is retained by the portions of the organs, which are still connected with the brain by the trunks that remain. Even slices of the organ, entirely separated from the body except by a nervous fibre, may exhibit electrical properties. Discharges may be produced, by. irritating the part of the nervous centres from which the trunks proceed, so long as the latter are entire ; or by irritating the portions of the divided trunks, which remain in connection with the electric organs; or even by irritating portions of the electric organs themselves, when separated from the nervous centres.—In all these respects, there is a strong analogy between the action of the nerves on the Electric organs, and their action on the Muscles. And as the latter contract by their own inherent powers, when stimulated by nerv- ous influence, so does there seem reason to believe, that the evolution of Electricity takes place from some peculiar changes in the electric organs, of which changes the agency of the nerves is one of the con- ditions. To the idea that the nervous centres produce the electricity, that the nervous trunks convey it, and that the electric organs serve merely to store it up, there are several objections, and especially these;—that there is nothing whatever in the structure of the brains of the Electrical Fishes, that marks them out as essentially differ- ent from those of the species to which they are otherwise allied;— that the power of the nerve-trunk, in conveying the requisite stimu- lus to the electric organs, is destroyed as completely by tying, ejs by dividing the trunk, which would not be the case if the nervous 442 MANIFESTATIONS OF ELECTRICITY. agency were itself of the nature of ordinary electricity (§ 396);— and that electric manifestations may be procured from the electric organs, by stimuli applied to themselves, after the complete severance of their connection with the brain,—just as muscles may be thrown into contraction by direct stimulation, under the same circumstances (§348). 775. It is another interesting point of analogy between the action of Muscles, and that of the Electrical organs, that the former (as is now fully proved by the elaborate and exact researches of Matteuci), is attended with electrical disturbance. In any fresh vigorous mus- cle, in a state of passive or tonic contraction, there is a continual electric current from the interior to the exterior, sufficient to excite the leg of a frog to energetic contraction, when its nerve is so ap- plied to the muscle, as to receive the influence of this current. And a much more powerful current is produced, when the muscle is thrown, by a stimulus applied to its own nerve, into a state of energetic con- traction. "The explanation of the constant direction of the current, from the interior towards the exterior of the muscle, seems to be, that the changes connected with the nutrition and disintegration of the muscular tissue go on more energetically in its interior, than they do nearer its surface, where the proper muscular fibres are mingled with a large proportion of areolar and tendinous substance. 776. It has also been shown that there exists in the Frog, during its whole life, a continual current of Electricity, passing from its extremities towards its head. The conditions on which this current depends do not seem very evident; and as it has been detected in no other animal, it has been termed the courant propre, or peculiar current, of the Frog. It bears this curious analogy to the electric discharges of Fishes; that it is not manifested, if the connection be made between corresponding points of the opposite sides; but that it shows itself, when the communication is made between points higher or lower in the body, whether on the same or on opposite sides. 777. Manifestations of Electricity may be produced, in most animals having a soft fur, by rubbing the surface, especially in dry weather; this is a fact sufficiently well known in regard to the domestic Cat. Some individuals of the Human race exhibit spontaneous manifesta- tions of electricity, which are occasionally of very remarkable power. There are persons, for instance, who scarcely ever pull off articles of dress, which have been worn next their skin, without sparks and a crackling noise being produced, especially in dry weather. This is partly due, however, to the friction of these materials with the surface, and with each other. But the case of a lady has been recently put on record, who was for many months in an electric state so different from that of surrounding bodies, that, whenever she was but slightly insulated by a carpet or other feebly-conducting medium, sparks passed between her person and any object which she approached. When she was most favourably circumstanced, four sparks per minute would pass between her finger and the brass ball of a stove at a dis- REPRODUCTION. 443 tance of \\ inch. Various experiments were tried, with the view of ascertaining if the Electricity was produced by the friction of articles of dress; but no change in these seemed to modify its intensity. From the pain wdiich accompanied the passage of the sparks, this condition was a source of much discomfort to the subject of it. CHAPTER XI. OF REPRODUCTION. 1. General View of the Nature of the Process. 778. There is no one of the functions of living beings, that dis- tinguishes them in a more striking and evident manner from the inert bodies which surround them, than the process of Reproduction. By this function, each race of Plants and Animals is perpetuated; whilst the individuals composing it successively disappear from the surface of the earth, by that death and decay which are the common lot of all. There are certain tribes, in which the death of the parent is necessary for the liberation of the Fig. 12a germs, from which a new race is to spring up. This is the case, for example, in some of the simplest Cellular Plants; in which every cell lives for itself alone, and performs its whole series of vital operations independently of the rest; and in which the process of reproduction consists in the rupture of the parent-cell, and the emission of the contained reproductive particles, every one of which is capable of developing simple isolated ceiis.con- itself into a new cell, resembling that of its Sg reproductive mole- parent and capable of going through the same series of changes (§ 31). But as, in more complex organisms, we find certain cells set apart for Absorption, others for Secretion, &c, so do we find a particular group of cells set apart for Reproduction; and these go through a series of changes analogous to those just described, yet without interfering with the general life of the structure. 779. Among many of the lower Animals, a multiplication of indi- viduals takes place by a process that closely resembles the budding of Plants; this must be regarded, however, not as a proper act of Repro- duction, but as a modification of the ordinary Nutritive process. The same may be said of the powers of reparation, which every Animal body possesses in a greater or less degree, but which are by far the most remarkable among the lower tribes; for when an entire member is renewed (as in the Star-fish), or even the whole body is regenerated 444 SIMPLEST FORMS OF REPRODUCTIVE PROCESS. from a small fragment (which is the case in many Polypes), it is by a process exactly analogous to that which is concerned in the repara- tion of the simplest wound in our own bodies, and which, as already explained (§ 636), is but a modification of the process that is con- stantly renewing, more or less rapidly, every portion of their fabric. The essential character of the special function of Reproduction, con- sists in the entire separation of certain germs from the parent structure; which are capable, by their own inherent powers, of developing themselves into new individuals: the only conditions requisite, being a proper supply of nutriment, and a certain amount of warmth. In the case of the simple Cellular Plants just now adverted to, the germs, when set free from the parent-cell, are thrown at once upon their own resources, and draw from the surrounding elements the materials of their growth and development (§ 32). But in the higher Plants, we find not only a set of germ-preparing organs, or reproductive cells (the pollen-grains), but also a set of germ-nourishing organs (the ovules); into which the reproductive granules are received, and in which they are supplied with nutriment previously elaborated by the parent, that serves to nourish them during the early stages of their development. 780. This is the universal method in which the Reproductive pro- cess is effected in Animals ; the concurrence of two sets of organs being always necessary,—the germ-preparing organs, or seminal cells; and the germ-nourishing organs, or ova. These maybe united in the same individual, as they are in most plants; and the ova may be fer- tilized from the seminal cells of the same being;—as happens in some of the lowest tribes of Mollusca. Or, the two sets of organs being present in each individual, it may not be capable of self-impregnation; but, in the congress of two individuals, each impregnates, and is im- pregnated by the other;—as may be observed in the Snail, and many of the higher Mollusks. Or the sexes may be altogether distinct; one individual possessing only the male or germ-preparing organs; and the other the female, or germ-nourishing apparatus. 781. The early Development of Animals may be so much better understood, when the general history of that of Plants is compre- hended, that it is desirable here to give an outline of the latter sub- ject.—Where, as in the simplest Cellular Plants, each individual consists, even in its adult state, but of a single cell, the development of one of the reproductive granules into the complete cell constitutes the whole history of its growth. In other cases, however, we have an extension of the original structure by a process of budding; so that, from the first-formed cell, a cluster, or filament, may be pro- duced, according to the mode in which this budding takes place. Thus it may occur, as in Palmella, very much in the manner of or- dinary Cartilage-cells, (§ 267, Fig. 37,) so as to produce a cluster of 2, 4, 8 or more ; or it may proceed in a linear direction, as in Carti- lage-cells near the ossifying surface, (Fig. 48,) so that a filament is the result, as the ordinary Confervce. Now if the cells of one of the sim- REPRODUCTIVE PROCESS IN PLANTS. 445 pie Conferva?, which is composed of single rows, bud out laterally, as well as longitudinally, a leaf-like expansion is formed, like that of the Sea-weeds. This simple organ has the power of performing the functions of absorption, digestion, respiration, &c, as well as that of reproduction ; and as it differs from the leaves of the higher plants (to which it otherwise bears a close resemblance), in its power of per- forming the last-named function, it is distinguished by the name of frond. 782. Although, in the highest Cryptogamia, the character of the Plant is ultimately to become very different from this, its formation commences in precisely the same manner; so that the young Fern, which is afterwards to send a woody stem and beautifully-formed leaves into the air, and to transmit its solid roots deep into the ground, might be readily mistaken for an humble Liverwort, whose frond is not destined to raise itself from the ground, but creeps along its sur- face, and obtains its nourishment by the slight fibres which insinuate themselves into the soil. In both cases, the primary frond is evolved, in a precisely similar manner, by the budding of the original cell; but the Liverwort remains upon the lower grade, beyond which it is never destined to pass,—the primary frond being, in that class, the permanent plant; whilst in the Fern, the primary frond is a tempo- rary organ merely, the purpose of which is to obtain and elaborate the nutriment, that is destined for the evolution of the permanent structure. It is from the centre of this leafy expansion, that the true stem and roots of the Fern are subsequently put forth; and the whole of the primary frond decays away, as soon as the first true leaf has unfolded itself. 783. Although the embryo of the Flowering Plant is developed under different conditions,—that, is, at the expense of the nutriment provided for it in the seed, within which it is contained,—yet the history of its growth is essentially the same. The mass of cells, which originates from the pollen-granules that fertilize the ovule, does not at first take the form which the young plant is afterwards to present; but spreads itself out into a single or double cotyledon, which is a leaf-like expansion, closely resembling the primary frond of the Fern. It is by this organ, that the nourishment provided in the ovule is ab- sorbed and prepared for the development of the young plant; the permanent fabric of which, even when the seed is mature, forms but a very small proportion of it. The development of the permanent structure takes place rapidly, however, during the process of germi- nation; in which all the nourishment contained in the seed is pre- pared for the embryo by the cotyledons ; these serving the purpose of leaves, until the stem and roots have been developed, and the true leaves unfolded. By the time that this store has been exhausted, the development of the embryo has advanced sufficiently far, to enable it to support itself; and the cotyledons then decay away. 784. Thus we see that even the highest Plants have to pass through the conditions, which are permanently shown in the lower; and that 446 REPRODUCTION IN ANIMALS.—ACTION OF MALE. the parts which are first formed, are destined for a temporary purpose only. We shall find, in tracing the history of the development of Animals, that exactly the same general fact may be observed, in even a higher degree;—the number of different stages being greater; and an even larger proportion of the parts first formed, in the embryo of the higher tribes, having a merely temporary purpose, and being destined to an early decay, as soon as the more permanent portions of the fabric have been evolved. Fig. 129. 2. Action of the Male. 785. The share in the Reproductive function, which belongs to the Male Sex, essentially consists in the formation and liberation of the reproductive particles. These are prepared within peculiar cells, as already described (§ 240); and the cells are either scattered through the soft parenchyma of the body, as hap- pens among some of the lowest ani- mals ; or they are confined to certain parts of it, as in those a little more elevated in the scale; or they are formed within follicles or tubes, clus- tered together into an organ of a gland- ular character, known as the Testis. Such an organ is found in all Insects and Mollusca; as well as in Verte- brated Animals. In the first of these classes, it is formed on the general plan of their proper glands (§ 720); being usually composed of tubes, more or less elongated, and sometimes ter- In the Mollusks, on the other hand, it is almost invariably composed of clusters of follicles. In either case, the seminal cells are developed within the tubes or follicles, as are the ordinary secreting cells of the Liver or Kidney within the tubes or follicles of those glands; and their contents are discharged by an excretory duct, which terminates in an organ that conveys them out of the body, either emitting them into the surrounding water (as happens with many Mollusca), or depositing them within the body of the female. It is curious that, in some of the lowest Fishes, we should return to one of the simplest conditions of this organ,—a mass of vesi- cles, without an excretory duct. In these cases, the secretion formed within the vesicles escapes, by their rupture, into the abdominal cavity; whence it passes out by openings that lead directly to the exterior. 786. The Testis in Man is formed, in every essential particular, upon the plan of the ordinary Glands. It consists of several distinct lobules, separated by processes of the fibrous envelop, or tunica Formation of Spermatozoa within semi- nal cells:—a, the original nucleated cell; 6. the same enlarged, with the formation of the Spermatozoa in progress ; c, the Sper- matozoa nearly complete, but still enclosed within the cell. minating in enlarged follicles. STRUCTURE OF THE TESTIS. 447 albuginea, which pass down between them ; and each lobule consists of a mass of convoluted tubuli seminiferi, through which blood-ves- sels are minutely distributed. The diameter of these tubuli is tolerably uni- form ; being, when they are not over- distended, from l-195th to l-170th of an inch. They form frequent anasto- moses with each other; and on this account it is difficult to trace out their free or csecal extremities. The tubuli of each testis discharge their contents into an efferent duct, the Vas-deferens; and by this the product is conveyed into the Vesicula seminalis on each side, which, like the gall-bladder and Urinary bladder, serves to store up the secretion until the proper time for dis- charging it. The product of the action of the Testis consists of a fluid, through which the Spermatozoa are diffused,— these last bodies being usually set free by the rupture of the seminal cells be- fore they leave the tubuli of the testis. It is difficult to determine the precise characters of the fluid portion of the secretion ; as this is mingled with other secretions (such as that of the Prostate gland, and of the mucous lining of the Vesiculse seminales and spermatic ducts) before it( is emitted. And an exact analysis is not of much consequence; since there can be no doubt that the peculiar powers of the fluid depend upon the Spermatozoa. It may be stated, how- ever, that the Spermatic fluid has an alkaline reaction, and that it contains albumen, together with a peculiar animal principle termed Spermatine ; and that it also includes saline matter, consisting chiefly of the muriates and phosphates, especially the latter, which form crys- tals when the fluid has stood for some little time. 787. The minute filamentous bodies set free by the rupture of the spermatic cells, are distinguished by their power of spontaneous move- ment, which occasioned them to be long regarded as proper Animal- cules. It is now clear, however, from the history of their develop- ment, as well as from other considerations, that they cannot be justly regarded in this light; and that they are analogous to the reproductive particles of Plants, which, in many cases, exhibit a spontaneous mo- tion of extraordinary activity, after they have been set free from the parent structure. The human Spermatozoon consists of a little oval flattened body, from the l-600th to the l-800th of a line in length; from which proceeds a filiform tail gradually tapering to a very fine Fig. 130. Anatomy of the Testis:—1, 1. The tu- nica albuginea. 2, 2. The mediastinum testis. 3, 3. The lobuli testis. 4, 4. The vasa recta. 5. The rete testis. 6. The vasa efferentia, of which six only are represented in this diagram. 7. The coni vasculosi, constituting the globus major of the epididymis. 8. The body of the epi- didymis. 9. The globus minor of the epididymis. 10. The vas deferens. 11. The vasculum aberrans. 448 FORMATION OF SEMINAL SECRETION. point, of l-50th or at most l-40th of a line in length. The whole is perfectly transparent; and nothing that can be called structure can be satisfactorily distinguished within it. The movements are principally excited by the undulations of the tail; which give a propulsive action to the body. They may continue for many hours after the emission of the fluid; and they are not checked by its admixture with other secretions, such as the urine and the prostatic fluid. When the semi- nal fluid remains in contact with a living surface (as when deposited in the generative organs of the female) the Spermatozoa may retain their vitality for some days ; and an instance has already been referred to (§ 240), in which the later stages of the development of the Sper- matozoa actually take place in this situation,—the seminal fluid emitted by the male (among many Crustacea) not containing any Spermatozoa completely formed, but numerous spermatic cells, which undergo the remainder of their development, and then rupture and set free their contents, within the oviducts of the female. 788. The power of procreation does not exist in the Human Male (except in rare cases) until the age of from 14 to 16 years; at wThich epoch, the sexual organs undergo a much increased development; and the instinctive desire, which leads to the use of them, is awakened in the mind. From that time, to an advanced age, the procreative power remains, in the healthy state of the system ; unless it be exhausted by * excessive use of it, or by too energetic a direction of the mental or corporeal powers to some other object. The formation of Seminal fluid being, like the proper acts of Secretion, very much influenced by conditions of the nervous system, is increased by the continual direc- tion of the mind towards objects which arouse the sexual-propensity; and thus, if sexual intercourse be very frequent, a much larger quan- tity of the fluid will be produced, than if it is more rarely emitted, although the amount discharged on each occasion will be less. The formation of this product is evidently a great tax upon the corporeal powers; and it is a well-known fact, that the highest degree of bodily and mental vigour is inconsistent with more than a very moderate indulgence in sexual intercourse; whilst nothing is more certain to reduce the powers, both of body and mind, than excess in this respect. 789. It may be stated as a general law, prevailing equally in the Vegetable and Animal kingdoms,—that the development of the indi- vidual, and the reproduction of the species, stand in an inverse ratio to each other. We have seen that, in many organized beings, the death of the parent is necessary to the production of a new generation ; and even in numerous species of Insects, it follows very speedily upon the sexual intercourse. It is a curious fact, that Insects which usu- ally die, the male almost immediately after the act of copulation, and the female very soon after the deposition of the eggs, may be kept alive for many weeks or even months, by simply preventing their copulation. And there can be no doubt, that, in the Human race, early death is by no means an unfrequent result of the excessive or premature employment of the genital organs; and where this does DEPENDENCE OF ANIMAL EMBRYO ON FEMALE PARENT. 449 not produce an immediately fatal result, it lays the foundation of future debility, that contributes to produce any forms of disease to which there may be a constitutional predisposition, especially those of a Scrofulous nature. 790. The emission of the Spermatic fluid is an act of a purely reflex nature; the Will having no power either to effect or to restrain it. The stimulus is given by the friction of the surface of the Glans Penis against the rugous walls of the Vagina; the sensibility of the organ being at the same time much increased, by the determination of blood to it. The impression is at last sufficiently strong to produce, through the medium of the lower part of the Spinal cord, (which is the gangli- onic centre of the circle of afferent and efferent nerves connected with this organ,) a reflex contraction of the muscles surrounding the vesi- culse seminales. These receptacles discharge their contents (which consist partly of the Spermatic fluid, and partly of a secretion of their own), into the Urethra; and from this they are expelled with some degree of force and with a kind of spasmodic action, by its own Compressor muscles. Although the sensations concerned in this act are ordinarily most acutely pleasurable, yet there appears to be suffi- cient evidence that they are by no means essential to its performance; and that the impression conveyed to the Spinal cord may excite the contraction of the Ejaculator muscles, like other reflex operations, without producing sensation (§ 394). 3. Action of the Female. 791. As it is the office of the Male to prepare the germ of the future being, and then to set it free, so is it the part of the Female to receive this germ, and to supply it with the materials for its develop- ment, up to the condition in which it can support its own life. The mode in which this is accomplished, is essentially the same with that, in which the process is effected in Plants. In certain parts of the female structure are developed peculiar bodies termed ova ; which contain a store of nutriment, adapted to supply the wants of the germ. The reproductive particles find their way into these, and begin to grow at the expense of the materials which they meet with in their interior. This may enable the embryo to develop itself, without any further assistance (save a warm temperature), into the form it is per- manently to assume ; as in the case of Birds and Reptiles, which do not come forth from the investments of the egg, until they have at- tained the form characteristic of the group to which they belong. Or it may only serve for the early part of the process ; and one of two methods may then be employed to complete it. Either a new con- nection is formed between the parent and embryo, by which the former continues to supply the latter with nutriment, more directly frora its blood ; as is the case with Mammalia—or the embryo issues from the egg, in a condition very unlike that which it is permanently to attain but in a form which enables it to acquire its own nourish- 29 ' 450 DEVELOPMENT OF OVA IN OVARIUM. ment, and to pass through the latter stages of its evolution quite inde- pendently of any assistance from its parent; this is the case with a large proportion of the Invertebrata. 792. Sometimes the permanent form of the latter is elaborated, as it were, out of the temporary, by the gradual development of new parts; as happens in most of the Worm tribe,—the animal, at the time of its first emersion from the egg, possessing but a few segments, or even but a single one ; but afterwards, by the progressive develop- ment of new segments, to the number in some instances of several hundreds, acquiring a great length. In other cases, there is a com- plete metamorphosis or change of form ; the animal at its emersion from the egg, not merely having an aspect which is entirely different frora that which it is ultimately to present, but possessing organs which it is afterwards to lose. Thus the Frog emerges in the state of a Fish ; and in this, its Tadpole condition, it breathes by gills, swims by its tail, and has all the essential characters of the class below its own. Certain of its organs gradually disappear altogether, whilst others are as gradually developed; and in this manner the temporary Fish is converted into the permanent Reptile. The metamorphosis is even more striking in Insects ; which come forth from the egg as Worms; and which attain their complete form by what appears to be a sudden change,—this change being really, however, of a very gra- dual character, the organs characteristic of the perfect Insect being slowly developed, during the preceding state of quiescence which usually characterizes the life of the Chrysalis, but being displayed and brought into use only when the Chrysalis-skin is thrown off. Thus the whole life of the Insect, up to this last change, may be re- garded as one of embryonic development; and the same may be said of the condition of the Frog, up to the time when its permanent organs are fully evolved. 793. The Ova, like the seminal cells, are scattered through the soft parenchyma of the body, in animals of the lowest class ; but they are more commonly developed in certain distinct portions of the fabric ; being sometimes formed in the midst of solid masses of areolar or cellular texture ; whilst in other instances they are developed, like the spermatic cells, in the interior of tubes and vesicles resembling those of glands, and furnished with an excretory duct. The latter condition obtains in the greater proportion of the higher Invertebrated animals, and in some Fishes ; but in the Vertebrated classes we return to the type which characterizes the egg-producing organs in many Zoophytes,—namely, the development of the egg in the midst of a mass of solid parenchyma, from which it gradually makes its way, to escape into the abdominal cavity. The Ovarium of the Mammal, Bird or Reptile, as well as that of most Fishes, differs entirely, there- fore, from that of the Invertebrata ; for the latter have all the essen- tial characters of true glands; whilst the former are nothing else than masses of parenchyma, copiously supplied with blood-vessels, and having dispersed through their substance certain peculiar cells, termed STRUCTURE AND DEVELOPMENT OF OVUM. 451 ovisacs, within which the ova are developed. In order that the latter may be set free, not only must the ovisac itself burst (like parent-cells in general), but the peculiar tissue, and the envelops, of the ovarium must likewise give way. When the ova thus escape into the ab- dominal cavity, they may lie there for some time, at last to be dis- charged through simple openings in its walls, as happens in those Fishes which have this form of ovarium ; or they may be at once received into the trumpet-shaped expansions of tubes, that shall con- vey them to these orifices. These tubes are termed oviducts, in com- mon with the excretory ducts of the glandular ovaria of Invertebrated animals ; for their function is the same,—that of conveying the ova to the outlet by which they are extruded from the body. They are represented in Mammalia by the Fallopian tubes, which are true ovi- ducts ; but these unite and enlarge to form a Uterine cavity, in which the embryo may be retained, whilst it is receiving the further assist- ance to its development, in the manner to be presently explained. This uterine cavity is peculiar to the Mammalia; but there are many cases among the lower classes, in which the ovum is retained within the oviduct, so that the young comes into the world alive ; and a few in which, during this delay, it receives a direct supply of additional nourishment from the fluids of its parent. 794. The essential structure of the ovule, or unfertilized egg, ap- pears to be the same in all animals. It consists externally of a mem- branous sac, termed, from the nature of its contents, the yelk-bag. The yelk, or contained fluid, consists chiefly of albumen and oil- globules ; and it is this substance, which, like the starchy and oily matter laid up in the seed of the Plant, is destined to afford support to the embryo, until it is able to obtain its own nutriment, or, as in Mammalia, forms a new connection with the parent. Floating in this fluid is a cell of peculiar aspect, termed the germinal vesicle; and upon its wall is a very distinct nucleus, termed the germinal spot.— The layer of albumen surrounding the yelk, and termed the white of the Bird's egg, together with the membrane which envelops this and forms the basis of the shell, are not added until after the ovum has left the ovarium. They are not present in the eggs of many of the lower Invertebrata; these consisting merely of the parts which are formed within the ovarium. 795. The structure of the ovule in Mammals differs in no essential particular from that just described ; but the yolk is much less in amount, than in the ovules of Invertebrated animals; since only the very earliest stages of the development of the embryo are to take place at its expense. We shall find that the ovule, after leaving the ovarium and receiving the fertilizing influence, becomes enclosed, whilst passing through the Fallopian tube, with a layer of albumi- nous matter, which represents the white of the Bird's egg; and with an additional fibrous envelop, which corresponds with the membrane enveloping the latter. This fibrous membrane, termed the Chorion, afterwards becomes subservient, however, to various important 452 PUBERTY.—MENSTRUAL DISCHARGE. changes; by means of which the ovum is again brought into con- nection with the parent, to derive from the blood of the latter the materials requisite for the continued development of the embryo. These changes will be described hereafter (§811). 796. The Ovisac of Mammalia forms the inner layer of what is termed the Graafian follicle, after the name of its discoverer; and instead of closely enveloping the ovulum, as it does in oviparous animals, it contains, in addition to it, a quantity of granular matter, consisting of cells arranged in membranous layers, together with fluid. Of these layers, one surrounds the ovulum, and is termed the tunica granulosa; another lines the ovisac, and is named the membrana granulosa; whilst, to certain bands passing from the former to the latter, and suspending the ovule (as it were) in the cavity of the ovisac, the name of retinacula has been given by their discoverer, Dr. Barry.—The outer layer of the Graafian follicle is formed by a thickening and condensation of the surrounding parenchyma of the ovarium; and it is quite distinct from the ovisac which it envelops. It is extremely vascular, and is evidently destined to afford to the structures within the materials for their development, which they re- ceive and appropriate by their own powers of absorption and assimi- lation. 797. The Mammalian Ovarium may be seen, even in the fcetal animal, to contain immature ova, enclosed within their ovisacs ; and the several parts of the former may be clearly distinguished, in those which are in the more advanced stages of development. It appears that, during the period of childhood, there is a continual rupture of the ovisacs (or parent-cells), and a discharge of ova, at the surface of the ovarium ; but these ova never attain so high a degree of develop- ment, as to render them fit for impregnation. Their evolution takes place more completely, as well as more rapidly, at the period of puberty, when there is a greatly increased determination of blood to the genital organs, and a correspondingly augmented energy in their nutritive operations. At this epoch, the parenchyma of the ovarium is crowded with ovisacs ; which are still so minute, that in the Ox, according to Dr. Barry's computation, a cubic inch wTould contain 200 millions of them. Some of those nearest the surface, however, are continually attaining increased development; and a rupture of some of the Graafian follicles, and a discharge of ova prepared for impregnation, from the exterior of the ovarium, thenceforth take place, with more or less tendency to periodicity, during the whole time that the female is in a state of aptitude for procreation. 798. In the Human female, the period of Puberty usually occurs between the 13th and 16th year. The differences in the time of its advent partly depend upon individual constitution, and partly upon various external circumstances, such as temperature, habits of life, &c. As a general rule, habitual exposure to a warm atmosphere, an inert life, sensual indulgence, and circumstances that excite the sexual feelings, favour the approach of Puberty; whilst a cold climate and MENSTRUAL DISCHARGE.—MATURATION OF OVA. 453 hardy life retard it. The appearance of the Catamenial discharge usually takes place whilst the evolution of the genital organs is in progress ; and it is a decided indication, when it occurs, that the aptitude for procreation has been attained. It is not unfrequently delayed much longer, however; and its absence is by no means to'be regarded as a proof of inability to conceive. The Catamenial secre- tion, which proceeds from the lining membrane of the Uterus, seems to consist of the elements of Blood, in an altered condition. It con- tains a considerable amount of red colouring matter; but the albu- minous and fibrinous constituents seem to be present in smaller proportion than in Blood. The coagulating power is for the most part wanting, when the function is performed in a healthy manner ; the appearance of clots being an indication that blood is escaping from the secreting surface. The coagulation of the fibrin normally present in the secretion, appears to be prevented by admixture with the vaginal mucus ; but when an increased araount is poured forth, this admixture is not sufficient to destroy its power of forming a clot. In some cases of difficult Menstruation, which seem to depend upon a state of low inflammation in the Uterus, the fibrin has such a tend- ency to become organized, as to form shreds, or layers of false merabrane, which sometimes plug up the os uteri. The healthy Menstrual secretion is remarkable for its very acid character. 799. This flux of altered blood from the lining membrane of the Uterus, is not confined to the Human female, as was formerly sup- posed ; but occurs in most of the lower Mammalia in the state of heat, or periodical aptitude for procreation, at which time the ova- rium contains ova ready for impregnation. The chief peculiarity attending its appearance in the Human female, is its regular monthly return. In the natural condition of many of the lower Mammalia, as in Oviparous animals, the period of heat recurs at some one time of the year,—usually in the spring; or, in the smaller and more prolific species, from two to six times. And in those which have undergone a change by domestication, the recurrence is usually irregular, de- pending upon various circumstances of regimen, temperature, &c. The general analogy between the Menstruation of the Human female and the heat of the lower Mammalia,—consisting in the peculiar apti- tude for impregnation which then exists, in consequence of the matu- ration of ova in the ovarium,—cannot now be questioned ; but it appears that, in the Human female, ova may be matured and impreg- nated at any part of the period, wdiich elapses between the occur- rences of the Catamenial discharge; though it is certain that the apti- tude for conception is much greater, during the few days which precede and follow the menstrual period, than at any intervening time. The duration of the period of aptitude for procreation, which is marked by the continued appearance of the Catamenia, is more limited in Women than in Men ; usually terminating at about the 45th year. It is sometimes prolonged, however, for ten or even fif- teen years longer; but cases are rare, in which women above 50 454 MATURATION OF OVA. years of age have borne children. There is usually no menstrual flow during pregnancy and lactation ; in fact, the cessation of the Catamenia is usually one of the first signs indicating that conception has taken place. It is by no means uncommon, however, for them to appear once or twice subsequently to Conception ; and their ap- pearance during Lactation, especially if it be much prolonged, is still more frequent; hence it might be inferred, that the continuance of Lactation would not prevent a fresh conception,—which is found to be true in practice. 800. We shall now take a brief survey of the changes which occur in the Ovulum, when it is being prepared for fecundation ; and of the principal features of its subsequent development.—Up to the period when the Ovule is nearly brought to maturity, it remains suspended in the centre of the cavity of-the Ovisac; but it then begins to move towards that side of the Graafian follicle, which is nearest the surface of the ovarium. An important change is at the same time occurring in the Graafian follicle itself; for whilst the part with which the ovule comes in contact gradually thins away, the outer or vascular layer of the remainder, especially on that side most deeply imbedded in the ovary, becomes much increased in thickness; and a deposition of fibrinous matter seems to take place at that part, between this layer and the inner layer or proper ovisac. This fibrinous matter is destined subsequently to become more or less completely organized; receiv- ing vessels, which are prolonged into it from its enveloping mem- brane: and it then forms the corpus luteum. The escape of the ovule from the ovarium involves processes which are essentially the same, whether it be impregnated or not; but the subsequent changes differ in the two cases, so that the corpus luteum which accompanies the pregnant state is a much more highly organized body than that which is found in the ovary of the unimpregnated female. This difference may be due in part to the absence, in the latter case, of that special determination of blood to the genital organs, which takes place in the former. 801. When the ovule is being thus brought near the surface of the Ovary, a series of remarkable changes takes place in its interior. The yelk becomes filled with cells; which, after passing through several generations (during which the transparency of the yelk is much interfered with), completely disappear, leaving the fluid appa- rently in the same condition as before. This process of cell-develop- ment in the substance of the yelk, continues for some time after fecundation ; and it probably has for its purpose, to prepare the mat- ter of the yelk for its subsequent functions ; just as we have seen reason to believe, that the albuminous matter of the chyle is ren- dered fit for the nutrition of the body, by the development of floating cells in its current (§213). But the most curious changes are those, which take place within the germinal vesicle. Though previously in the centre of the yelk, it now moves up towards one side of it, and becomes flattened against the yelk-bag. At the same time, the FERTILIZATION OF OVA. 455 edge of its nucleus begins to resolve itself into a ring of cells; which sprout forth, as it were, from its inner wall, into its cavity. These cells enlarge; and another ring is developed nearer the centre of the nucleus, pushing the former one outwards. A third ring is next formed internally to the second ; and a similar development of suc- cessive annuli of minute cells, one within another, continues, until the whole germinal vesicle is filled with minute cells; of which those constituting the outer and first formed rings are the largest, whilst those forming the central rings are very minute. The centre of what was the germinal spot remains transparent; and into this the germ finds its way in the act of fertilization, by the means to be presently described. All these cells, like those of the yelk, have a merely temporary existence, and speedily deliquesce again ; and their func- tion appears to be, to prepare the contents of the germinal vesicle for being applied to the nutrition of the germ, which is to be subsequently introduced into it. 802. By the changes in the position of the Ovulum and of its con- tained parts, which have been already noticed, the germinal vesicle is brought into very close proximity with the surface of the Ovary. It is still covered, however, by the peritoneal coat of the ovary; by a thin layer of the fibrous substance of that organ ; by the ovisac ; and by the yelk-bag, which, in the Mammalian ovum, is known as the zona pellucida. The three former of these envelops gradually thin away, and at last rupture, and give passage to the ovule; which thus escapes from the surface of the ovarium. At about the same time, a chink or fissure is formed in the part of the yelk-bag, that covers the central pellucid space of the germinal spot; and into this space the fertilizing influence appears to be introduced ; for we find it afterwards occupied by two new cells, of very different appearance from the rest, from which the whole of the embryonic structure is subsequently to be developed. 803. Much discussion has taken place, with regard to the exact point at which the fertilization of the ovulum takes place ; but this does not seem to be a matter of much consequence, as we find the order of the different steps to vary considerably in different classes of animals. Thus in many aquatic Mollusca, and even in a large proportion of the class of Fishes, there is no act of copulation whatever; but the spermatic fluid, when emitted by the male, is diffused^through the water, and fertilizes the ova, which have been deposited by the female in his neighbourhood. In the Frog, again, and in other Rep- tiles, the spermatic fluid is emitted upon the ova, at the time that they are bein^ extruded by the female. In many Insects and Crustacea,— in which° a single congress often serves to fertilize many thousand eggs, the deposition of which occupies a period of several weeks or months,—the spermatic fluid is received and stored up in a saccular dilatation of the oviduct of the female, which is termed the spermo- theca ; and in this manner it serves to impregnate the ova, as they are successively developed, and are conveyed to the outlet of the oviduct. 456 EARLY CHANGES IN FERTILIZED OVUM. In Birds, we find that ova are often set free from the ovarium in a state of full maturity, but without fertilization; and that they receive their additional layer of albumen and their shelly envelop, in passing down the oviduct, so as, at the time of their deposition, to differ in no obvious particular from fertile eggs. It is doubtful, in regard to Mammalia, whether the act of fertilization takes place before the ovum has been actually discharged from the ovisac, or subsequently to its finally quitting the ovarium and being received into the Fallopian tube. It is quite certain that the spermatozoa frequently, if not inva- riably, find their way to the surface of the ovary, being carried thither by their own spontaneous movements ; and it seems on the whole most probable, that the fertilization of the ova usually takes place before they have been discharged from the ovisac, or whilst they are still in the commencement of the Fallopian tube. It is not unlikely that the place of the act of fecundation varies, according to the point at which the ovule and the seminal fluid first come into contact,— which may depend upon the degree of maturity of the ova at the period of copulation. 804. Everything indicates that the contact of the Spermatozoon with the Ovulum is the one thing needful in the act of fecundation ; and there is strong reason to believe, that the large end of the Sper- matozoon finds its way into the fissure just described, which is formed in the Zona pellucida ; and that it there deposits the germs of the two new cells, which are afterwards seen within the germinal vesicle. We have seen that this is the essential nature of the fecundating pro- cess in the Flowering Plant; the reproductive granules, prepared within the pollen-cell, being conveyed by the pollen-tube within the ovule, where they speedily develop themselves into the first cells of the embryonic structure. The manner in which the reproductive germs of the Animal find their way to the ovary, is different, as we have seen ; a power of spontaneous movement (which finds its resem- blance in that of the sporules of the Confervee, &c.) being imparted to them, by which they bring themselves into contact with the ovum. 805. From this stage, an entirely new set of changes begins to take place in the interior of the Ovum, during its passage along the Fallo- pian tube or oviduct. The two new cells, which at first occupy only the pellucid centre of the germinal spot, rapidly increase in size, and begin to develop new cells in their own interior. At the same time they press upon the cells, which filled the germinal vesicle previously to its fertilization; and these gradually liquefy or dissolve away, until all trace of them is lost; and the twin-cells, with their offspring, are alone contained within the germinal vesicle. Each of the first-formed cells gives birth, by the usual process of cell-development, to a new generation of two ; so that the number is now four; from these four is produced a third generation of eight; and these go on progressively doubling, until at last a mass is produced, closely resembling a mul- berry, in which the number of cells is too great to admit of being counted. This " mulberry mass" is obviously analogous to the col- GERMINAL MEMBRANE.—CICATRICULA. 457 lection of cells, which is first developed within the seed of the Flower- ing Plant (§783); and between the condition of the Animal and that of the Vegetable embryo, at this period, there would not seem to be any essential difference. 806. In the next stage, however, a marked difference shows itself, which is very charac- Fig. m. teristic of the two kingdoms respectively. The mass of cells, which is the rudiment of the Vegetable embryo, spreads itself out into a flat leaf-like expansion,—the primary frond, or co- tyledon,—which remains as the permanent form of the lowest plants, but is only tempo- rary in the higher (§ 782). But in the embryo of the Animal, the " mulberry mass," having moved up to the side of the yelk, and having ^^ggrJgS become flattened against its enveloping mem- zona peiiucida, are the serous , „. „ ° . , l r 11 lamina, a; the yelk, b; and brane, sends off from its edges a layer ot cells, the incipient embryo, c. which passes round the yelk, so as completely ... to enclose it within a membranous envelop, the exterior of which is in contact with the yelk-bag. A second layer is afterwards formed within the preceding, from the central part of the mulberry mass; and, in the higher animals, a third is subsequently formed between them. This membranous formation, as a whole, is known as the germinal membrane; its external pellicle is termed the serous layer; the internal is termed the mucous layer; and the intermediate one, which gives origin to the first vessels of the embryonic structure, is termed the 807. Thus the first development of the Animal embryo is into a sac enclosing the store of nutriment that has been prepared for it,— in fact, a stomach; and we shall presently see, that it is by the agency of the walls of this sac, that the nutrient materials which it encloses are prepared for being appropriated to the development of the more permanent part of the fabric, which is to be evolved from the centre of the mulberry mass. But we may here stop to notice the interesting fact that the development of the ovum in the lowest classes of animals may be said almost to stop at this point; the external layer of the germinal membrane remaining as the integument; the internal layer becoming the lining of the stomach; and the space occupied by the yelk forming the digestive cavity, into which an entrance or mouth is formed, by the thinning-away of the germinal membrane at a certain point, round which tentacula or prolonged lips are usually developed. This is the essential part of the history of development m the simpler Polypes • and we see how remarkably it corresponds with the history of development of the lower Cryptogamic plants, in which the first- formed membranous expansion, or primary frond, remains as the per- manent leaf. , , i ,, *. c 808 In the.higher Animals, on the other hand, the greater part ot the germinal membrane, and of the cavity which it forms, have a 458 FORMATION OF CHORION. merely temporary purpose; being cast off, when they have performed their function, like the cotyledons of Flowering Plants. Nearly the whole of the permanent structure of the embryo is formed from a single large cell; which at first occupies the centre of the "mulberry mass;" but which is seen at the surface of the latter, when this under- goes the flattening already described. This cell, together with the cluster of ordinary cells that surrounds it, is that which forms the cicatricula or germ-spot upon the surface of the yelk-bag, in the im- pregnated ovum of the Fowl; and, whilst still retaining its clearness, it forms a large round transparent space in the centre of the cicatricula, which is known as the Areapellucida. The nucleus of this Embryonic cell, which was at first annular, changes its form into that of'a pear, and then into that of a violin ; and consists at last of two long parallel lines, enclosing a narrow space between them, but separating and enclosing a wider space at one extremity. In this state, it is called the Primitive Trace. The same process then takes place within the Embryonic cell, which has been described as occurring within the Germinal vesicle ; the granules forming the outer border of the nucleus being first developed into cells; these being pushed o utwards by a new series subsequently generated nearer the centre; and these being displaced, in their turn, by a continuance of the same process. It is from the peripheral cells originating in this primitive trace, that the inner layers of the germinal membrane (§ 806) seem to be developed ; the cells that originate nearer its centre, are those frora which the more permanent portions of the embryonic fabric are evolved. The principal steps of that process will be presently noticed; we must now stop to consider the changes which take place in the female gene- rative apparatus, subsequently to the liberation of the ovum from the ovarium, but having relation to the new connection, which is to be afterwards formed between the embryo and its parent. 809. During the time which is occupied by these important changes, the Ovum passes through the Fallopian tubes, and makes its way into the Uterus. During its transit through the Fallopian tubes, the Mam- malium ovum,—like the ovum of Birds in its passage through the ovi- duct,—receives an additional layer of albuminous matter secreted from the walls of the tube; and this is surrounded by a fibrous membrane, whose structure and mode of formation have been described on a for- mer occasion (§ 181). The outer layer of this envelop, in the egg of the Bird, is further consolidated by the deposition of particles of car- bonate of lime in its areola; but it undergoes no higher organization. In the Mammal, however, this new envelop (termed the Chorion) is a formation of great importance; being the medium through which the whole subsequent nutrition of the embryo is derived. This is at first taken in by means of a number of villous processes, proceeding from the entire surface of the Chorion, and giving it a spongy or shaggy appearance ; these processes (which are composed of nucleated cells) serve as absorbing radicles, which draw in the fluids afforded by the parent; and they thus make up for the early exhaustion of the small FORMATION OF DECIDUA, AND VILLI OF CHORION. 459 supply of nutritious matter stored up in the ovum itself. The contained embryo appropriates the fluid which is thus imbibed, by simple ab- sorption through its surface; and thus it is nourished, until a more special provision for its development comes into action. The struc- ture of this organ, termed the Placenta, cannot be understood, until the concurrent changes in the lining membrane of the Uterus have been considered. 810. This membrane, in its natural condition, presents on its free surface the orifices of numerous cylindrical follicles ; which are ar- ranged parallel to each other, and at right angles to the surface. In the spaces between these follicles, the blood-vessels form a dense capillary network. When impregnation takes place, this mucous membrane swells and becomes lax ; its capillaries increase in size ; the follicles are turgid with a white epithelium ; and the interfollicular spaces are crowded with nucleated cells, which fill up the meshes of the capillary network. In this peculiar condition, the uterine mucous merabrane is termed the Decidua. At a later period, the decidua may be found to consist of two distinct layers; the decidua vera, lining the uterus; and the decidua reflexa, covering the exterior of the ovum. It was formerly supposed that the latter was a portion of the former, which had been pushed before the ovum at its entrance into the uterus; but the two layers are now known to be so different in texture, that they cannot be supposed to have the same origin; and there seems much probability in Mr. Goodsir's view, that the decidua vera is chiefly formed by the highly vascular mucous membrane itself, and the deci- dua reflexa by the abundant production of epithelial cells from its follicles. 811. When the ovum has arrived in the Uterus, therefore, and the villous tufts of its chorion are developed, these come into contact, in the first instance, with the layer of cellular decidua, which intervenes between them and the vascular decidua. Through this cellular mem- brane, therefore, the ovum must derive its nutriment from the vascular surface; and it cannot be deemed improbable, that the office of the cellular decidua is to draw from the subjacent vessels the materials which are to serve for the nutrition of the ovum, and to present it to the villous tufts of the chorion. Each of these, as already mentioned, is composed of an assemblage of nucleated cells, which are found in various stages of development; and these are always enclosed within a layer of basement-membrane, which seems itself to be composed of flattened cells united by their edges. At the free extremity of each villus, is a bulbous expansion, the cells composing which are arranged round a central spot; and it is at this point that the most active pro- cesses of growth take place, the villus elongating by the development of new cells from its germinal spot, and, like the spongiole of the plant, drawing in nutriment from the soil in which it is imbedded.— In its earliest grade of development, as already remarked, the chorion and its villi contain no vessels; and the fluid drawn in by the tufts is communicated to the embryo, by the absorbing powers of its germinal 460 FORMATION OF VERTEBRAL COLUMN, AND OF VESSELS. membrane. But when the tufts are penetrated by blood-vessels, and their communication with the embryo becomes much more direct, the means by which they communicate with the parent are found to be essentially the same;—namely, a double layer of cells, one layer belonging to the foetal tuft, the other to the vascular maternal surface. (See § 819.) 812. We now return to the Embryo itself; the general history of whose development has been already traced, up to the period at which the inner part of the elongated nucleus of the embryonic cell is begin- ning to give origin to the permanent structures of the foetus. The parts first formed in the embryo of Vertebrated animals, are such as most characteristically distinguish them from all others;—namely, the Vertebral column, and Spinal Cord. The latter is formed in the groove, which runs along the median line of the primitive trace; and it is surrounded, when first developed, by a tubular structure, which has but a temporary existence in the higher Vertebrata, but which is permanent in the lower Fishes. This structure, termed the Chorda dorsalis, is found to be composed, wherever it exists, of nucleated cells. From the cells exterior to this, the vertebral column is deve- loped ; and this makes its first appearance in the condition of two rows of minute opaque plates, imbedded in ridges that rise up on either side of the central groove, and constituting the arches of the incipient vertebrae. These ridges incline towards each other; and at last meet and cover-in the groove, so as to complete the bony cylinder protecting the spinal cord. Towards the anterior extremity, how- ever, they do not at once close in; and the large cells, in which the great divisions of the Encephalon originate, may be seen between them. 813. During the progress of this change, another very im- portant one is taking place, which has reference to the nutrition of the embryo during its further de- velopment. This is the formation of vessels in the substance of the germinal membrane; which ves- sels serve to take up the nourish- ment supplied by the yelk, as well as that derived from the chorion externally, and to con- vey it through the tissues of the embryo. These vessels are first seen in that part of the vascular lamina of the germinal mem- brane, which immediately sur- rounds the embryo; and they form a network, bounded by a circular channel, which is known Vascular Area of Fowl's egg, at the beginning of the third day of incubation;—a, a, yelk; 6, 6, b, b, venous sinus bounding the area: c, aorta; d, punc- tual saliens, or incipient heart; e, «, area pellucida; /,/, arteries of the vascular area; g,g, veins; A, eye. FORMATION OF VESSELS AND DIGESTIVE CAVITY. 461 under the name of the Vascular Area (Fig. 132). This gradually extends itself, until the vessels spread over the whole of the germinal membrane. The vessels are probably formed by the coalescence of the original cells of the layer; and the first blood-discs which they contain seem to originate in the nuclei of these cells. This network of vessels serves to receive the nutritious matter contained in the yelk-bag, and to convey it to the embryo; but the act of absorption seems to be performed here as elsewhere, by cells,—a layer of which always intervenes between the vascular layer and the yelk itself. These cells probably correspond in function with those of the villi of the intestinal canal in the adult (§ 242); as the vessels of the yelk- bag, or temporary digestive cavity, represent those of the alimentary canal, to be afterwards developed from a portion of it. The vessels of the yelk-bag terminate in two large trunks, which enter the embryo at the point that afterwards becomes the umbilicus, and which are known as the Omphalo-Mesenteric, Meseraic, or Vitelline vessels. The first movement of fluid takes place towards the embryo; and this may be witnessed before any distinct heart is evolved. 814. The formation of the Heart takes place in the substance of the Vascular layer; by a dilatation of the trunk, into which the blood- vessels unite. At first it appears as a mere excavation, surrounded by cells ; but its walls gradually acquire firmness and consistency, and are endowed with a contractile power that enables them to exe- cute regular pulsations. In this early condition, the heart is known as the punctum saliens (d, Fig. 132). The first appearance of the heart in the Chick is at about the 27th hour; the time of its forma- tion in Mammalia has not been distinctly ascertained. 815. Concurrently with the formation of the Vascular system, the production of the permanent Digestive cavity commences. This originates in the separation of a small portion of the yelk-bag, lying immediately beneath the embryo, from the general cavity, in the fol- lowing manner.—At about the 25th hour of incubation, in the Fowl's egg, the parts of the germinal membrane which lie beyond the extre- mities, and which spread out from the sides of the embryo, are doubled in, so as to make a depression upon the yelk; and their folded edges gradually approach one another under the abdominal surface of the embryo, so as at last to meet and enclose a cavity, which is at first simple in its form, but which is subsequently rendered more eomplex by the prolongation and involution of its walls in various parts, so as to form the stomach and intestinal tube (Figs. 133, 134, 135). This digestive cavity communicates for some time with the yelk-bag (from which it has been thus pinched off, as it were), by a wide opening, that is left by the imperfect meeting of the folds of the germinal membrane that constitute its walls. In the Mammalia, this orifice is gradually narrowed, and is at last completely closed ; and the yelk- bag, thus separated, is afterwards thrown off. It may be detected, however, upon the umbilical cord, up to a late period of pregnancy, and is known as the Umbilical vesicle. In Birds, and other oviparous ex- of the serous layer nsing up to form the Amnion. ternal and internal folds of the serous layer form- ing the amnion; g, incipient allantois. these gradually approach one another, at last meeting in the space be- tween the general envelop and the embryo, so as to form an addi- tional investment to the latter. As each fold contains two layers of membrane, a double envelop is thus formed ; of which the outer layer (Fig. 134, d, e, and Fig. 135, h), afterwards adheres to the inner sur- face of the chorion,—the original yelk-bag, or Zona pellucida, being now lost sight of; whilst the inner one (Fig. 134,/,/, and Fig. 135 /), remains as a distinct sac, to which the name of Amnion is given' This takes place during.the third day in the Chick ; but the period at which it occurs in the Human Ovum has not yet been clearly ascer- tained. J 817. The embryo, like the adult, has need of Respiration ; in or- der that the carbonic acid set free in the Nutritive operations may be removed from its fluids. In Fishes, the surrounding water acts with sufficient power upon the vessels of the yolk-bag, to produce the required aeration, up to the time when the gills of the young animal are ready to come into play But in the higher oviparous animals, whose development proceeds further before they leave the ego- a more special provision is made for the purpose. A bag, termed the allantois, sprouts (as it were) from the lower end of the intestine (Fio- RESPIRATION OF EMBRYO ;—ALLANTOIS. 463 134, g); and gradually enlarges, passing round the embryo (Fig. 135, g), so as in Birds almost com- pletely to enclose it, intervening be- tween the germinal membrane and the shell, and receiving the direct influence of the air that penetrates the latter. It is thus the temporary lung of the air-breathing oviparous animal; and it serves for the aera- tion of its fluids, up to the time when it quits the egg. In the Ovum of the Mammal, the chief office of the Allantois is to convey the ves- sels of the Embryo to the Chorion; and its extent bears a pretty close correspondence with the extent of surface, through which the Chorion comes into vascular connection with the Decidua,—this extent varying considerably in the different orders of Mammalia. Thus in the Car- nivora, whose placenta extends like a band around the whole ovum, the allantois also lines the whole inner surface of the chorion, except where the umbilical vesicle comes into contact with it. On the other hand, in Man and the Quadrumana, whose placenta is restricted to one spot, the allantois is small, and conveys the fcetal vessels to one portion only of the Chorion. When these vessels have reached the Chorion, they ramify in its substance, and send filaments into its villi; and in proportion as these villi form that connection with the uterine structure which has been already described (§ 811), do the vessels increase in size. They then pass directly from the foetus to the chorion ; and the allantois, being no longer of any use, shrivels up like the Yelk-bag, and remains as a minute vesicle, only to be detected by careful examination. The lower part of it, however, pinched off (as it were) from the rest, remains as the Urinary blad- der; and the Urachus or suspensory ligament of the latter represents the duct, by which the Allantois was originally connected with the abdominal cavity. 818. The connection which is thus formed between the Vascular system of the foetus and that of the parent, is the only one that exists in the lower Mammalia ; which are thus properly designated as " non- placental." Each villus of the Chorion contains a capillary loop; this is enclosed in a layer of cells; and this again in a lamina of base- ment-membrane ;—the whole forming the foetal tuft. This comes into contact with the cellular decidua, which lies upon the basement- membrane covering the vascular layer of the decidua. Now the Placenta is composed of these very elements, arranged in a more complex manner. It is formed by an extension of the vascular tufts Fig. 135. Diagram representing a Human Ovum in second month:—a, 1, smooth portion of cho- rion; a, 2, villous portion of chorion; k, k, elongated villi, beginning to collect into Pla- centa; 6, yolk-sac or umbilical vesicle; ^em- bryo; /, amnion (inner layer); g, allantois; h, outer layer of amnion, coalescing with chorion. 464 STRUCTURE OF THE PLACENTA. of the chorion at certain parts; and a corresponding adaptation, on the part of the Uterine structure, to afford to-these an increased supply of nutritious fluid. These specially prolonged portions are scattered, in the Ruminants and some other Mammalia, over the whole surface of the Chorion, forming what are termed the Cotyle- dons; but in the higher orders, and in Man, they are concentrated in one spot, forming the Placenta. In some of the lower tribes, the maternal and fcetal portions of the placenta may be very easily sepa- rated ; the former consisting of the thickened decidua ; and the latter being composed of the prolonged and ramifying vascular tufts of the Chorion, which dip down into it. But in the Human placenta, the two elements are mingled together through its whole substance. 819. The Fcetal portion of the placenta consists of the branches of the umbilical vessels ; which subdivide at the point at which they enter the mass, and form, by their minute ramifications, a large part of its substance. Each villus contains a capillary vessel, which forms a series of loops, communicating with an artery on the one side, and with a vein on the other; but the same capillary may enter several villi, before re-entering a larger vessel. The vessels of the villi (Fig. 136, g) are covered, as in the chorion, by a layer of cells, (/) enclosed in basement-membrane (e); but the fcetal tuft thus Fig. 136. Fig. 137. Extremity of a placental villus t^-a, external Portion of the external membrane, with the membrane of the villus, continuous with the external cells, of a placental villus:—a, cells lining membrane of the vascular system of the seen through the membrane ; b, cells seen from mother; 6, external cells of the villus, belonging within the villus; e, cells seen in profile along to the placental decidua; c, c, germinal centres the edge of the villus. of the external cells; d, the space between the maternal and fcetal portions of the villus; e, the internal membrane of the villus, continuous with the external membrane of the chorion;/, the in- ternal cells of the villus, belonging to the cho- rion; g, the loop of umbilical vessels. formed is enclosed in a second series of envelops (a, b, c), derived from the maternal portion of the placenta,—a space (d) being left between the two, however, at the extremity of the tuft. The vas- cular tufts not unfrequently extend beyond the uterine surface of the placenta, and dip down into the uterine sinuses, where they are bathed in the maternal blood. The Maternal portion of the Placenta may be regarded as a large sac, formed by a prolongation of the internal coat of the great Uterine vessels. Against the fcetal surface of this sac, the tufts just described may be said to push themselves, STRUCTURE OF THE PLACENTA. 465 so as to dip down into it, carrying before them a portion of its thin wall, which constitutes a sheath to each tuft. In this manner, the whole interior of the placental cavity is intersected by nume- rous tufts of fcetal vessels, dis- posed in fringes, and bound down by the membrane that forms its proper wall,—just as the intestines are covered and held in their places by the peritoneum. Now as this di- latation of the uterine blood- vessels carries the decidua before it, every one of the vascular tufts that dips down into it will be covered with a layer of the cellular structure of the latter (Fig. 137, and Fig. 138, e); and this will also form a part of all the bands that connect and tie down the tufts (Fig. 138, g). The blood is conveyed into the cavity of the placenta by the " curling arteries," so named from their peculiar course (Fig. 138, c), which proceed from the arteries of the uterus; and it is returned by large short straight trunks, which pass obliquely through the decidua (Fig. 138, b), and discharge their con- tents into the great uterine sinuses. 820. There is no more direct communication between the Mother and Foetus than this; all the observations, which have been supposed to prove a direct vascular continuity, being certainly fallacious. The function of the Placenta is manifestly double. The foetal tufts draw, from the maternal blood, the materials which are required for the nutrition of the embryo,—these materials having been first selected and partially elaborated by the two sets of intervening cells: and in this character, the fcetal tufts resemble the villi of the intestinal sur- face, which dip down into the fluids of the alimentary canal, and absorb the nutritive material which they furnish. But the Placenta also serves as a respiratory organ ; aerating the blood of the fcetus, by exposing it to the influence of the oxygenated blood of the Mother : and in this respect the fcetal tufts bear a close correspondence with the gills of aquatic animals, bringing the blood into relation with a surrounding fluid medium containing oxygen, which is imbibed by the blood in exchange for the carbonic acid given off. 821. The formation of the Human Placenta commences in the latter part of the second month of utero-gestation ; during the third, it ac- quires its proper character; and it subsequently goes on increasing, in 30 Fig. 138. Diagram illustrating the arrangement of the pla- cental decidua: — a, decidua in contadt with the interior of the uterus; 6, venous sinus passing ob- liquely through it by a valvular opening; c, a curling artery passing in the same direction; d, the lining membrane of the maternal vascular system, passing in from the artery and vein lining the bag of the pla- centa, and covering e, e, the foetal tufts, passing on to them from their stems from the foetal side of the cavity, also by the terminal decidual bars/,/, from the uterine side, and from one tuft to the other by the lateral bar, g; h,h, separated fcetal tufts, showing the internal membrane and cells, which, with the loops of umbilical vessels, constitute the true foetal portion of the tufts. 466 CIRCULATION IN FCETUS. Fig. 139. o o The fcetal circulation 1. The umbilical cord, consisting of the umbilical vein and two umbilical arteries ; proceeding from the placenta (2). 3. The umbilical vein dividing into three branches; two (4, 4; to be distributed to the liver; and one (5), the ductus venosus, which enters the inferior vena cava (6). 7. The portal vein, returning the blood from the intestines, and uniting with the right hepatic branch. 8. The right auricle ; the course of the blood is denoted by the arrow, proceeding from 8 to 9, the left auricle. 10. The left ventricle ; the blood following the arrow to the arch of the aorta (11). to be distributed through the branches given off by the arch to the head and upper ex- tremities. The arrows 18 and 13, represent the return of the blood from the head and upper extre- mities through the jugular and subclavian veins, to the superior vena cava (14), to the right auricle (3). and in the course of the arrow through the right ventricle (15), to the pulmonary artery (16). 17. The ductus arteriosus, which appears to be a proper continuation of the pulmonary artery ; the offsets at each side are the right and left pulmo- nary artery cut off; these are of extremely small pize as compared with the ductus arteriosus. The ductus arteriosus joins the descending aorta (18, 18), which divides into the common iliacs, and these into the internal iliacs, which become the hypogastric arteries (19), and return the blood alono-the umbilical cord to the placenta; while the other divisions, the external iliacs (20), are continued into the lower extremities. The arrows at the terminations of these vessels mark the re- turn of the venous blood by the veins to the inferior cava. accordance with the growth of the ovum. The vessels of the Ute- rus undergo great enlargement throughout; but especially at the part to which the Placenta is at- tached ; and the blood, in moving through them, produces a peculiar murmur, which is usually audible with distinctness at an early period of pregnancy, and which may be regarded (when due care is taken to avoid sources of fallacy) as one of its most unequivocal physical signs. The sound is most com- monly heard near the situation of the Fallopian tube of the right side ; and it corresponds with the pulse of the mother. 822. It would be inconsistent with the character and objects of this Treatise, to follow, in any detail, the history of the develop- ment of the Fcetus, during its intra-uterine life; and a general account of the evolution of most of the chief organs, is given in connection with that of their struc- ture. The condition of the Cir- culating apparatus, however, at the period of birth, deserves espe- cial notice. A general account of the development of the simple pulsating trunk, which constitutes its first form, into the four-cavitied heart of the higher Vertebrata,— and of the conversion of the single trunk proceeding from it, with its four pairs of branchial arches, into the aorta and pulmonary arte- ries, with their chief subdivisions, has been already given (§ 566). Up to the time of birth, however, the partition between the Auricles is incomplete ; a large aperture, the foramen ovale, existing in it. There is also a direct communi- cation between the pulmonary artery and the aorta, by the ductus arteriosus; and another direct FCETAL CIRCULATION. 467 channel between the umbilical vein and the vena cava, by the ductus venosus. 823. The following is the course of the Circulation of the Blood in the Fcetus. The fluid brought from the placenta by the umbilical vein (Fig. 139, 3), is partly conveyed at once to the vena cava as- cendens, by means of the ductus venosus (5), and partly flows through two trunks (4, 4), that unite with the portal vein (7), returning the blood from the intestines, into the substance of the liver, thence to be returned to the vena cava by the hepatic vein. Having thus been transmitted through the two great depurating organs, the placenta and the liver, the blood that enters the vena cava is purely arterial in its character; but being mixed in the vessels with the venous blood that is returned from the trunk and lower extremities, it loses this character in some degree, by the time that it reaches the heart. In the right auricle, which it then enters, it would also be mixed with the venous blood which is brought down from the head and upper extremities by the descending cava ; were it not that a very curious provision exists, to impede (if it does not entirely prevent) any further admixture. This consists in the arrangement, of the Eustachian valve; which directs the arterial current (that flows upwards through the ascending cava) into the left side of the heart, through the foramen ovale : whilst it directs the venous current (that is being returned by the descending cava) into the right ventricle. When the ventricles con- tract, the arterial blood contained in the left is propelled into the ascending Aorta, and supplies the branches that proceed to the head and upper extremities, before it undergoes any further admixture : whilst the venous blood contained in the right ventricle, is forced into the Pulmonary artery, and thence through the ductus arteriosus (17) which is like a continuation of its trunk, into the descending aorta, mingling with the arterial current which that vessel previously conveyed, and thus supplying the trunk and lower extremities with a mixed fluid. A portion of this is conveyed, by the umbilical arteries, to the Placenta; in which it undergoes the renovating influence of the maternal blood ; and from which it is returned in a state of purity. 824. Hence the head and superior extremities, whose development is required to be in advance of that of the lower, are supplied with blood nearly as pure as that which returns from the placenta ; whilst the rest of the body receives a mixture of this, with what has pre- viously circulated through the system. The Pulmonary arteries con- vey little or no blood through the lungs; the current of blood, pro- pelled from the right ventricle, passes directly onwards through the ductus arteriosus, into the aorta.—At birth, however, the course of the Circulation undergoes great changes, that it may be adapted to the new mode, in which the infant is henceforth to obtain its nutrition and to carry on its respiration. As soon as the lungs are distended by the first inspiration, a portion of the blood of the pulmonary artery is diverted into them, and there undergoes aeration ; and, as this pro- portion increases, with the full activity of the lungs, the ductus arte- 468 LENGTH OF GESTATION. riosus gradually shrinks, and its cavity finally becomes obliterated. At the same time, the foramen ovale is closed by a valvular fold; and thus the direct communication between the two auricles is cut off. When these changes have been accomplished, the circulation which was before carried on upon the plan of that of the higher Reptiles (§ 563) becomes that of the complete warm-blooded animal; all the blood which has been returned in a venous state to the right side of the heart, being transmitted through the lungs, before it can reach the left side, or be propelled from its arterial trunks.—It is by no means unfrequent, however, for some arrest of development to pre- vent the completion of these changes ; and various malformations, involving an imperfect discharge of the circulating and respiratory functions, may hence result. 825. The average length of time, which elapses between Concep- tion and Parturition, in the Human female, appears to be 280 days, or 40 weeks. There can be little doubt, however, that Gestation may be occasionally prolonged for one, two, or even three weeks, beyond that period; such prolongation not being at all unfrequent amongst the lower animals ; and numerous well-authenticated in- stances of it, in the Human female, being upon record. Upon what circumstances this departure from the usual rule is dependent, has not yet been ascertained ; but it is a remarkable circumstance, ascer- tained by the observations of cattle-breeders, that the male has an influence upon the length of gestation,—a large proportion of cows in calf by certain bulls exceeding the usual period, and a small pro- portion falling short of it. Hence we must attribute the prolongation of the period to some peculiarity in the embryo, derived from its male parent. 826. The shortest period at which Gestation may terminate, con- sistently with the life of the child, has not been precisely ascertained ; the difficulty of determining the precise date of conception being usually such, in this case as in the preceding, as to prevent the exact length of the Gestation from being known. Thus, the commence- ment of pregnancy being fixed by the time of the cessation of the Catamenia, when there is no more definite guide, it is obvious that the act of Conception may have taken place during any part of the interval that has elapsed since the last monthly period ; and thus a doubt may exist as to the length of the Gestation, to the extent of from one to three weeks. There are very satisfactory cases on record, in which, from the degree of development of the infant at birth, as well as from other circumstances, it might be certainly known not to have attained 26 or 27 weeks, or little more than six months ; and in which, by careful treatment, the infant was reared in a con- dition of health and vigour. And there is reason to believe, that infants have lived for some time, and might probably have been reared under better management, that were born as early as the 24th or 25th week. 827. The act of Parturition, by which the fcetus is expelled frora ACT OF PARTURITION. 469 the Uterus, is accomplished in part by the contractile power of the Uterus itself; and in part by the combined operation of the various muscles, which press upon the abdominal cavity, and which effect the expulsion of the feces and urine. No account can be given of the reasons, why this change should take place at the period, which has been mentioned as its usual date ; but we are as much in the dark in regard to other periodic phenomena of Animal life. For some days previously to the commencement of labour, there is usually a slow contraction of the fibres of the fundus and body of the uterus; and a yielding of those of the cervix ; so that the child lies lower, and the size of the abdomen diminishes. This slow contraction is probably not dependent upon any act of the nervous system ; but upon the direct excitement of the contractility of the muscular sub- stance of the uterus. When labour properly commences, however, the Spinal system of nerves comes into play, and the uterine contrac- tions are of a reflex nature. As before, however, the act of contrac- tion is confined to the fundus and body of the uterus ; the fibres of the cervix uteri, and of the vagina, being in a state of relaxation, which allows them to yield to the pressure of the child's head. In the first stage of labour, the Uterine contractions appear to be alone concerned ; and it is not until the head of the child is passing through the os uteri, and is entering the vagina, that the assistance of the abdominal mus- cles is called in. These act, in the first instance, as in ordinary expiration ; but their power is much increased by the voluntary reten- tion of the breath, so that the whole of their contractile force may be applied to the expulsion of the fcetus. In a latter stage of labour, this retention of the breath becomes involuntary, during the accession of the " pains;" and the expulsion of the fcetus is commonly effected with considerable force, especially.if the previous resistance has been considerable. 828. The same action which expels the fcetus, usually detaches the placenta; and if the uterus contract with sufficient force, after this has been thrown off, the orifices of the vessels which communicated with it are so effectually closed, that little or no hemorrhage from that source takes place. When efficient contractions do not occur, they may frequently be excited by pressure upon the uterus itself; by the application of cold to the abdominal surface, to the extremities, and (in severe hemorrhage) to the entire body: or by the application of the child to the nipple, which will frequently at once succeed in producing the desired effect. The efficacy of these means,—the latter in parti- cular,—obviously depends upon the influence of the spinal system of nerves upon the muscular fibres of the uterus; the application of cold to the surface, or the irritation of the nipple, occasioning a reflex action in the uterus. But it is probable that this organ has also con- siderable power of contracting, independently of the nervous system; thus there are well-authenticated cases on record, in which the fcetus has been expelled after the somatic death (§ 65) of the parent; which must have been in consequence of the persistence of the independent 470 MAMMARY GLANDS. contractility of the Uterus, and the relaxed state of all the parts through which the child had to make its exit. 829. The cause of the occasional occurrence of the parturient efforts at an unusually early period, is as little understood as that of their ordinary action. There are some individuals, in whom this regularly happens at a certain month; so that it seems to be an action natural to them. In many cases, however, it may be traced to some undue exertion of body, or mental excitement; and not unfrequently to a general constitutional irritability, which renders the system liable to be deranged by very trifling causes. Premature labour is almost always to be prevented, if possible; being injurious alike to both mother and child; and for this prevention we have chiefly to rely upon rest and tranquillity of mind and body, and upon the careful avoidance of all those exciting causes, which are liable to produce uterine contractions by their operation upon the nervous system; whilst, at the same time, any measures which will invigorate the body, without stimulating it, should not be overlooked. 830. A peculiar preparation is made, in the females of the class Mammalia, for the sustenance of the infant, for a long period after birth. This consists in the secretion of a fluid, from the glands, termed Mammary, which contains all the elements that are required for the development of the body of the infant, during the first year. These glands present themselves in an almost rudimentary state, in some of the non-placental animals of the class ; consisting only of a few large follicles, which open separately upon the surface (Fig. 106). In the higher Mammalia, however, we find it composed of vast numbers of minute follicles, clustered together upon excretory ducts. The general arrangement of these, in the human subject, is seen in Fig. 140; and in Fig. Ill, the character of the follicles them- Fi° 14°- selves, and of the secreting epithelial cells they contain, as seen under a much higher magni- fying power, has been already shown. Each Mammary gland consists of a number of glan- dulae, which are held together by areolar and fibrous tissue ; this arrangement may probably have reference to the mobility, which it is re- miik-d'ucTin'fonicieTrfrom^ quisite that the different parts of the mass should ■c^operjain\argedTurytimers.A' possess, one upon the other, in consequence of its situation upon the pectoralis muscle. The ducts converge and unite together; so as at last to form ten or twelve principal trunks, which terminate in the nipple. At the base of the nipple, these tubes dilate into reservoirs, which extend beneath the areola, and to some distance into the gland, when the breast is in a state of lactation. These, which are much larger in many of the lower Mammalia than they are in the Human female, seem to have for their office to contain a store of milk, sufficient to supply the imme- diate wants of the child when it is first applied to the breast; so that Termination of portion COMPOSITION OF MILK. 471 it shall not be disappointed, but shall be induced to proceed with sucking, until the draught be occasioned (§ 836). 831. The Mammary gland may be detected at an early period of fcetal existence, and it then presents no difference in the male and female; and it continues to grow, in each sex, in proportion to the body at large, up to the period of puberty. At that epoch, however, the gland begins to undergo a great enlargement in the female; and by the age of twenty, it attains its full size previous to lactation. Even then, however, the milk-follicles cannot be injected from the tubes. During pregnancy, the mammary glands receive a greatly-increased quantity of blood. This determination often commences very early ; and produces a feeling of tenderness and distension, which is a valuable sign (where it occurs in conjunction with others) of conception having taken place. The vascularity of the gland continues to increase during pregnancy; and, at the time of parturition, its lobulated character can be distinctly felt. The follicles cannot be readily injected, however, until the gland is in a state of complete functional activity; i. e., during lactation.—The Mammary gland of the Male does not undergo this increase of development, except under certain peculiar circumstances to be presently noticed (§ 836); and it remains a sort of miniature picture of that of the female, varying in diameter from that of a large pea to an inch or even two inches. 832. The Milk, secreted by the Mammary glands, consists of Water, holding in solution the peculiar albuminous substance termed Casein (§ 176), and various Saline ingredients, together with (in most cases) a certain form of Sugar ; and having Oleaginous globules suspended in it. These globules appear to be surrounded by a thin pellicle, which keeps them asunder, so long as the milk remains at rest.—The existence of these elements in ordinary Milk, as that of the Cow, is made apparent by the processes to which it is subjected in domestic economy. If it be allowed to stand for some time, exposed to the air, a large part of the oleaginous globules come to the surface, in consequence of their inferior specific gravity; and thus is formed the cream, which includes also a considerable amount of casein, with the sugar and salts of the milk. These may be partly separated by the continued agitation of the cream, as in the process of churning; this, by rupturing the envelops of the oil-globules, separates it into butter, formed by their aggregation, and buttermilk, containing the casein, sugar, &c. A considerable quantity of casein, however, is still entangled with the oleaginous matter; and this has a tendency to decompose, so as to render the butter rancid. It maybe separated by keeping the butter melted at a temperature of 180°; when the casein will fall to the bottom, leaving the butter pure and much less liable to change,—an operation which is commonly known as the clarifying of butter. The Milk, after the cream has been removed, still contains the greater part of its casein and sugar. If it be kept long enough, a spontaneous change takes place in its composition ; an incipient change in the casein being the cause of the conversion of 472 COMPOSITION OF MILK. the sugar into lactic acid ; and this coagulating the casein, by pre- cipitating it in small flakes. The same precipitation may be accom- plished at any time by the agency of various acids, especially the acetic, which does not act upon Albumen ; but Casein cannot be coagulated like albumen, by the influence of heat alone. The most complete coagulation of Casein is effected by the agency of the dried stomach of the calf, known as rennet; which exerts so powerful an influence, as to coagulate the casein of 1,800 times its weight of milk. It is thus that, as in the making of cheese, the curd is sepa- rated from the whey; the former consisting chiefly of the casein; whilst the latter contains a large proportion of the saline and saccha- rine matter, which entered into the original composition of the milk. These may be readily separated by evaporation. 833. The chief characters of Casein have been already stated (§ 176).—Its Oleaginous matter consists, like the fats in general, of the two substances, elaine and stearine; but it also contains another substance peculiar to it, which is termed butyrine. This last (to which the characteristic smell and taste of butter are due) is converted by saponification into three volatile acids, of strong animal odour, to which the names of butyric, capric, and caproic acids have been given. This change may be effected, at any period, by treating the butyrine with alkalies; but it may also take place by spontaneous decompo- sition, which is favoured by time and moderate warmth.—The Sugar of Milk is peculiar as containing nearly 12 per cent, of water; so that it may be considered as really a hydrate of sugar. It is nearly iden- tical in its composition with starch; and may, like it, be converted into true sugar by the agency of sulphuric acid. But it is chiefly re- markable for its proneness to conversion into lactic acid, under the influence of a ferment or decomposing azotized substance.—The Sa- line matter contained in Milk appears to be nearly identical with that of the blood; with a larger proportion, however, of the phosphates of lime and magnesia, which amount to 2 or 2^ parts in 1000. These are held in solution chiefly by the Casein, which has a remarkable power of combining with them. 834. Thus ordinary Milk contains the three classes of organic principles, which form the chief part of the food of animals,—namely, the albuminous, the saccharine, and the oleaginous; together with the mineral elements, which are required for the development and consoli- dation of the fabric of the infant. It would appear, however, that the combination of all these is not necessary; but rather has reference to the composition of the food, on which the animal is destined to be afterwards supported. Thus it has been lately shown that, in the Carnivora, the milk contains no sugar ; which principle is altogether wanting in the food of the adult. Amongst the different species of Herbivorous animals, the proportion of the several ingredients varies considerably ; and it is also liable to considerable variation in accord- ance with the nature of the food, the amount of exercise taken by the animal that affords it, and other circumstances. Thus in the milk of COMPOSITION AND PRODUCTION OF MILK. 473 the Cow, Goat, and Sheep, the average proportions of Casein, Butter, and Sugar are nearly the same one with another, each amounting to from 3 to 5 per cent. In the milk of the Ass and Mare, on the other hand, the proportion of Casein is under 2 per cent., the oleagi- nous constituents are scarcely traceable, whilst the sugar and allied substances rise to nearly 9 percent. In the Human female, the sac- charine and oleaginous elements are both present in large araount; whilst the Casein forms a moderate proportion.—The proportion of the saccharine and oleaginous elements appears to be considerably affected by the amount in which these are present in the food; and by the degree in which the quantity ingested is consumed by the respiratory process. Thus, a low external temperature, and out-door exercise, by increasing the production of carbonic acid from the lungs, occasion the consumption of the oleaginous and saccharine matters, which might otherwise pass into the milk, and thus diminish the amount of cream. On the other hand, exercise favours the secretion of casein ; wdiich would seem to show, that this ingredient is derived from the disintegration of the azotized tissues. Thus in Switzerland, the cattle which pasture in exposed situations, and which are obliged to use a great deal of muscular exertion, yield a very small quantity of butter, but an unusually large proportion of Cheese ; yet the same cattle, when stall-fed, give a large quantity of butter, and very little cheese. 835. The Milk first secreted after parturition, known as the Colos- trum, is very different from ordinary milk; and possesses a strongly purgative action, which is useful in clearing the bowels of the infant from the various secretions which have accumulated in them at birth, constituting the meconium. The Colostrum, when examined with the Microscope, is found to contain a multitude of large yellow granulated corpuscles; each of which seems composed of a number of small grains aggregated together. The colostric character is sometimes retained for some time after birth, and severely affects the health of the infant. This may happen without any peculiarity in the ordinary characters of the secretion, which has all the appearance of healthy milk ; but the Microscope at once detects the difference, by the presence of the colostric corpuscles. 836. The formation of this Secretion is influenced by the Nervous system, to a greater degree, perhaps, than that of any other. The process may go on continuously, to a slight degree, during the whole period of lactation; but it is only in animals that have special reser- voirs for the purpose, that any accumulation of the fluid can take place. In the Human female, as we have seen, these are so minute as to hold but a trifling quantity of milk ; and the greater part of the secretion is actually formed, whilst the child is at the breast. The irritation of the nipple produced by the act of suction, and the mental emotion connected with it, concur to produce an increased flow of blood into the gland, which is known to Nurses as the draught; and thus the secretion is for the time greatly augmented. The draught 474 INFLUENCE OF FEELINGS ON MAMMARY SECRETION. may be produced simply by the emotional state of mind, as by the thought of the child when absent; and the irritation of the nipple may alone occasion it; but the two influences usually act simulta- neously. The most remarkable examples of the influence of such stimuli on the Mammary secretion, are those in which milk has been produced by girls and old women, and even by men, in quantity suffi- cient for the support of an infant. The application of the child to the nipple in order to tranquilize it, the irritation produced by its efforts at suction, and the strong desire to furnish milk, seem in the first instance to occasion an augmented nutrition of the gland, so that it becomes fit for the performance of its function; and then to produce in it that state of functional activity, the result of which is the pro- duction of Milk. 837. It is not only in this way, that the Mammary secretion is in- fluenced by the condition of the mind ; for it is peculiarly liable to be affected as to quality, by the habitual state of the feelings, or even by their temporary excitement. Thus a fretful temper not only lessens the quantity of milk, but makes it thin and serous, and gives it an irritating quality; and the same effect will be produced for a time by a fit of anger. Under the influence of grief or anxiety, the secretion is either checked altogether, or it is diminished in amount, and dete- riorated in quality. The secretion is usually checked altogether by terror; and under the influence of violent passion, it may be so changed in its characters, as to produce the most injurious and even fatal con- sequences to the infant. So many instances are now on record, in which children, that have been suckled within a few minutes after the mothers have been in a state of violent rage or terror, have died sud- denly in convulsive attacks, that the occurrence can scarcely be set down as a mere coincidence ; and certain as we are of the deleterious effects of less severe emotions upon the properties of the milk, it does not seem unlikely that, in these cases, the bland nutritious fluid should be converted into a poison of rapid and deadly operation. 838. Of the quantity of Milk ordinarily secreted by a good Nurse, it is impossible to form any definite idea; as the amount which can be artificially drawn, affords no criterion of that which is ordinarily secreted at the time of the draught. The quantity which can be squeezed from either breast at any one time, and which, therefore, must have been contained in its tubes and reservoirs, is about two ounces. The amount secreted will depend upon several circum- stances; such as the nature and amount of the ingesta; the state of bodily health ; and the condition of the mind. An adequate but not excessive amount of nutritious food, in which the farinaceous, olea- ginous, and albuminous principles are duly blended; a vigorous but not plethoric constitution, regular habits, and moderate exercise ; together with a cheerful and tranquil temper; altogether produce the most beneficial influence upon the secretion. It is seldom that stimu- lating liquors, which are so commonly indulged in, are anything but prejudicial; but the unmeasured condemnation of them, in which ELEMENTS OF MILK PRE-EXISTENT IN BLOOD. 475 some writers have indulged, is certainly injudicious; as experience amply demonstrates the improvement in the condition both of mother and infant, which occasionally results from the moderate employment of them.—In the administration of medicines to the mother, it is very desirable that the tendency of soluble saline substances to pass into the milk, and thus to affect the child, should be borne in mind. The vegetable substances used in medicine seem to have much less disposition to pass off by this secretion; and they are consequently to be preferred during lactation. 839. From the close correspondence which exists between the ele- ments of the Milk and those of the Blood, it is evident that we cannot expect to trace the existence of the former, as such, in the circulating current. It is interesting, however, to remark, that a preparation ap- pears to be taking place in the laboratory of the system, for the pro- duction of this secretion, long before the period of parturition. The Urine of pregnant women almost invariably contains a peculiar sub- stance termed /destine, which is nearly related to casein, and which disappears from the urine as soon as lactation has fully commenced. It wrould seem, therefore, that a compound of this nature is in course of preparation during pregnancy; and that it is eliminated by the kidney until the Mammary Gland is prepared for the active perform ance of its function.—That the kidney may relieve the system from the accumulation of other constituents of the mammary secretion, ap- pears from a case recently put on record ; in which the urine of a par- turient female, who did not suckle her infant, was found to contain a considerable quantity of butyric acid, during several days. There can be no doubt that in ordinary states of the system, this secretion cannot be required for the depuration of the blood ; since it does not occur in the male at all, and is present in the female at particular times only. But these facts afford ground to believe that, when the process is going on, certain products are generated in the system, which are not found there at other times. And it is quite certain that the sud- den checking of the secretion, or the re-absorption of the fluid already poured out, occasioning an accumulation of these substances in the cir- culating current, may give rise to very injurious consequences. Some very curious instances are on record, in which a transference of the secreting power to some other surface has taken place under such cir- cumstances ; so as to relieve the system from the accumulation in question. 476 GENERAL FUNCTIONS OF NERVOUS SYSTEM. CHAPTER XII. OF THE NERVOUS SYSTEM. 1. General View of the operations, of which the Nervous System is the instrument. 840. We have now considered the entire series of those operations, which make up the vegetative or organic life of the Animal; including the functions by which the germ is prepared, by which it is nourished until it can be left to its own powers, by which its continued deve- lopment is effected until the fabric characteristic of the adult has been built up, and by which the normal constitution is maintained through a lengthened period,—so long as the necessary materials are supplied, and no check or hindrance is interposed, by external influences, to that regular sequence of change, on which the continuance of its powers depends. In this survey it will have been perceived, that the essential parts of these operations are, in Animals as in Plants, completely independent of the influence of that, which constitutes the peculiar endowment of Animals; namely, the Nervous System. a. The Reduction of the food in the Stomach, by the solvent powTer of the gastric fluid, is a purely chemical operation, with which the Nervous System has nothing whatever to do, excepting that it per- haps accelerates the process, by stimulating the Muscular coat of the stomach to that peculiar series of contractions, which keeps the con- tents of the cavity in continual movement, and favours the action of the solvent upon it. b. With the process of Absorption, by which the nutritive materials, with other substances, are introduced into the vessels, the Nervous System has nothing to do ; this being a purely vegetative operation, partly dependent upon the simple physical conditions which produce Endosmose, and partly on a process of cell-growth. c. The Assimilation of the new material, effected, as we have seen reason to believe, by another set of independent cells, can receive but little influence from the Nervous System, and is obviously capable of taking place without its aid. d. The Circulation of the Blood, again, though dependent in part upon the impulsive power of a Muscular organ, the Heart, is not on that account brought into closer dependence upon the Nervous System; for we have seen that the contractions of the heart result from its own inherent powers, so as to continue after it has been completely de- tached from the body; and that the capillary power, which is the chief agent in the movement of the blood in the lower animals, and which exerts an important subsidiary action in the higher, is the result of GENERAL FUNCTIONS OF NERVOUS SYSTEM. 477 the exercise of certain affinities, between the blood and the surround- ing tissues, in which the Nervous System can have no immediate concern. e. The act of Nutrition, in which every tissue draws from the cir- culating blood the materials for its own continued growth and deve- lopment, and by which it incorporates these with its own substance, is but a continuance of the same kind of operation, as that which takes place in the early development of the embryo, long anteriorly to the first appearance of the nervous system,—namely, a process of cell- development and metamorphosis, which must be, from its very nature, independent of Nervous agency. / The same may be said of the Secreting operation in general; for this essentially consists of the separation of certain products from the blood, by cells situated upon free surfaces; which thus remove those products from the interior of the fabric. g. And the interchange of oxygen and carbonic acid, which takes place between the atmosphere and the venous blood, when brought into mutual relation in the lungs, and which is the essential part of the function of Respiration, is an operation of a merely physical character, with which the Nervous system can have no direct concern. h. Finally, the development of the reproductive germs in the one sex, and of the ova within which these are to be evolved in the other, the subsequent fertilization of the latter by the former, and the changes consequent upon that act, together making up the function of Repro- duction, may be all regarded as modifications of the ordinary Nutri- tive processes; and are effected, like these, by the inherent powers of the parts concerned in them, at the expense of the materials sup- plied by the blood, without any direct dependence upon the Nervous system. 841. Still, although the various processes, which make up the essen- tial part of the nutritive operations, in Animals as in Plants, are no more dependent on any peculiar influence derived frora a Nervous system, in the former, than they are in the latter, it must be evident, from the details already given, that there must be in Animals various accessory changes, which are requisite for the continuance of the former, and which can only be effected by the peculiar powers with which Animals are endowed.—Thus, to commence with Digestion ;— this preliminary process, which the nature of the food of the plant renders unnecessary for its maintenance, can only be accomplished by the introduction of the food into a cavity or sac, in which it may be submitted to the action of the solvent fluid. The operation of grasp- ing and swallowing the food, wherever it is performed, is accomplished through the agency of the Nervous system; and if it be checked by the loss of Nervous power, the Digestive process must cease for want of material.—So, again, although interchange of gaseous ingredients between the atmosphere and the circulating fluid may take place with sufficient energy in Plants and the lower Animals, through the mere exposure of the general surface to the atmosphere, yet we find that, 478 SHARE OF NERVOUS SYSTEM IN ORGANIC FUNCTIONS. in all the higher Animals, certain movements are requisite, for the con- tinual renewal of the air or water which are in contact with one side of the respiratory surface, and of the blood wThich is in relation with the other: for the direction of which movements, a Nervous system is requisite.—In the excretory processes, moreover, the removal of the effete matters from the body can only be accomplished, in the higher Animals, by certain combined movements; the object of which is to take up the products that are separated by the action of the proper secreting cells, and to carry them to the exterior of the body, there to be set free ; and these combined movements can only be effected by the agency of the Nervous system.—Lastly, in the act of Repro- duction, the arrangement of the sexual organs in Animals, requires that a certain set of movements should be adapted to set free the germ from the body of the male, and to convey it to the ovule of the fe- male ; and further, that the ovum should be expelled from the body of the latter, in a state of more or less advanced development. For these movements a special arrangement is made, in the construction of the Nervous system, and in the application of its peculiar powers. 842. Thus we see that, although the Organic functions of the Ani- mal are essentially independent of the Nervous System, this system affords the conditions which are requisite for their continued main- tenance ; being the instrument by which the muscles are called into action for the performance of the various combined actions that con- stitute the mechanism (so to speak), by which the Vegetative part of the fabric is combined with the Animal portion of the organism. We are not to suppose, however, that every movement which takes place in the Animal body is dependent upon the Nervous System ; for we have seen that the Muscular tissue may be employed to perform con- tractions excited by stimuli applied to itself, and that it may thus execute a set of movements in which the nervous system has no di- rect participation. And it is desirable that the Student should ob- serve, that these are, in all instances, those most directly connected with the Vegetative functions, and, at the same time, those of the simplest and most straightforward character.—Thus, the peristaltic movement, by which the alimentary and fecal matters are propelled along the Intestinal tube, results from the direct excitement of the contractility of its muscular walls, and is entirely independent of Nervous agency; and this movement is accomplished by the succes- sive contraction of the different fasciculi surrounding the tube, which take up (as it were) each other's action (§ 352). So, again, the suc- cessive contractions and dilatations of the cavities of the Heart, which perform so important a part in the Circulation of the blood, are the result of the properties inherent in that organ ; the muscular fibres of which are excited to a peculiar rhythmical and consentaneous con- traction, by the flow of blood into the cavities when dilating. More- over, in the Excretory ducts of various glands, we find a Muscular coat, by which the fluids secreted in the glands, are propelled towards GENERAL FUNCTIONS OF NERVOUS SYSTEM. 479 their outlet on the exterior of the body, or on one of its free internal surfaces. 843. In these instances, then, we observe that the simple Contrac- tility of Muscular structure, excited by direct stimulation, is applied to effect the movements most closely connected with the Organic functions. With the processes, therefore, which take place in the penetralia of the system, the Nervous System has no direct concern. Its office is to guard the portals for entrance and exit; and to fill those chambers, which admit the new materials from the external world ; or to empty the receptacles, which collect from the interior of the system the effete matters that are to be cast out from it. And we find that, for these offices, the Nervous system is employed in its very simplest mode of operation ;—that which does not involve Sensation, Intelligence, Will, or even Instinct (in the proper sense of that term), but which may take place independently of all consciousness,—by the simple reflexion of an impression, conveyed to a ganglionic centre by one set of fibres proceeding towards it from the circumference along another set which passes from it to the muscles, and calls them into operation (§394). This reflex function, therefore, is the simplest application of the Nervous System in the Animal body. We shall presently see reason to believe, that a very large proportion of the movements of many of the lower animals are of this reflex character ; and that they are not necessarily accompanied by sensation, although this may usually be aroused by the same cause which produces them. As we rise, however, in the scale of Animal existence, we find the reflex movements forming a smaller and smaller proportion of the whole ; until, in Man, they constitute so limited a part of the entire series of movements of which the Nervous system is the agent, that their very existence has been overlooked. 844. But the main purpose of the Nervous System is to serve as the instrument of the Psychical* powers, which are the distinguishing attribute of the Animal. It has been already pointed out, that the pos- session of Consciousness (or of the capability of receiving sensations), and the power of executing Spontaneous Movements (that is, move- ments which are not immediately dependent upon external stimuli), con- stitute the essential features in which the Animal differs frora the Plant. All the other differences in structure, that respectively characterize these two classes of living beings, are subordinate to this one leading distinction,—the presence of a Nervous system, and of its peculiar attributes in the one,—and its absence in the other. Now when we attempt to analyze these peculiar attributes, we may resolve them, like the properties of the material body, into different groups. We find that the first excitement of all mental changes, whether these involve the action of the feelings or of the reason, depends upon sen- sations ; which are produced by impressions made upon the nerves of * This term, derived from the Greek 4uxfl> *s used t0 designate the sensorial and mental endowments of Animals, in the most comprehensive acceptation of those terms. 480 DEPENDENCE OF MENTAL ACTIONS UPON SENSATION. certain parts of the body, and are conveyed by these to a particular ganglionic centre, which is termed the sensorium,—being the part in which Sensation, or the capability of feeling external impressions, especially resides. 845. Now there are numerous actions, especially among the lower Animals, which seem to be as far removed from the influence of the Will, and as little directed by Intelligence, as the Reflex movements themselves ; but which, nevertheless, depend upon sensation for their excitement. The sensation may immediately direct the movement; or it may excite an emotion or desire, which, without any calculation of consequences, any intentional adaptation of means to ends, any exertion of the reason, or any employment of a discriminating Will, may produce an action, or train of actions, as directly and obviously -adapted to the well-being of the individual, as we have seen those of the reflex character to be. It is impossible to say, in regard to many of the actions of the lower animals, to what extent they involve feel- ings or emotions, at all analogous to those which we experience ; and it would seem better to apply the generic term Consensual to those in which the Sensation excites the motor action, either immediately, or through the agency of an indiscriminating Desire excited by the sen- sation. This class will include all the purely Instinctive actions of the lower Animals; which make up, with the reflex, nearly the whole of the Animal functions in many tribes; but which are found to be gradually brought under subordination to the Intelligence and Will, as we rise towards Man, in whom those faculties are most strongly developed, so as to keep the Consensual as well as the Reflex actions quite in subordination to the more elevated purposes of his existence. 846. There are many sensations, however, which do not thus im- mediately give rise to muscular movements; their operation being rather that of stimulating to action the Intellectual powers. There can be little doubt that all Mental processes are dependent, in the first instance, upon Sensations; which serve to the Mind the same kind of purpose, that food and air fulfil in the economy of the body. If we could imagine a being to come into the world with its mental faculties fully prepared for action, but destitute of any power of receiving sen- sations, these faculties would never be aroused from the condition in which they are in profound sleep; and the being must remain in a state of complete unconsciousness, because there is nothing of which it can be made conscious, no kind of idea which can be aroused within it. But after the mind has once been in active operation, the destruction of all future power of receiving sensations would not reduce it again to the inactive condition. For sensations are so stored up in the mind, by the power of Memory, that they may give rise to ideas at any future time; and thus the mind may feed (as it were) upon the past. Now the ideas which are excited by sensations, and which are coloured by the state of Feeling which accompanies them, become the subjects of Reasoning processes more or less complex, sometimes of the utmost brevity and simplicity, sometimes of the most CLASSIFICATION OF ACTIONS OF NERVOUS SYSTEM. 481 refined and intricate nature. These reasoning processes, when they result in a determination to execute a particular movement, execute that movement by an act of Volition ; the peculiar character of which is, that it is the expression of a definite purpose, of a designed adapta- tion of means to ends, on the part of the individual performing it; instead of being the result of mere blind indiscriminating impulse, which seems to be the main-spring of the instinctive operations. It is in Man, that we find the highest development of the reasoning faculties ; but it is quite absurd to limit them to him, as some have done ; since no impartial observer can doubt, that many of the lower animals can execute reasoning processes, as complete in their way as those of Man, though much more limited in their scope. 847. Thus, then, we have to consider the Nervous system under three heads ;—first, as the instrument of the Reflex actions;—second, as the instrument of the Consensual and Instinctive actions;—third, as the instrument of the Intellectual processes, and of Voluntary movements. Now there is good reason to believe, that to each of these groups of actions a particular portion of the Nervous Centres, with its afferent and efferent nerves, may be assigned ;—one ganglion, or collection of ganglia, being the instrument of the Reflex actions; another of the Consensual and Instinctive operations; and a third of the Intellectual powers, and of the Voluntary movements to which they give rise. In order that the relations of these subdivisions may be better understood, it will be desirable to take a brief survey of the comparative structure of the Nervous system in the principal groups of Animals; and to inquire what actions may be justly attributed to its several parts in each instance ; commencing with those in which the structure is the simplest, and the variety of actions the smallest; and passing on gradually to those, in which the structure is increased in complexity by the addition of new and distinct parts, and in which the actions present a corresponding variety. 2. Comparative Structure and Actions of the Nervous System. 848. From what has been already said (§ 373-9) of the characters of the two elementary forms of the Nervous Tissue, it is evident that no Nervous System can exist, in which both these forms should not be present. We look, therefore, for ganglia, composed of the vesi- cular nervous substance, and serving as the centres of nervous power; and for cords or trunks, composed of the tubular substance, and serving to communicate between the ganglia and the parts with which they are to be functionally connected. Now it is quite certain that, at present, no such Nervous apparatus can be detected in many of the lowest Animals ; and some Physiologists have had recourse to the supposition of their possessing a "diffused" nervous system,— that is, of their possessing nervous particles, in a separate form, in- corporated as it were with their tissues. But we have seen, that each tissue possesses its own properties, and can perform its own actions, 31 482 NERVOUS SYSTEM OF LOWEST ANIMALS. independently of the rest;—that even the contractility of Muscular fibre is by no means dependent upon the Nervous system, though usually called into play through its means ;—and that the simplest office of a Nervous System is to produce a muscular movement in respondence to a certain impression ; which action requires that it should have an internuncial or communicating power, only to be ex- ercised (so far as we at present know) by continuous fibres. The apparent absence of a Nervous system is doubtless to be attributed, in many instances, to the general softness of the tissues of the body, which prevents it from being clearly made out among them. And it is to be remembered, that, on the principles already stated, we should expect to find it bearing a much smaller proportion to the entire structure, in the lowest Animals, whose functions are chiefly Vegeta- tive,—than in the highest, in which the vegetative functions seem destined merely for the development of the Nervous and Muscular systems, and for the sustenance of their powers. 849. Among the Radiated classes, the parts of whose bodies are arranged in a circular manner around the mouth, and repeat each other more or less precisely, the Nervous system presents a corre- sponding form. In the Star-fish, for example, which is one of the highest of these animals, it forms a ring, which surrounds the mouth ; this ring consists of nervous cords, which form communications between the several ganglia, one of which is placed at the base of each ray. The number of these ganglia corresponds with that of the rays or arms ; being five in the common Star-fish; and from nine to fifteen, in the species possessing those several numbers of members. The ganglia appear to be all similar to one another in function, as they are in the distribution of their branches; every one of them sending a large trunk along its own ray, and two small filaments to the organs in the central disk. The rays being all so similar in structure, as to be exact repetitions of each other, it would appear that none of the ganglia can have any controlling power over the rest. All the rays (in certain species) have at their extremities what seem to be very imperfect eyes ; and so far as these can aid in direct- ing the movements of the animal, it is obvious that they will do so towards all sides alike. Hence there is no one part, which corre- sponds to the head of higher animals ; and the ganglia of the nervous system, like the parts they supply, are but repetitions of one another, and are capable of acting quite independently. Each would perform its own individual functions if separated from the rest; but, in the entire animal, their actions are all connected with others by the circular cord, which passes from every one of the five ganglia to those on either side of it. We shall find that, in Articulated and Vertebrated animals, there is a similar repetition of corresponding ganglia, on the two sides of the median plane of the body; and that these are con- nected by transverse bands, analogous in function to the circular cord of the Star-fish. Moreover, we shall see a like repetition of ganglia, almost or precisely similar in function, in passing from one extremity NERVOUS SYSTEM OF RADIATA AND MOLLUSCA. 483 of these animals to the other ; and these ganglia are connected by lon- gitudinal cords, whose function is in like manner commissural.—From the best judgment we can form of the actions of the Star-fish, by com- paring them with the corresponding actions of higher animals, we may fairly regard the greater number of them as simply reflex ; being performed in direct respondence to external stimuli, the impression made by which is propagated to one or more of the ganglia, and ex- cites in them a motor impulse. How far the movements of these aninials are indicative of sensation, we have not the power of deter- mining; but it may be safely affirmed, that they afford very little indication, if any, of the exercise of reasoning faculties, or of volun- tary power. 850. Perhaps the simplest form of a Nervous system is that pre- sented by certain of the lower Mollusca; for here, the body not pos- sessing any repetition of similar parts, the nervous system is destitute of that multiplication of ganglia which we see in the Star-fish ; whilst the limited nature of the animal powers involves a corresponding sim- plicity in the integral parts of their instrument. The animals, to which reference is here made, form the class Tunicata; which is inter- mediate, in many respects, between the ordinary Mollusks and the Zoophytes. They consist essentially of an external membranous bag or tunic ; within which is a muscular envelop ; and within this, again, a respiratory sac, which may be considered as the dilated pharynx of the animal. At the bottom of this last, is the entrance to the stomach; which, with the other viscera, lies at the lower end of the muscular sac. The external envelops have two orifices ; a mouth, to admit water into the pharyngeal sac; and an anal orifice, for the expulsion of the water which has served for respiration, and of that which has passed through the alimentary canal, together with the fecal matter, the ova, &c. A current of water is continually being drawn into the pharyngeal sac, by the action of the cilia that line it; and of this, a part is driven into the stomach, conveying to it the necessary supply of aliment in a very finely divided state; whilst a part is destined merely for the aeration of the circulating fluid, and is transmitted more directly to the anal orifice, after having served that purpose. These animals are for the most part fixed to one spot, during all but the earliest period of their existence; and they give but little external manifestation of life, beyond the continual entrance and exit of the currents already adverted to, which, being effected by ciliary action, is altogether independent of the nervous system (§ 224). When any substance is drawn in by the current, however, the entrance of which would be injurious, it excites a general contraction of the mantle or muscular envelop; and this causes a jet of water to issue from one or both orifices, which carries the offending body to a distance. And, in the same manner, if the exterior of the body be touched, the mantle suddenly and violently contracts, and expels the contents of the sac. 851. These are the only actions, so far as we know, which the 484 NERVOUS SYSTEM OF MOLLUSCA. Nervous system of these animals is destined to perform. They do not exhibit the least trace of eyes, or of other organs of special sense; and the only parts that appear peculiarly sensitive, are the small tentacula or feelers, that guard the oral orifice. Between the two apertures in the mantle, we find a solitary ganglion, which receives branches from both orifices, and sends others over the muscular sac. This, so far as we know at present, constitutes the whole nervous system of the animal; and it is fully sufficient to account for the movements which have been described. For the impression produced by the contact of any hard substance with the tentacula, or with the general surface of the mantle, being conveyed by the afferent fibres of this ganglion, will excite in it a reflex motor impulse; which, being transmitted to the muscular fibres of the contractile sac, as well as to those circular bands that surround the orifices and act as sphincters, will produce the movements in question. 852. In the Conchifera, or Mollusks inhabiting bivalve shells, there are invariably two ganglia, having different functions. The larger of these (Plate II, Fig. 1, c), corresponding to the single ganglion of the Tunicata, is situated towards the posterior end of the body (that is, the end most distant from the mouth), in the neighbourhood of the posterior muscle that draws the valves together; and its branches are distributed to that muscle, to the mantle, to the gills (d, d), and to the siphons (e, e), by which the water is introduced and carried off. But we find another ganglion, or rather pair of ganglia (a, a), situated near the front of the body, either upon the oesophagus, or at its sides; these ganglia are connected with the very sensitive tentacula, which guard the mouth ; and they may be regarded as presenting the first approach, both in position and functions, to the brain of higher ani- mals. In the Oyster, and others of the lower Conchifera, which have no foot, these are the only principal ganglia; but in those hav- ing a foot,—which is a muscular tongue-like organ,—we find an addi- tional ganglion (b) connected with it. This is the case in the Solen, or animal of the Razor-shell ; whose foot is a very powerful boring- instrument, enabling it to penetrate deeply into the sand.—Here, then, we have three distinct kinds of ganglionic centres; every one of which may be doubled, or repeated on the two sides of the body. First, the cephalic ganglia, a, a, which are probably the sole instruments of sen- sation and of the consensual movements; as well as of whatever volun- tary power the animal may possess : these are almost invariably double, being connected together by a transverse band, which arches over the oesophagus. Second, the pedal ganglion, b, which is usually single, in conformity with the single character of the organ it supplies; but in one very rare Bivalve Mollusk, the foot is double, and the pedal ganglion is double also. Third, the respiratory ganglion, c, which frequently presents a form that indicates a partial division into two halves, corresponding with the repetition of the organs it supplies on the two sides of the body. Besides these principal centres, we meet with numerous smaller ones upon the nervous cords, (//, and NERVOUS SYSTEM OF MOLLUSCA. 485 g, g,) which proceed from them to the different parts of the general muscular envelop or mantle. 853. Now it will be observed, that the two cephalic ganglia a, a, are connected with the pedal ganglion b, by means of a pair of trunks proceeding from the former to the latter; and that they are, in like manner, separately connected with the respiratory or branchial gan- glion, c. It is found, upon careful dissection, that these cords do not serve merely to bring the ganglia into relation ; but that a part of them pass through the ganglion into the trunks proceeding from it. Thus of the nerves which supply the large fleshy foot, and which appear to proceed from the pedal ganglion b, a part are undoubtedly connected with that ganglion alone, coming into relation with its vesicular substance ; but a part also pass on to the cephalic ganglia, by the connecting trunks,—'so that these, rather than the pedal gan- glion, constitute their centre. The same may be said of the nerves proceeding from the branchial ganglion: a portion of them having their centre in the vesicular matter of that ganglion ; whilst another portion has no relation to it whatever (beyond that of proximity), but passes through or over it, to become connected with the cephalic ganglia. There is good reason to believe, that the pedal and branchial ganglia minister to the purely reflex actions of the organs they re- spectively supply ; and that they would serve this purpose as well, if altogether cut off from connection with the cephalic ganglia: whilst the latter, being the instruments of the actions which are called forth by sensation (whether these be of a consensual or of a voluntary na- ture), exert a general control and direction over the movements of the animal. 854. It is difficult to make satisfactory experiments upon this sub- ject in these animals ; their movements being for the most part slow and feeble, and their nervous system not readily accessible ; and our idea of the respective functions of their ganglia is chiefly founded upon the distribution of their nerves, and upon the analogous opera- tions of the ganglia that correspond to them in other animals. In ascending through the series of the Mollusca, we find the Nervous sys- tem increasing in complexity, in accordance with the general organi- zation of the body: the addition of new organs of special sensation, and of new parts to be moved by muscles, involving the addition of new ganglionic centres, whose functions are especially adapted to these purposes. But we find no other multiplication of similar cen- tres than a doubling on the two sides of the body; excepting in a few cases, where the organs they supply are correspondingly multi- plied. We have a very characteristic example of this in the arms of the Cuttle-fish, which are furnished with great numbers of contractile suckers, every one possessing a ganglion of its own. Here we can trace very clearly the distinction between the reflex actions of each individual sucker, depending upon the powers of its own ganglion; and the consensual or voluntary movement, which results from its connection with the cephalic ganglia. The nervous trunk, which 486 NERVOUS SYSTEM OF MOLLUSCA. proceeds to each arm, may be distinctly divided into two tracts; in one of which there is nothing but fibrous structure, forming a direct communication between the suckers and the cephalic ganglia; whilst in the other are contained the ganglia, which peculiarly appertain to the suckers, and which are connected with them by distinct filaments: so that each sucker has a separate relation with a ganglion of its own, whilst all are alike connected with the cephalic ganglia, and are placed under their control. We see the results of this arrangement, in the modes in which the contractile power of the suckers may be called into operation. When the animal embraces any substance with its arm (being directed to this action by its sight or other sensa- tion) it can bring all the suckers simultaneously to bear upon it; evi- dently by a voluntary or instinctive impulse, transmitted along the motor cords, that proceed from the cephalic ganglia to the suckers. On the other hand, any individual sucker may be made to contract and attach itself, by placing a substance in contact with it alone; and this action will take place equally well, when the arm is separated frora the body, or even in a small piece of the arm when recently severed from the rest,—thus proving that, when it is directly excited by an impression made upon itself, it is a reflex act, quite independ- ent of the cephalic ganglia, not involving sensation, and taking place through the medium of its own ganglion alone. 855. In the Molluscous classes, generally speaking, the Nervous system bears but a small proportion to the whole mass of the body; and the part of it which ministers to the general movements of the fabric, is often small in proportion to those which serve some special purpose, such as the actions of respiration. This is what we should expect from the general inertness of their character, and from the small amount of muscular structure which they possess. On the other hand, in the Articulated classes, in which the locomotive apparatus is highly developed, and its actions of the most energetic kind, we find the Nervous system almost entirely subservient to this function. In its usual form, it consists of a chain of ganglia, connected by a double cord; commencing in the head, and passing backwards through the body (Plate II., Fig 2). The ganglia, though they usually appear single, are really double ; being composed of two equal halves, some- times closely united on the median line, but occasionally remaining separate, like the cephalic ganglia of the Solen (Fig. 1, a, a), and being united together by a transverse commissural trunk. In like manner, the longitudinal cord, though really double (as seen in the upper part of Fig. 2), often appears to be single, in consequence of the close approximation of its lateral halves (as in the lower part of Fig. 2). In general we find a ganglion in each segment; giving off nerves to the muscles of the legs, as in Insects, Centipedes, &c. ; or to the muscles that move the rings of the body, where no extremities are developed, as in the leech, worm, &c. In the lower Vermiform (or worm-like) tribes, especially in the marine species, the number of segments is frequently very great, amounting even to several hundreds; NERVOUS SYSTEM OF AR-TICULATA. 487 and the number of ganglia follows the same proportion. Whatever be their degree of multiplication, they seem but repetitions of one another; the functions of each segment being the same with those of the rest. The cephalic ganglia, however, are always larger and more important; they are connected with the organs of special sense; and they evidently possess a power of directing and controlling the move- ments of the entire body; whilst the power of each ganglion of the trunk is confined to its own segment.—The longitudinal ganglionic cord of Articulata occupies a position, which seems at first sight altogether different from that of the nervous system of Vertebrated animals; being found in the neighbourhood of the ventral or inferior surface of their bodies; instead of lying just beneath their dorsal or upper surface. There is reason, however, for regarding the whole of the body of these animals as having an inverted position; so that they maybe considered as really crawling upon their backs. On this view, their longitudinal nervous tract corresponds with the spinal cord of Vertebrata in position, as we shall find that it does in function. 856. We shall draw our chief illustrations of the structure of the nervous system in the Articulated series, from the class of Insects; in which it has been particularly examined. In these animals, the num- ber of segments never exceeds twelve (exclusive of the head), either in their larva, pupa, or imago states ; and the total number of pairs of ganglia, therefore, never exceeds thirteen, including the cephalic ganglia. These, in the larva, are nearly equal in size, one to another (Plate II., Fig. 2, a, and 1—12); the functions of the different seg- ments of the body being almost uniform; and the development of the organs of special sense not being such, as to involve any considerable predominance in the size of the cephalic ganglia. We observe, at the anterior extremity, the pair of cephalic ganglia (a); from which proceeds, on each side, a cord of communication to the first ganglion (1) of the trunk. This double cord, with the ganglia above and below, thus forms a ring, which embraces the oesophagus; the cephalic ganglia being situated on the upper side of it, whilst the ganglionic column of the trunk lies beneath the alimentary canal along its whole length. In the Sphinx Hgustri, or Privit Hawk-moth, the nervous system of whose larva is here represented, the last two segments of the body are drawn together, as it were, into one; and instead of distinct 11th and 12th ganglia, we find but a single mass, nearly double the size of the rest, and obviously formed of the elements that would have otherwise gone to form the two. 857. When the structure of the chain of ganglia is more particularly inquired into, it is found to consist of two distinct tracts; one of which is coraposed of nervous fibres only, and passes backwards from the cephalic ganglia, over the surface of all the ganglia of the trunk, giving off branches to the nerves that proceed from them; whilst the other includes the ganglia themselves. Hence, as in the Mollusca, every part of the body has two sets of nervous connections ; one with the cephalic ganglia; and the other with the ganglion of its own seg-. 4SS REFLEX ACTIONS OF ARTICULATA. Fig. 141. Portion of the ganglionic tract of Poly- desmus maculalus;—b, inter-ganglionic cord; c, anterior nerves; d, posterior nerves;'/, k, fibres of reflex action ; g-, h, commissural fibres; t, longitudinal fibres, softened and enlarged, as they pass through ganglionic matter. ment. Impressions made upon the afferent fibres, which proceed from any part of the body to the cephalic ganglia, become sensations when conveyed to the latter; whilst, in respondence to these, the influence of the instincts, or of the will, operating through the cephalic ganglia, harmonizes and directs the general movements of the body, by means of the efferent nerves proceeding from them. For the reflex operations, on the other hand, the ganglia of the ventral cord are sufficient; each one ministering to the actions of its own segment, and, to a certain extent also, to those of other segments. It has been ascertained by the careful dissections of Mr. Newport, that of the fibres con- stituting the roots, by which the nerves are implanted in the ganglia, some pass into the vesicular matter of the gan- glion, and, after coming into relation with its vesicular substance, pass out again on the same side (Fig. 141,/ k); whilst a second set, after traversing the vesicular matter, passes out by the trunks proceeding from the oppo- site side of the same ganglion ; and a third set runs along the portion of the cord which connects the ganglia of different segments, and enters the nervous trunks that issue from them, at a distance of one or more ganglia above or below. Thus it appears, that an impression conveyed by an afferent fibre to any ganglion, may excite a motion in the muscles of the same side of its own segment; or in those of the opposite side; or in those of segments at a greater or less distance, according to the point at which the efferent fibres leave the cord. 858. The general conformation of Articulated animals, and the ar- rangement of the parts of their nervous system, render them peculiarly favourable subjects for the study of the reflex actions ; some of the principal phenomena of which will now be described. If the head of a Centipede be cut off, whilst it is in motion, the body will continue to move onwards by the action of the legs; and the same will take place in the separate parts, if the body be divided into several distinct por- tions. After these actions have come to an end, they maybe excited again by irritating any part of the nervous centres, or the cut extre- mity of the nervous cord. The body is moved forwards by the regu- lar and successive action of the legs, as in the natural state; but its movements are always forwards, never backwards, and are only di- rected to one side, when the forward movement is checked by an interposed obstacle. Hence, although they might seem to indicate consciousness and a guiding will, they do not so in reality; for they are carried on, as it were, mechanically: and show no direction of ob- ject, no avoidance of danger. If the body be opposed in its progress REFLEX AND CONSENSUAL ACTIONS OF INSECTS. 489 by an obstacle of not more than half its own height, it mounts over it, and moves directly onwards, as in its natural state; but if the ob- stacle be equal to its own height, its progress is arrested, and the cut extremity of the body remains forced up against the opposing sub- stance,—the legs still continuing to move.—If, again, the nervous cord of a Centipede be divided in the middle of the trunk, so that the hinder legs are cut off from connection with the cephalic ganglia, they will continue to move, but not in harmony with those of the fore part of the body; being completely paralyzed, as far as the animal's con- trolling power is concerned; though still capable of performing reflex movements, by the influence of their own ganglia, which may thus continue to propel the body, in opposition to the determiriation of the animal itself.—The case is still more remarkable, when the nervous cord is not merely divided, but a portion of it is entirely removed from the middle of the trunk; for the anterior legs still remain obe- dient to the animal's control; the legs of the segments, from which the nervous cord has been removed, are altogether motionless; whilst those of the posterior segments continue to act, through the reflex powers of their own ganglia, in a manner which shows that the animal has no powrer of checking or directing them. 859. The stimulus to the reflex movements of the legs, in the fore- going cases, appears to be given by the contact of the extremities with the solid surface on which they rest. In other cases, the appropriate impression can only be made by the contact of liquid; thus a Dytiscus (a kind of water-beetle), having had its cephalic ganglia removed, remained motionless, so long as it rested upon adry surface ; but when cast into water, it executed the usual swimming motions with great energy and rapidity, striking all its comrades to one side by its vio- lence, and persisting in these for more than half an hour. Other movements, again, may be excited through the respiratory surface. Thus, if the head of a Centipede be cut off, and while it remains at rest, some irritating vapour (such as that of ammonia or muriatic acid) be caused to enter the air tubes on one side of the trunk, the body will be immediately bent in the opposite direction, so as to withdraw itself as much as possible from the influence of the vapour; if the same irritation be then applied on the other side, the reverse movement will take place; and the body may be caused to bend in two or three different curves, by bringing the irritating vapour into the neighbourhood of different parts of either side. This movement is evidently a reflex one, and serves to withdraw the entrances of the air- tubes from the source of irritation; in the same manner as the acts of coughing and sneezing in the higher animals cause the expulsion, from the air-passages, of solid, liquid or gaseous irritating matters, which may have found their way into them. 860. From these and similar facts it appears, that the ordinary movements of the legs and wings of Articulated animals are of a reflex nature, and may be effected solely through the ganglia with which these organs are severally connected; whilst in the perfect being, they 490 CEPHALIC AND RESPIRATORY GANGLIA OF INSECTS. are harmonized, controlled, and directed by its instinct or its will, which act through the cephalic ganglia and the nerves proceeding from them. There is strong reason to believe, that the operations to which these ganglia are subservient, are almost entirely of a consensual nature; being immediately prompted by sensations, chiefly those of sight, and seldom involving any processes of a truly rational character. When we attentively consider the habits of these animals, we find that their actions, though evidently directed to the attainment of cer- tain ends, are very far frora being of the same spontaneous nature, or from possessing the same designed adaptation of means to ends, as those performed by ourselves, or by the more intelligent Vertebrata, under like circumstances. We judge of this by their unvarying cha- racter,—the different individuals of the same species executing pre- cisely the same movements, when the circumstances are the same ; and by the very elaborate nature of the mental operations, which would be required, in many instances, to arrive at the same results by an effort of reason. Of such we cannot have a more remarkable example, than is to be found in the operations of Bees, Wasps, and other social In- sects; which construct habitations for themselves, upon a plan which the most enlightened human intelligence, working according to the most refined geometrical principles, could not surpass; but which yet do so without education communicated by their parents, or progressive attempts of their own, and with no trace of hesitation, confusion, or interruption,—the different individuals of a community all labouring effectively for one common purpose, because their instinctive or con- sensual impulses are the same. 861. It is interesting to remark that, in the change from the Larva to the perfect or Imago state of the Insect, the Cephalic ganglia undergo a great increase in size. (Plate II., Fig. 3, a, a.) This evidently has reference to the increased development of the organs of special sense in the latter; the eyes being much more perfectly formed ; antennas and other appendages used for feeling being evolved ; and rudimentary organs of hearing and smell being added. In respondence to the new sensations, which the animal will thus acquire, a great number of new instinctive actions are manifested ; indeed, it may be said, that the instincts of the perfect Insect have frequently nothing in common with those of the Larva. The latter have reference to the acquirement of food; the former chiefly relate to the acts of reproduc- tion, and to the provisions requisite for the deposit and protection of the eggs and the early nutrition of the young.—We find another im- portant change in the nervous system of the adult or perfect Insect; . namely, the concentration of the ganglionic matter of the ventral cord in the thoracic region (e,f); with the three segments of which, the three pairs of legs and the two pairs of wings are connected. The nine segments of the abdomen, in the perfect Insect, give attachment to no organs of motion, and are seldom themselves very movable; and we find that the ganglia which correspond with them have undergone no increase in size, but have rather diminished, and have sometimes STOMATO-GASTRIC AND OTHER NERVES OF INVERTEBRATA. 491 almost completely disappeared. Where the last segment, however, is furnished with a particularly movable appendage, such as a sting, or an ovipositor, we always find a large ganglion in connection with it. 862. These ganglia of the ventral cord evidently correspond in function with the pedal ganglion of the Mollusca; being so many repetitions of it; in accordance with the number of members. We have now to speak of a system of respiratory ganglia, which also are repeated in like manner, in accordance with the condition of the respi- ratory apparatus; this being diffused through the whole body, in most of the Articulata, instead of being restricted to one spot as in the Mollusca. The system of respiratory nerves consists of a chain of minute ganglia, lying upon the larger cord, and sending off its delicate nerves between those that proceed from the ganglia of the latter, as seen in Fig. 2. These respiratory ganglia and their nerves are best seen in the thoracic portion of the cord, where the cords of communi- cation between the pedal ganglia diverge or separate from one another. And this is particularly the case in the Pupa state, when the whole cord is being shortened, and their divergence is increased. The thoracic portion of the cord, in the Pupa of the Sphinx ligustri, is shown in Plate II., Fig. 4; where a, b, and c, represent the 2d, 3d, and 4th double ganglia of the ventral cord; d, d,the cords of connec- tion between them, here widely diverging laterally; and e, e, the small respiratory ganglia, which are connected with each other by delicate filaments that pass over the ganglia of the ventral cord, and which send off lateral branches, that are distributed to the air-tubes and other parts of the respiratory apparatus, and communicate with those of the other system. 863. Besides the Respiratory system of ganglia and nerves, there is in Insects, as in some Mollusks, a set of minute ganglia, which is especially connected with the acts of mastication and swallowing, its filaments being distributed to the muscles of the mouth and pharynx, and some of its ganglia being even found on the stomach, where that organ is remarkable for its muscular powers. The number and arrangement of these ganglia vary considerably in different animals, even in those of the same group; but some traces of this distinct system, which is designated as the stomato-gastric, may always be found. One of the minute ganglia appertaining to it, and forming its anterior termination, is seen to lie on the median line, in front of the great cephalic ganglia, in Plate II., Fig. 3, c. Frora this a trunk passes backwards along the oesophagus; which may be likened to the oesophageal branches of the Par vagum in Vertebrata. Two other small ganglia, communicating with this, are seen at d, d. 864. We are not without traces, moreover, among Invertebrated animals, of the Sympathetic system of the higher classes; though it is quite a mistake to compare the entire system of nerves and ganglia in the former, with the Sympathetic system of the latter,—as was for- merly done. The chief distribution of the branches of the Sympa- 492 NERVOUS SYSTEM OF VERTEBRATA. thetic of Vertebrata is upon the walls of the blood-vessels, and upon the muscular substance of the heart and alimentary canal; and it is by the passage of some of the filaments, from the system of minute ganglia just pointed out, to the dorsal vessel, that we recognize it as combining the functions of the Sympathetic with those of the gastric and cardiac portions of the Par vagum. It will be remembered that there is a frequent inosculation between these two nerves, even in the highest animals. 865. Thus we have seen that, in Invertebrated animals, the Nervous System consists of a series of isolated ganglia, connected together by fibrous trunks. The number of these ganglia, and the variety of their function, entirely depend upon the number and variety of the organs to be supplied. In the lowest Mollusca, the regulation of the ingress and egress of water seems almost the only function to be performed; and here we have but a single ganglion. In the Star-fish, we have five or more ganglia ; but they are all repetitions, one of another. In the higher Mollusca, and in Articulata, we have a ganglion, or more commonly a pair of ganglia, situated at the anterior extremity of the body, connected with the organs of special sensation, and evidently exerting a dominant influence over the rest. In the lower Mollusca, we have but a single ganglion for general locomotion; but this is doubled laterally, and repeated longitudinally in the Articulata, in accordance with the multiplication of their locomotive organs, so as to form the ventral cord. In like manner, the Mollusca possess a single ganglionic centre for the respiratory movements; and this is repeated in every segment of the Articulata, forming a chain of respiratory ganglia, which regulates the actions of the extensively-diffused respi- ratory apparatus of these animals. The acts of mastication and deglu- tition, again, in both sub-kingdoms, are under the control of a distinct set of ganglionic centres; which are connected, however, like the preceding, with the cephalic ganglia; and it is probable that, as in other cases, some filaments from the latter enter into all the branches, which they transmit to the muscles. And we have further seen, that, wherever special organs are developed, whose operations depend upon muscular contraction, ganglionic centres are developed in immediate relation with them ; so as to enable them to act by their simple reflex power, as well as under the direction of the cephalic ganglia;—as in the case of the suckers of the Cuttle-fish. 866. When we direct our attention to the Nervous system of the Vertebrated series, we perceive that it differs from that of the Inver- tebrated classes we have been considering in two remarkable fea- tures. In these last, it has seemed but as a mere appendage to the rest of the system, designed to bring its several parts into more advan- tageous relation. On the other hand, in the Vertebrata, the whole structure appears subservient to it, and designed but to carry its pur- poses into operation. Again, in the Invertebrata, we do not find any special adaptation of the organs of support for the protection of the Nervous System. It is either enclosed, with the other soft parts of NERVOUS SYSTEM OF VERTEBRATA. 493 the body, in one general hard tegument, as in the Star-fish and other Echinodermata, and in Insects, Crustacea, and other Articulata; or it receives a still more imperfect protection, or even none at all, as in the Mollusca. Now in the Vertebrata, we find a special and com- plex bony apparatus, adapted in the most perfect manner for the pro- tection of the Nervous system ; and it is, in fact, the possession of a jointed spinal column, and of its cranial expansion, which best cha- racterizes the group. 867. The Nervous System of Vertebrata is not merely remarkable for its high development, relatively to the remainder of the structure. It is also distinguished by the possession of parts, to which we have nothing analogous in the lower tribes; and by the mode in which these are concentrated and combined, so as to form one continuous mass, instead of consisting of a series of scattered ganglia.—The chief parts which are newly introduced (so to speak) in this sub- kingdom, are the Cerebral Hemispheres and Cerebellum; of which there are no traces whatever in the lower Articulata and Mollusca, and but very minute representations in the highest. These are super- imposed, as it were, upon the cephalic ganglia connected with the organs of special sense, and upon the cords that connect them with the first ganglion of the trunk.—Again we find that the locomotive ganglia, which formed the long-knotted cord of the Articulata, are united with the centres of the respiratory system, and with those of the stomato-gastric system; to form one continuous tract, which com- mences anteriorly from the ganglia of special sense, and runs back- wards* without interruption, in the canal of the Vertebral column, forming the Spinal Cord. This is a continuous instead of an inter- rupted ganglionic mass; it is composed of two lateral halves, precisely similar to each other; and each of these consists of two parts, as dis- tinct from each other as the two tracts in the ventral cord of the Arti- culata,—namely, a fibrous structure, which is continuous between the Encephalon (or collection of nervous masses within the cranium) and certain fibres of the roots of the spinal nerves, and which also serves to connect together the different parts of the cord itself,—and a vesi- cular portion, which forms the proper centre of another set of fibres entering into the roots of those nerves. The anterior portion of the Spinal cord, which is prolonged into the cranium, and comes into immediate relation with the encephalon, is termed the Medulla Ob- longata. It is in this that the centres of the respiratory and stomato- gastric nerves are found ; the situation of these important ganglia within the cranium, being obviously destined to protect them from those injuries to which the Spinal Cord itself is liable. 868. Thus, then, we recognize in the Nervous system of Vertebrata the following fundamental parts.—1. A system of ganglia subservient to the reflex actions of the organs of locomotion, and corresponding * When we speak of the Vertebrata generally, their bodies are of course supposed to be in a horizontal position,—not vertical as in Man. 494 NERVOUS CENTRES OF FISHES. with the chain of pedal or locomotive ganglia that makes up the chief part of the ventral cord of the Articulata; in this system, the gray or vesicular matter forms one continuous tract, which occupies the inte- rior of the Spinal Cord.—2. A ganglionic centre for the movements of respiration, and another for those of mastication and deglutition; these, with part of the preceding, make up the proper substance of the Medulla Oblongata.—3. A series of ganglia, in immediate con- nection with the organs of Special Sense; these are situated within the cranium, at the anterior extremity of the Medulla Oblongata; and, in the lowest Vertebrata, they constitute by far the largest portion of the entire Encephalon.—4. The Cerebellum, which is a sort of off-shot from the upper extremity of the Medulla Oblongata, lying behind the preceding.—5. The Cerebral Hemispheres, a pair of ganglionic masses, which lie upon the ganglia of special sense, capping them over more or less completely, according to their relative development.—These two last organs exist in the lowest Vertebrata, as in Invertebrated animals generally, in quite a rudimentary state ; but their develop- ment, relatively to other parts of the Encephalon, and to the entire bulk of the animal, increases as we ascend the scale; so that in Man and the higher Mammalia they constitute by far the largest portion of the Nervous centres, and are essential to the greater part of the ope- rations of the Nervous system. The development of the Cerebral Hemispheres holds a close relation with the increase of the Intelli- gence, and with the predominance of the Will over the involuntary impulses. The increased size of the Cerebellum, on the other hand, seems connected with the necessity which exists, for the adjustment and combination of the locomotive powers, when the variety in the movements performed by the animal is great, and a more perfect harmony is required among them.—A sketch of the mode in which these different parts are combined and arranged in the several classes of Vertebrata, and of their relative development in each, will aid us in the subsequent more detailed examination of their functions. 869. In the class of Fishes, taken as a whole, the Encephalon bears a much smaller proportion to the Spinal Cord, than in the higher Vertebrata. In the curious Amphioxus, or Lancelot, there is no dis- coverable nervous mass anterior to the Medulla Oblongata ; and we have here, therefore, an animal regularly formed upon the plan, which occasionally presents itself as a monstrosity in Man, namely, having the Spinal Cord and Medulla Oblongata for the whole of the nervous centres, and being anencephalous, or destitute of any proper ence- phalon. In some of the lowest Vermiform (worm-like) Fishes, such as the Lamprey, the cephalic masses are very little more developed in proportion to the Spinal Cord, than are the cephalic ganglia of Insects in reference to their chain of ventral ganglia. But as the organs of special sense acquire a more complete evolution, we find the ganglia connected with them presenting a greatly-increased size. On opening the cranial cavity of a Fish, we usually observe four ENCEPHALON OF FISHES AND OF HUMAN EMBRYO. 495 nervous masses (three of them in pairs) lying, one in front of the other, nearly in the same line with the Spinal cord. The first or most anterior of these are the Olfactory ganglia (Plate II., Figs. 5, 6, 7, a), or the ganglia of the nerves of smell; the nature of which is known, from their being situated at the origin of the Olfactory nerves. In the Shark and some other Fishes, these are separated from the rest by peduncles or foot-stalks; a fact of much interest, as explaining the arrangement which we find in Man. What is commonly termed the trunk of his Olfactive nerve is reajly the commissure connecting the Olfactive ganglion,—known as the bulbous enlargement that lies upon the cribriform plate of the Ethmoid bone,—with the other por- tions of his Encephalon ; the proper fibres of the nerve being those, which come off from this ganglion, in the numerous branches that proceed from it into the nasal cavity.—Behind the Olfactive ganglia is a pair of masses, b, b, of which the relative size varies greatly in different Fishes. Thus in the Perch, whose Encephalon is here figured, their size is intermediate between that of the first and third pairs; being as much inferior to that of the third, as it is superior to that of the first. On the other hand, in the Shark and several other Fishes, they are considerably larger than the succeeding pair. These second ganglia are the Cerebral Hemispheres.—Behind them, and forming the third pair of ganglionic masses, c, c, are two large bodies, from which the optic nerves arise ; these are evidently the Optic ganglia, corresponding to the principal mass of the cephalic ganglia in Insects, and the higher Mollusca.—And at the back of these, over- lying the top of the spinal cord, is a single mass, d, the Cerebellum. This, also, varies greatly in its relative dimensions; being much more highly developed in the Active and rapacious Sharks, than it is in Fishes of inferior muscular energy and variety of movement.—The Spinal Cord e, is divided at the top by a fissure, which is most wide and deep beneath the cerebellum, where there is a complete sepa- ration between its two halves. This opening corresponds to that, through which the oesophagus passes in the Invertebrata ; but as the entire nervous mass of Vertebrated animals lies above the alimentary canal (or nearer the dorsal surface), it does not serve the same pur- pose in them ; and in the higher classes, the fissure is almost entirely closed by the union of the two halves on the median plane,—the fourth ventricle, however, being a remnant of it. This cavity is partly seen in Fig. 7, which is a vertical section of the brain whose upper and under surfaces are shown in Figs. 5 and 6. 870. The Optic lobes of Fishes must not be confounded with the Thalami optici of the higher Vertebrata, with which they have only a slight analogy, as is proved by their position and connections. They are rather to be compared with the Corpora Quadrigemina, which are the real ganglia of the optic nerve. Their analogy is not so complete, however, to these bodies in their fully-formed brain of the higher Vertebrata; as to the certain parts which occupy their place at an earlier period. In the Human embryo, at about the 6th week, the 496 NERVOUS SYSTEM OF FISHES AND REPTILES. Encephalon consists of a series of vesicles arranged in a line with each other; of which those that represent the cerebrum (b, Fig. 142) are the smallest, whilst that which represents the cerebellum (d) is the largest. Between the cerebral and cerebellic vesicles, are two others, (c, and a), of which the pos- terior one is the Optic ganglion, and answers to the Tubercula quadrige- mina ; whilst the anterior contains the third ventricle, and corresponds in some degree to the thalami optici. This condition is precisely represent- ed in the Lamprey; but in most Fishes, the Optic ganglia, and the parts surrounding the third ventricle, form but one lobe ; so that the third ventricle seems hollowed out of the optic ganglia, as shown in Fig. 7, c (Plate II.).—Besides the Olfactive and Optic ganglia, there are in many Fishes distinct Auditory ganglia, from which the nerves that minister to the sense of hearing originate; these are frequently blended, how- ever, with the Medulla oblongata, their vesicular substance forming a part of its gray matter.—It is curious to notice the very large com- parative size of the Pineal gland (/), and of the Pituitary body (h), in this class; the functions of these organs are entirely unknown. 871. The Encephalon of Reptiles does not show any considerable advance in its general structure, above that of the higher Fishes. The Cerebral Hemispheres (Figs. 8, 9, 10, b) are always much larger than the Olfactive and Optic ganglia ; and they generally cover in the latter (c, e) in part, by their posterior extremities. The Cerebellum is almost invariably of small proportional dimensions ; and this is espe- cially the case in the Frog, in which it does not even cover in the fourth ventricle. This low development of the Cerebellum in Rep- tiles, is what might be anticipated from the general inertness of these animals, and the want of variety in their movements. The Spinal Cord is still very large, in proportion to the nervous masses contained in the skull; and, as we shall hereafter see, its power of keeping up the movements of the body, after it has been cut off from all connec- tion with the brain, is very considerable.—We find that, in Reptiles, as in Fishes, the Spinal Cord may have a nearly uniform size from one extremity to the other, like the ventral cord of the lower Articu- lata ; or it may present considerable enlargements at particular spots, like the ganglionic cord in the thoracic region of Insects. This dif- ference depends upon the degree of development of the special loco- motive organs. Thus in the Eel and Serpent, whose movements are accomplished by the undulations of the entire trunk, and which are Fig. 142. Human embryo of sixth week, enlarged about three times:—a, vesicle of corpora quadrigemina; 6, vesicle of cerebral hemi- spheres; c, vesicle of thalami optici and third ventricle ; d, vesicle for cerebellum and me- dulla oblongata; e, auditory vesicle;/, olfac- tory fossa ; h, liver; ** caudal extremity. NERVOUS CENTRES OF BIRDS AND MAMMALS. 497 destitute of members, we find an uniform development of ganglionic matter in the spinal cord. On the other hand, in the Flying-fish, in which the pectoral fins or anterior extremities effect the greater part of the propulsion of the body, we find a great ganglionic enlargement of the Spinal cord, at the part with which the nerves of those mem- bers are connected: in the Frog, whose movements are chiefly effected by the posterior extremities, we find a similar enlargement at the roots of the crural nerves; and in the Turtles and Lizards, the two pairs of whose members are nearly equal in function, and serve to effect the principal movements of the body, we find an anterior and posterior enlargement of the Spinal Cord, corresponding to the parts with which the nerves of these members are connected. 872. We find in Birds a considerable advance in the character of the Encephalon, towards that which it presents in Mammalia. The Cerebral Hemispheres (Plate II., Figs. 11, 12, 13, b) are greatly in- creased in size ; and then cover in, not merely the olfactory ganglia, but in great part also the optic ganglia. The former are of compara- tively small size ; the organ of smell in Birds not being much deve- loped. The latter are very large, in conformity with the acuteness of sight, which is highly characteristic of the class. The Cerebellum is of large size, as we should expect from the number and complexity of the muscular movements performed by animals of this class; but it is still undivided into hemispheres. The Spinal Cord is still of considerable size in comparison with the Encephalon ; and it is much enlarged at the points whence the legs and wings originate. In the species which have the most energetic flight, such as the Swallow, the enlargement is the greatest where the nerves of the wings come off; but in those which, like the Ostrich, move principally by running on the ground, the posterior enlargement, from which the legs are supplied with nerves, is much the more considerable. 873. In the Mammalia we find the size and general development of the Encephalon presenting a gradual increase, as we ascend the series, from the non-placental Monotremes and Marsupials, towards Man. In the former, the Hemispheres exhibit no convolutions; and the great transverse commissure, or connecting band of fibrous struc- ture,—termed the corpus callosum,—is deficient. As we rise through the true viviparous division of the class, we notice a gradually in- creasing prolongation of the Cerebral Hemispheres backwards; so that first the optic ganglia, and then the cerebellum, are covered in by them. The latter partly shows itself, however, in all but Man and the Quadrumana, when we look at the brain from above downwards; as we see in the Encephalon of the Sheep (Plate II., Figs. 14, 15, d). The Cerebral hemispheres increase, not only in size, but also in complexity of structure, both external and internal. Their exterior, instead of remaining smooth, is marked by convolutions ; which serve to extend very greatly the amount of surface, over which blood-ves- sels can pass into the gray substance. Their internal structure be- comes more complex, in the same proportion as their size and the 32 498 NERVOUS CENTRES OF MAMMALIA.—SPINAL CORD. depth of their convolutions increase; and in Man all these conditions present themselves in a far higher degree than in any other animal. The number of commissural bands, connecting the two hemispheres with each other transversely, and uniting their anterior and posterior portions, is very greatly increased ; and, in fact, a large proportion of their mass is composed, in Man and the higher Mammalia, of fibres of this character. In proportion to the increase of the Cerebral hemi- spheres, there is a diminution in the size of the ganglia of special sense; and this is seen when we compare them, not merely with the rest of the Encephalon, but even with the Spinal Cord. The Olfac- tive ganglia (Fig. 14, a), are always readily discoverable; being separated from the remainder of the encephalic masses by a peduncle on each side. The Optic ganglia, (Fig. 15, c,) on the other hand, are so completely covered in by the Hemispheres, that it is only when the latter are turned aside that we can discern them. They differ in external aspect from the optic ganglia of Birds and the lower Vertebrata ; being divided by a transverse furrow into anterior and posterior eminences,—whence they are known as the Corpora Quad- rigemina. The Cerebellum is chiefly remarkable for the develop- ment of its lateral parts or hemispheres, and for the intricate arrange- ment of the gray and white matter in them (Fig. 15, d); the central portion, sometimes called the vermiform process, is relatively less developed than in the lower Vertebrata, where it forms the entire organ. The Spinal Cord is much reduced in size, when compared with other parts of the nervous centres; the motions of the animal, in this class, being more dependent upon its will, or guided by its sensations ; and the simply reflex actions bearing a much smaller pro- portion to the rest. The development of ganglionic enlargements, in accordance with the presence or absence of high locomotive powers in the extremities, follows the same rule as in the preceding classes. 3. Functions of the Spinal Cord and its Nerves. 874. In commencing our more detailed examination into the func- tions of the different parts of the Nervous system in Vertebrated animals, it seems best to commence with the Spinal Cord; this being the portion whose presence is most essential to the continuance of life. As already mentioned, Infants are sometimes born without any Cerebrum or Cerebellum ; and such have existed for several hours or even days, breathing, crying, sucking, and performing various other movements. The Cerebrum and Cerebellum have been experiment- ally removed from Birds and young Mammalia, thus reducing these beings to a similar condition ; and all their vital operations have, nevertheless, been so regularly performed, as to enable them to live for weeks, or even months. In the Amphioxus, as already remarked, we have an example of a completely-formed adult animal; in which no rudiment of a Cerebrum or Cerebellum can be detected. And in ordinary profound sleep, or in apoplexy, the functions of these or- REFLEX ACTIONS OF SPINAL SYSTEM. 499 gans are so completely suspended, that the animal is, in all essential particulars, in the same condition for a time as if destitute of them. It is possible, indeed, to reduce a Vertebrated animal to the condition (so far as its nervous system is concerned) of an Ascidian Mollusk (§ 850); for it may continue to exist for some time, when not merely the Cerebum and Cerebellum have been removed from above, but when nearly the whole Spinal Cord has been removed from below, —that part only of the latter being1 left, which is the centre of the respiratory actions, and which corresponds to the single ganglion of the Tunicata. On the other hand, no animal can exist by its En- cephalon alone, the Spinal Cord being destroyed or removed ; for the reflex actions of the latter are so essential to the continuance of its respiration, and consequently of its circulation, that if they be sus- pended (by the destruction of the portion of the cord wThich is con- cerned in them), all the organic functions must soon cease. 875. Although the Spinal Cord was formerly regarded as little else than a bundle of nerves proceeding from the Brain, yet its true rank, as a distinct centre of nervous action, is now universally admitted. That the actions performed by it are of a purely reflex nature,—con- sisting in the excitement of muscular movements, in respondence to external impressions, without the necessary intervention of sensation, —appears to be a necessary inference, from the facts that have been brought to light by experiment and observation. Experiments on the nature of this function are best made upon cold-blooded animals; as their general functions are less disturbed by the effects of severe injuries of the nervous system, than those of Birds and Mammals. When the Cerebrum has been removed, or its functions have been suspended by a severe blow upon the head, a variety of motions may be excited by their appropriate stimuli. Thus, if the edge of the eye- lid be touched with a straw, the lid immediately closes. If a candle be brought near the eye, the pupil contracts. If liquid be poured into the mouth, or a solid substance be pushed within the grasp of the muscles of deglutition, it is swallowed. If the foot be pinched, or burned with a lighted taper, it is withdrawn ; and (if the animal ex- perimented on be a Frog) the animal will leap away, as if to escape from the source of irritation. If the cloaca be irritated with a probe, the hind legs will endeavour to push it away. 876. Now the performance of these, as well as of other movements, many of them most remarkably adapted to an evident purpose, might be supposed to indicate, that sensations are called up by the impres- sions ; and that the animal can not only feel, but can voluntarily di- rect its movements, so as to get rid of the irritation which annoys it. But such an inference would be inconsistent with other facts.—In the first place, the motions performed by an animal under such circumstances are never spontaneous, but are always excited by a stimulus of some kind. Thus, a decapiated Frog, alter the first vio- lent convulsive movements occasioned by the operation have passed away, remains at rest until it is touched ; and then the leg, or its 500 REFLEX ACTIONS OF SPINAL SYSTEM. whole body, may be thrown into sudden action, which immediately subsides again. In the same manner, the act of swallowing is not performed, except when it is excited by the contact of food or liquor; and even the respiratory movements, spontaneous as they seem to be, would not continue, unless they were continually re-excited by the presence of venous blood in the vessels. These movements are all necessarily linked with the stimulus that excites them ;—that is, the same stimulus will always produce the same movement, when the condition of the body is the same. Hence it is evident, that the judg- ment and will are not concerned in producing them ; and that the adaptiveness of the movements is no proof of the existence of con- sciousness and discrimination in the being that executes them,—the adaptation being made for the being, by the peculiar structure of its nervous apparatus, which causes a certain movement to be executed in respondence to a given impression,—not by it. An animal thus circumstanced may be not unaptly compared to an automaton ; in which particular movements adapted to produce a given effect, are produced by touching certain springs. Here the adaptation was in the mind of the maker or designer of the automaton; and so it evi- dently is, in regard to the reflex or consensual movements of animals, as well as with respect to the various operations of their nutritive system, over which they have no control, yet which concur-most ad- mirably to a common end. 877. Again, we find that such movements may be performed, not only when the Brain has been removed, the spinal cord remaining entire, but also when the Spinal cord has been itself cut across, so as to be divided info two or more portions,—each of them completely isolated from each other, and from other parts of the nervous centres. Thus, if the head of a Frog be cut off, and its spinal cord be divided in the middle of the back, so that its fore legs remain connected with the upper part, and its hind legs with the lower, each pair of members may be excited to movement by a stimulus applied to itself; but the two pairs will not exhibit any consentaneous motions, as they will do when the spinal cord is undivided. Or, if the Spinal cord be cut across, without the removal of the Brain, the lower limbs may be excited to movement, by an appropriate stimulus, though they are completely paralyzed to the will; whilst the upper remain under the control of the animal, as completely as before. Now it is not con- ceivable that, in this last case, sensation and volition should exist in that portion of the spinal cord which remains connected with the nerves of the posterior extremities, but which is cut off from the brain. For, if it were so, there must b.e two distinct centres in the same ani- mal, the attributes of the brain not being affected ; and, by dividing the spinal cord into two or more segments, we migfyt thus create in the body of one animal two or more distinct centres of sensation, independent of that which still holds its proper place in the Encepha- lon. To say that two or more distinct centres of sensation are pre- sent in such a case, would really be in effect the same as saying, that REFLEX ACTIONS OF SPINAL SYSTEM. 501 there are two or more distinct minds in one body,—which is mani- festly absurd. 878. But the best proofs of the limitation of the endowments of the Spinal Cord, are derived from the phenomena presented by the Human subject, in cases where that organ has suffered injury, by disease or accident, in the middle of the back. We find that, when this injury has been severe enough to produce the effect of a complete division of the Cord, there is not only a total want of voluntary control over the lower extremities, but a complete absence of sensation also,—the individual not being in the least conscious of any impression made upon them. When the lower segment of the Cord remains sound, and its nervous connections with the limbs are unimpaired, distinct reflex movements may be excited in the limbs by stimuli, directly applied to them,—as, for instance, by pinching the skin, tickling the sole of the foot, or applying a hot plate to its surface;—and this with- out the least sensation, on the part of the patient, either of the cause of the movement, or of the movement itself. This fact, taken in connection with the preceding experiments, both upon Vertebrated and Articulated animals, distinctly proves that Sensation is not a necessary link in the chain of reflex actions; but that all which is required is an afferent fibre, capable of receiving the impression made upon the surface, and of conveying it to the centre; a ganglionic cen- tre, composed of vesicular nervous substance, into which the afferent fibre passes; and an efferent fibre, capable of transmitting the motor impulse, from the ganglionic centre, to the muscle which is to be thrown into contraction. 879. These conditions are realized in the Spinal Cord. We may have reflex actions excited through any one isolated segment of it, as through a single ganglion of the ventral cord of Articulata; but they are then confined to the parts supplied by the nerves of that segment. Thus, if the spinal cord of a Frog be divided just above the origin of the crural nerves, the hind-legs may be thrown into reflex contrac- tion by various stimuli applied to themselves; but the fore legs will exhibit no movement of this kind. But when the brain has been removed, and the Spinal Cord is left entire, movements may be excited in distant parts,—as, for example, in the fore legs, by any power- ful irritation of the posterior extremities,—and vice versa. This is particularly well seen in the convulsive movements, which take place in certain disordered states of the nervous system ; a slight local irrita- tion being sufficient to throw almost any muscles of the body into a state of energetic action (§ 885). And a similar state may be arti- ficially induced, by applying Strychnine (in solution) to the Spinal Cord of a decapitated Frog. 880. The minute Anatomy of the Spinal Cord is a subject of great difficulty ; and our notions of the course of the fibres within it are rather founded upon physiological phenomena, and upon the more evident structure of the ventral column in Articulata, than upon what can be clearly demonstrated in Vertebrated animals. The roots of 502 FUNCTIONS OF ROOTS OF SPINAL NERVES. the Spinal nerves are all distinctly separable into an interior and a posterior fasciculus; and it is certain that these fasciculi have entirely opposite functions. If they be laid bare, and the anterior fasciculus of" any spinal nerve be touched, violent contractions are immediately seen in the muscles supplied by that nerve; these contractions are as strongly manifested, if the anterior roots be be divided, and their sepa- rated ends be irritated ; whilst no such result follows, whatever amount of irritation be applied to the ends still in connection with the cord. Notwithstanding these violent movements, the animal shows lit- tle or no sign of pain.—On the other hand, if the posterior roots be irritated, the animal gives signs of acute pain, and no vigorous mus- cular contractions are produced. The movements which are witness- ed are evidently of a reflex nature, being called forth through the ante- rior roots; as is proved by their cessation when these are divided. Further, if the posterior roots be divided, and the separated ends be irritated, no effect whatever is pro- duced ; no movement is excited ; and no sensation is occasioned ; but if the ends still in connection with the cord be irritated, the animal shows signs of pain as before.— Hence it is evident, that the poste- rior roots are made up of afferent fibres ; that is, of the fibres which convey impressions towards the nervous centres: which impressions, if confined to the cord itself, excite reflex actions; whilst, if conveyed to the brain, they produce sensations. On the Other hand it is equally evident, that the anterior roots are composed of efferent or motor fibres, which serve to convey to the muscles the motor impulses originating in the nervous centres; these impulses may be occasioned by the reflex action of the Spinal cord; or they may descend from the Brain, where they have been generated by an act of the will. In the accompanying Diagram, the left side shows the supposed composition of the posterior roots of the nerves; and the right side, that of the anterior. 881. The Spinal Cord is a completely double tract; being composed of two distinct halves, united together on the median plane by nume- rous commissural fibres. This union is much closer in Man and the Mammalia than it is in the lower Vertebrata; but the division is still marked externally, by a deep fissure on the anterior surface of the cord, and by a shallower one on its posterior aspect. Its surfaee is traversed Fig. 143. Diagram of the origins and terminations of the different groups of nervous fibres:—a, a, vesicular substance of the spinal cord; 6, b, b, vesicular substance of the brain ; e, vesicular substance at the commencement of afferent nerve, which consists of el, the cerebral divi- sion, or sensory nerve passing on to the brain, and *i, the spinal division, or excitor nerve, which terminates in the vesicular substance of the spinal cord: on the other side we have the efferent or motor nerve proceeding to the mus- cle d, likewise consisting of two divisions*— e2, the cerebral portion, proceeding from the brain, and conveying the influence of the will or of instinct; and «>, the spinal division, con- veying the reflex power of the spinal cord. STRUCTURE OF THE SPINAL CORD. 503 moreover, by two furrows on each side ; so that each half is divided into three columns, the anterior, lateral, and posterior. The anterior roots of the spinal nerves join the Cord for the most part along the line of the'anterior furrow; and the posterior along the line of the posterior furrow; so that the middle or lateral column lies between them, the anterior column being altogether in front of them, and the posterior column behind them. When a transverse section of the Cord is made, it is seen to contain, on each side, a crescentic patch of gray or vesicular substance; the points of each crescent are directed towards the anterior and posterior furrows of its own side respectively; whilst the convexities of the two crescents approach one another near the median plane, and are connected by a transverse tract of gray matter. The remainder of the cord is made up of white or tubular substance, the course of whose fibres is, for the most part, longitudinal.—The pos- terior peak of the crescentic patch of gray matter approaches very closely to the bottom of the posterior furrow; whilst the anterior peak does not come into nearly the same degree of proximity with the bot- tom of the anterior furrow. Hence it is considered by some, that the lateral or middle columns of the cord, being much less completely isolated from the anterior columns than they are from the posterior, should be associated with the former, under the name of antero-lateral columns. 882. Upon tracing the roots of the nerves into the substance of the Cord, the connection of a part of their fibres with its gray or vesicular substance is easily made evident. Of these fibres, therefore, it serves as the proper ganglionic centre. There is reason to believe, both from anatomical investigation, and from physiological phenomena, that, as in the Articulata, (§ 857,) a part of the afferent or excitor fibres, after traversing the gray substance, pass out on the same side as the efferent or motor; whilst another portion crosses to the opposite side, and forms part of its efferent trunks. The continuity of other fibres, however, with the longitudinal fibres that form the white strands of the Spinal Cord, has not been yet clearly demonstrated; though the analogy of the ventral cord in Articulated animals, and the physiological pheno- mena, which show the direct connection between the sensory surfaces and the brain on the one hand, and between the brain and the mus- cles on the other, would seem to indicate that such continuity must exist.—The relative proportions of the gray and white matter in the Spinal Cord differ considerably at its different parts. Thus in the cervical region, there is an enlargement corresponding with the origins of the nerves that form the brachial plexus; this enlargement is partly caused by an increase in the amount of gray matter; but the whole of the cervical portion of the cord contains a very large amount of fibrous , structure also. On the other hand, there is a still greater enlargement of the cord in the lumbar region, at the part whence the nerves of the lower extremities arise ; and this enlargement is caused by the great increase in the amount of the gray matter at that point; the white or fibrous portion constituting but a comparatively small part of it. Now 504 REFLEX FUNCTION OF THE SPINAL CORD. these anatomical facts harmonize well with the physiological views just given; for the actions of the lower extremities being much more of a simply reflex nature than those of the upper, we find the gangli- onic portion of the spinal cord exhibiting a corresponding increase at the origins of their nerves; whilst the actions of the superior extremi- ties being for the most part of a voluntary character, we find that the cord mainly consists, at the part with which their nerves are connect- ed, of white fibrous structure, which appears to convey to those nerves the direct influence of the brain. 883. It was supposed by Sir C. Bell (who was the first to deter- mine the relative functions of the two roots of the spinal nerves in Vertebrated animals), that the anterior columns of the Spinal cord have a function corresponding to that of the anterior roots of the spi- nal nerves; and the posterior columns with the posterior roots. But from the difficulty of tracing the connection between the longitudinal fibres of the cprd and any portion of the roots, it is at present impos- sible to say how far there is any anatomical reason for the assumption of this correspondence; and it is quite certain, that the physiological facts at present known, from observation of the effects of disease or injury upon different tracts of the spinal cord, do not bear out the supposition. As to what the precise functions of the several columns are, however, it is not easy to form any other conjecture, that shall be consistent with all the phenomena at present known. 884. Of the particular Reflex actions to which the Spinal Cord (using that term in its limited sense, as excluding the Medulla Oblon- gata) is subservient, those most connected with the organic functions have already been noticed. They are chiefly of an expulsive kind ; being destined to force out the contents of various cavities of the body. Thus the ordinary acts of defecation and urination, the ejaculatio se- minis, and parturition, are all reflex actions, over which the will has a greater or less degree of control; being able to keep the two former ones in check, so long as the stimulus is not very violent, and being also capable of effecting them by itself; but having no control over the two latter, either by way of acceleration or prevention, when once the stimulus by which they are excited has come into full action.—The movements of the posterior extremities are among the most remarkable of those, which seem due to the action of the proper Spinal Cord. It has been already noticed, that these may be excited, even in Man, when the spinal cord has been severed in the middle without injury to its lower segment; and it is remarkable, that gentle stimuli, applied to the skin of the sole of the foot, appear the most capable of producing them. WTe have seen how completely, in the lower animals, the acts of progression maybe sustained, by the repeated stimulus of the con- tact of the ground, or of fluid, without any influence from the cephalic ganglia ; the power of these being limited, it would seem, to the con- trol and direction of them. And there is strong reason to believe that, so far as the ordinary acts of locomotion are concerned, the movements of the inferior extremities in Man may be performed on the ACTS OF LOCOMOTION.—CONVULSIVE DISORDERS. 505 same plan, being continued by reflex power, when once set in action by the will, whilst we are walking steadily onwards,—the mind being at the same time occupied by some train of thought which engrosses its whole attention. There are few persons to whom it has not occa- sionally happened that, on awaking (as it were) from their re very, they have found themselves in a place very different from that to which they had intended going ; and even when the mind is sufficient- ly on the alert to guide, direct, and control the motions of the limbs, their separate actions appear to be performed without any direct agency of the will. It is certain that, in Birds, the movements of flight may be performed after the removal of the Cerebrum. 885. There are many irregular or abnormal reflex actions, performed through the instrumentality of the Spinal Cord, the study of which is of the highest importance to the Medical Man. It is probable that all Convulsive movements are produced through its agency and that of the Medulla Oblongata; for it has been found, by repeated experiments, that these movements are never produced by injuries of the Cerebral hemispheres.—Convulsive movements may be of three kinds. 1. They may be simply reflex; being the natural result of some extraordinary irritation. 2. They may be simply centric ; depending upon a pecu- liar condition of the ganglionic centre of the Spinal Cord, which oc- casions muscular movements without any stimulation. 3. They may depend upon the combined action of both principles; the nervous cen- tres being in a very irritable state, which causes very slight irritations (such as would otherwise be inoperative) to excite violent reflex or convulsive movements. This last is by far the most common cause of the convulsive actions, that occur in various diseased conditions of the system. Thus, convulsions are not unfrequent in children, during the period of teething; being produced by the irritation which results from the pressure of the tooth as it rises against the unyielding gum. In this case, the stimulus would scarcely be sufficient to produce the violent result, were it not for a peculiarly excitable state of the Spinal Cord, brought about by various causes. In like manner, when such an excitable state exists, convulsions may be occasioned by the pre- sence of intestinal worms, of irritating substances, or even simply of undigested matters in the alimentary canal; and will cease as soon as they are cleared out—in the same manner as the convulsions of teething may often be at once checked—by the free lancing of the gums. 886. The" influence of the condition of the Spinal Cord itself is peculiarly seen in the convulsive diseases termed Hydrophobia, Te- tanus, Epilepsy, and Hysteria. In the first of these, not only the Spinal Cord, but the Medulla Oblongata, and the ganglia of Special Sense are involved ; their peculiar condition being the result, it would appear, of the introduction of a poison into the blood. It is most remarkable that the Brain should so completely escape its in- fluence. When the state of intense excitability in these centres is once established, the slightest stimulus is sufficient to bring about 506 PECULIARITIES OF CERTAIN SPINAL NERVES. convulsive movements of the utmost violence. It is characteristic of this complaint, that the stimuli most effectual in exciting the movements, are those which act through the nerves of special sense; thus the sight or the sound of water will bring on the paroxysm; and any attempt to taste it increases the severity of the convulsions.— In Tetanus there appears to be a similarly excitable state of the Spinal Cord and Medulla Oblongata, not involving the ganglia of special sense. This may be the result of causes altogether internal, as in the idiopathic form of the disease; in which the condition ex- actly resembles that which may be artificially induced by the admin- istration of Strychnine, or by its application to the cord. Or it may be first occasioned by some local irritation, as that of a lacerated wound ; the irritation of the injured nerve being propagated to the nervous centres, and establishing the excitable state in them. When the complaint has once established itself, the removal of the original cause of irritation (as by the amputation of the injured limb) is sel- dom of any avail; since the slightest impressions upon almost any part of the body, are sufficient to excite the tetanic spasm.—In like manner, Epilepsy, which consists in convulsive actions with tem- porary suspension of the functions of the brain, may result from the irritation of local causes, like the convulsions of teething; and may, like them, cease when the sources of irritation are removed. But when it becomes confirmed, it seems to involve a disorder of the nervous centres, which no local treatment can influence. 887. These and other forms of Convulsive disorder, when produc- tive of a fatal result, usually act by suspending the respiratory move- ments ; the muscles which effect these being fixed by the spasms, so that the air cannot pass either in or out, and suffocation takes place as completely as if the entrance to the air-passages were closed. It is remarkable that every one of them may be imitated by Hysteria; a state of the nervous system in which there is a peculiar excitability, but in which there is no such fixed tendency to irregular action as would indicate any positive disease,—one form of convulsion often taking the place of another, at short intervals, with the most wonder- ful variety. It will often be found, that the convulsions may be im- mediately traced to some local irritation ; thus they are particularly liable to occur at the catamenial periods, especially if the menstrual flux be deficient. But the liability to them, resulting from the pecu- liar excitability of the nervous system, can only be treated by such constitutional remedies as tend to increase its vigour and to promote its normal activity. 888. The statement that the spinal nerves arise by double roots, is not without exception as regards some, which arise from its cranial prolongation, and which are distributed to the parts of the head and neck. The first spinal nerve, or sub-occipital, (the 10th pair of Wil- lis,) not unfrequently arises by a single set of roots, from the anterior portion of the cord; and it is then purely motor except in virtue of its inosculation with other nerves. The Hypoglossal (9th pair of STRUCTURE OF THE MEDULLA OBLONGATA. 507 Willis) appears to be also a purely motor nerve; arising by one set of roots; and being distributed entirely to the muscles of the tongue; which organ derives its sensibility from other nerves. The Glosso- pharyngeal usually arises from a single set of roots, and these corre- spond with the posterior roots of the spinal nerves ; in some animals, however, and occasionally in man, there is a distinct anterior root, and the nerve acquires direct motor functions. It may in some re- spects be considered as making up, with the preceding, an ordinary spinal nerve. The Spinal Accessory, again, appears to be chiefly or entirely a motor nerve at its origin; and in like manner the Pneumo- gastric, or Par Vagum, seems at its roots to correspond with the pos- terior roots of the ordinary spinal nerves, and to execute functions analogous to theirs; but these two nerves exchange fibres, so that each acquires in part the endowments of the other. The Facial nerve, (or portio dura of the 7th,) which is the nerve that supplies the mus- cles of the head in general, arises by a single root, and is exclusively motor in its properties.—except in branches which have received sensory filaments by inosculation with other nerves. The same is the case, also, with the Motor Nerves of the Orbit, (the 6th, 4th and 3d, of Willis,) which arise by single roots, and which have no sen- sory endowments but those which they obtain by inosculation with the Fifth pair.—On the other hand, the Fifth pair arises by a double root; that which corresponds to the anterior or motor root to the spi- nal nerves is very small, however, and only enters the third division of the nerve, which supplies the muscles concerned in mastication ; the other root, corresponding with the posterior roots of the spinal nerves, is of large size, and its branches are distributed to the face and head, supplying them with sensibility. Thus the sensory division of the fifth pair being distributed, not merely to the same parts with its motor division but also to the parts which derive their motor endow- ments from the Facial nerve, and from the nerves of the orbit, may be regarded as making up, together with all of them, one ordinary Spinal nerve. 4. Functions of the Medulla Oblongata. 889. This portion of the nervous centres, as already stated, does not differ in any essential particular from the Spinal Cord, of which it may be considered as a cranial prolongation. But the arrangement of its constituent parts is peculiar; being the medium by which the various strands of the Spinal Cord are connected with the different portions of the Encephalon : and it is also remarkable as being the ganglionic centre, concerned in the maintenance of the action of re- spiration, and in the ingestion of food. Four principal tracts of nerv- ous matter may be distinguished in each of its lateral halves. These are anteriorly, the anterior pyramids; next, the olivary bodies; next, the' restiform bodies ; and, lastly, the posterior pyramids. The fol- lowing are the principal connections of these different strands, with 508 STRUCTURE AND FUNCTIONS OF MEDULLA OBLONGATA. the several parts of the Encephalon above, and of the Spinal Cord below. 890. The Anterior Pyramids, which consist entirely of fibrous structure, may be said to connect the motor fibres of the Cerebral Hemispheres with the antero-lateral columns of the Spinal Cord. Of the fibres of which they are composed, a large part decussate ; those that proceed from the right hemisphere, passing into the left side of the cord; and those from the left hemisphere into the right side of the cord,—an arrangement which fully explains the fact, that in Hemi- plegia, the paralytic affection of the body is on the opposite side to that of the face, the latter corresponding with the side of the brain in which the disease may exist. A small proportion of the fibres of the anterior pyramids does not decussate ; and this passes down, with fibres from the olivary columns, into the anterior columns of the cord; whilst the decussating fibres dip more deeply away from the anterior surface of the cord, and connect themselves rather with its middle columns. 891. The Olivary bodies are composed externally of fibrous struc- ture; their fibres being connected above with the Cerebral Hemi- spheres and Corpora Quadrigemina; and below with the antero-lateral columns of the Spinal Cord. But beneath the fibrous layer we find a large mass of vesicular matter, the presence of which gives to the Medulla Oblongata its ganglionic character. This seems to be the centre of the respiratory nefves; it is continuous with the gray matter of the Spinal Cord below, and with that of certain parts of the Ence- phalon above; and, from its peculiar aspect, it is known as the corpus dentatum. 892. The Restiform columns are continuous above with the fibres of the hemispheres of the Cerebellum; and below they pass, without decussation, chiefly into the posterior columns of the spinal cord,—a band of arciform fibres, however, crossing over to the anterior columns on each side. 893. The Posterior Pyramids are two small strands of fibrous structure, lying between the two restiform bodies; and occupying the portion of the Medulla Oblongata on either side of the posterior median furrow. They seem to stop short at the fourth Ventricle ; and it has not yet been ascertained whether they have any connec- tion with the higher parts of the Encephalon. Below, they assist in forming the posterior columns of the Spinal Cord ; and, if it be true that they have no connection with the brain, we may assign to them. the function of connecting the different segments of the cord with each other. 894. The functions of the Medulla Oblongata are, therefore, of a double character;—to bring the higher parts of the Encephalon into connection with the Spinal Cord and the Nerves that issue from it; —and to serve as a centre for the reflex movements, performed through the nerves that issue from it. In both respects it corresponds precisely with any segment of the Spinal Cord'itself; and there is no REFLEX ACTIONS OF MEDULLA OBLONGATA. 509 reason to believe, that it possesses any other or more special endow- ments. The importance, however, of the reflex acts of Respiration and Deglutition, over which it presides, causes this portion of the Medulla to be the one whose integrity is most essential to the pre- servation of life; and therefore it seems to possess a character more distinctive than it really has. 895. The chief excitor nerve of the respiratory movements, as al- ready stated (§§ 685-687) is the afferent portion of the Par Vagum; but the afferent portion of the Fifth pair is also a powerful excitor; and the afferent portions of all the spinal nerves, conveying impres- sions from the general surface of the body, are also capable of con- tributing to the excitement necessary for the production of the move- ment.—The chief motor nerves are the phrenic and intercostals; which, though issuing from the Cord at a considerable space lower down, probably originate in the Medulla oblongata. The motor portions of several other spinal nerves are also partly concerned ; as are also the Facial nerve, the motor portion of the Par Vagum, and the Spinal Accessory. The ordinary movements of Respiration involve little action of any motor nerves but the Phrenic and Inter- costal; and it is only when an excess of the stimulus (produced for example by too long a suspension of the aerating process) excites extraordinary movements, that the nerves last enumerated are called into action. 896. The acts of Prehension of food with lips, and of Mastication, though usually effected by voluntary power in the adult, seem to be capable of taking place as a part of the reflex operation of the Me- dulla Oblongata, in the Infant, as in the lower animals. This is particularly evident in the prehension of the nipple by the lips of the infant, and the act of suction wrhich the contact of that body (or of any resembling it) seems to excite. The experiments provided for us by nature, in the production of anencephalous monstrosities, fully prove that the integrity of the nervous connection of the lips and respiratory organs with the Medulla Oblongata, is alone sufficient for the performance of this action; and experiments upon young animals, from which the brain has been removed, establish the same fact. Thus Mr. Grainger found that, upon introducing his finger, moistened with milk, or with sugar and wTater, between the lips of a puppy thus mutilated, the act of suction was excited; and not merely the act of suction itself, but other movements having a relation to it; for as the' puppy lay on its side, sucking the finger, it pushed out its feet, in the same manner as young pigs exert theirs in compressing the sow's dugs. This action seems akin to many of those by which the lower animals take in their food ; and we may thus recognize in the Medulla Oblongata a distinct centre of reflex action for the reception and deglu- tition of aliment, analogous to the stomato-gastric ganglia of Inverte- brated animals. 897. In the movements of Deglutition, which, as formerly explained, (§ 453,) are purely reflex, the chief excitor is undoubtedly the ^tffe- 510 GANGLIA OF SPECIAL SENSE IN MAN. rent portion of the Glosso-pharyngeal nerve. It is found that, if the trunk of this nerve, or its pharyngeal (but not its lingual) branches, be pinched, pricked, or otherwise irritated, whilst still in connection with the Medulla Oblongata, the movements concerned in the act of swallowing are excited. The same occurs if, when the trunk of the Glosso-pharyngeal has been divided, the cut extremity in connection with the Medulla Oblongata is irritated ; but little or no muscular contraction is produced by irritation of the separated extremity ; whence it is apparent, that the Glosso-pharyngeal has little or no di- rect motor power, but acts as an excitor. In this it appears to be assisted by the branches of the Fifth pair distributed upon the fauces ; and probably, also, by the branches of the Superior Laryngeal distri- buted upon the Pharynx. The motor influence, which is generated in respondence to the stimulus thus conveyed, appears to act chiefly through the branches of the Par Vagum, which are distributed to most of the muscles concerned in swallowing; but the Facial, the Hypo- glossal, the motor portion of the Fifth, and perhaps also the motor portions of some of the Cervical nerves, are also concerned in the movement, and may effect it, though with difficulty, after the pha- ryngeal branches of the Par Vagum have been divided. 898. In the propulsion of the food down the Oesophagus, to which the glosso-pharyngeal nerve does not extend, the muscular contrac- tion, so far as it is of a reflex nature, (§ 455,) must depend upon the oesophageal branches of the Par Vagum alone ; their afferent portion being the excitor, and their motor portion giving the requisite stimu- lus to the muscles. The same must be the case in regard to the mus- cular contractions of the cardiac and pyloric sphincters, and of the walls of the stomach, so far as regards their dependence upon the nervous system at all; but the degree of this is doubtful. 899. There are other reflex actions of the Medulla oblongata, con- nected with the regulation of the aperture of the Glottis; these, which are effected through the superior and inferior laryngeal branches of the Par Vagum, will be better noticed when the actions of the La- rynx are under consideration.—In like manner, the reflex action con- cerned in the regulation of the aperture of the Pupil will be more conveniently noticed in the sketch to be presently given of the Physi- ology of Vision. 5. Functions of the Sensory Ganglia. 900. All the nerves of Sensation, both general and special, may be traced into a series of ganglionic masses lying at the base of the brain ; which seem to constitute their own particular centres. Thus we have seen in Fishes, the Olfactive, Optic, and Auditory ganglia, marked out as such, by the termination of the nerves proceeding from the organs of smell, sight, and hearing, in these masses respectively. These ganglia bear an evident correspondence with the cephalic gan- glia of the Invertebrata; which must chiefly, however, be regarded t GANGLIA OF SENSE IN MAN. 511 as optic ganglia, since the development of the eyes far surpasses that of the other organs of special sense. On the other hand, they find their representatives in certain organs at the base of the brain, in Man and the higher Mammalia; which, though small in proportion to the whole Encephalon, are capable of being clearly marked out, as the ganglionic centres of the several nerves of sense. Thus, anteriorly, we have the Olfactive ganglia, in what are commonly termed the bul- bous expansions of the Olfactive nerve; which, however, are real ganglia, containing gray or vesicular substance ; and their separation from the general mass of the Encephalon, by the peduncles or foot- stalks commonly termed the trunks of the olfactory nerves, finds its analogy in many species of Fish (§ 869). The ganglionic nature of these masses is more evident in many of the lower Mammalia, in which the organ of smell is highly developed, than it is in Man, whose olfactive powers are comparatively moderate.—At some distance be- hind these, we have the representatives of the Optic Ganglia, in the Tubercula Quadrigemina, to which the principal part of the roots of the Optic nerve may be traced. Although these bodies are so small in Man as to be apparently insignificant, yet they are much larger, and form a more evidently important part of the Encephalon, in many of the lower Mammalia; though still presenting the same general aspect.—The Auditory ganglia seldom form distinct lobes or projec- tions ; but are usually lodged in the substance of the Medulla Ob- longata. Their real character is most evident in certain Fishes, as the Carp ; in which we find the Auditory Nerve having as distinct a ganglionic centre as the Optic. In higher animals, however, we are able to trace the Auditory nerve into a small mass of gray matter, which lies on each side of the fourth Ventricle; and although this is lodged in the midst of parts whose function is altogether different, yet there seems no reason for doubting that it has a character of its own, and that it is really the ganglion of the auditory nerve.—We are not able to fix upon any such mass of gray matter as the distinct Gusta- tory ganglion ; nor is it necessary to attempt to do so; for, as we shall see hereafter, there is strong reason to regard the sense of Taste as only a refined kind of Touch. 901. At the base of the Cerebral Hemispheres, we find two gan- cdionic masses on either side, through which all the fibres pass that connect the Hemispheres with the Medulla Oblongata. These are the Corpora Striata, and Thalami Optici. Upon tracing forwards the tract of fibres that ascends from the anterior Pyramids, we find it passing chiefly into the Corpora Striata; whilst, if we follow the Olivary column, we shall find it to enter the Thalami. The anterior continuations of these two columns together form the Crura Cerebri, or peduncles of the Cerebrum; and the relative functions of the two layers of which it is composed (which may be very readily isolated) are clearly indi- cated by the characters of the nerves that are respectively connected with them. Thus along the tract that passes from the anterior Pyramids to the Corpora Striata, we have none but motor nerves; 512 THALAMI OPTICI, AND CORPORA STRIATA. Fig. 144. whilst along the tract that connects the Olivary columns with the Thalami, there are none but sensory nerves. The fibres of the Crura Cerebri, after entering these masses, seem to radiate towards all parts of the surface of the hemi- spheres, at whose base they are situated ; but some of them probably find a ganglionic centre in these bodies them- selves ; since their substance contains a considerable amount of gray matter. It may be regarded as not improbable, then, that we may consider the Thalami as the ganglionic centres of common sensation; standing in the same relation to the sensory nerves, that converge from various parts of the body towards the En- cephalon, as do the Optic and other ganglia to their nerves of special sensation. And as these last give origin (as will be presently shown) to motor fibres, so may we regard the ganglionic matter of the Cor- pora Striata as probably shar- ing in the same function ; giv- ing origin to the motor fibres, which produce the respondent consensual movements;—just as the anterior peak of gray matter in the Spinal Cord gives exit to the motor fila- ments, which effect the reflex movements excited through the afferent fibres that form part of the posterior roots.— It must be remembered, how- ever, that a large proportion of the fibres, that can be traced from the Medulla Oblongata, through the Crura Cerebri, into the Thalami Optici and Corpora Striata, pass through these latter masses, to become continuous with the fibres radiating from them to the surface of the Cerebral hemispheres. Upon these, there is no reason to believe that their ganglionic matter exerts any influence. 902. The functions of this group of ganglia may be partly inferred from the results of experiments; and these have been chiefly made The base of the Brain, upon which several sections have been made, showing the distribution of the diverg- ing fibres. 1. The medulla oblongata. 2. One-half of the pons Varolii. 3. The crus cerebri crossed by the optic nerve (4). and spreading out into the hemisphere to form the corona radiata. 5. The optic nerve near its origin. 6. The olfactory nerve. 7. The corpora albi- cantia. On the right side a portion of the brain has been removed to show the distribution of the diverging fibres. 8. The fibres of the corpus pyramidale passing through the substance of the pons Varolii. 9. The fibres passing through the thalamus opticus. 10. The fibres passing through the corpus striatum. 11. Their distri- bution to the hemispheres. 12. The fifth nerve; its two roots may be traced, the one forwards to the fibres of the corpus pyramidale, the other backwards to the fasciculi teretes. 13. The fibres of the corpus pyrami- dale, which pass outwards with the corpus restiforme into the substance of the cerebellum; these are the arci- form fibres of Solly. The fibres referred to are those below the numeral, the numeral itself rests upon the corpus olivare. 14. A section through one of the hemi spheres of the cerebellum, showing the corpus rhom- boideum in the centre of its white substance; the arbor vitae is also beautifully seen. 15. The opposite hemi- sphere of the cerebellum. FUNCTIONS OF SENSORY GANGLIA. 513 upon the Optic ganglia, or Corpora Quadrigemina. The partial loss of the ganglion on one side produces temporary blindness in the eye of the opposite side, and partial loss of muscular power on the oppo- site side of the body ; and the removal of a larger portion, or the com- plete extirpation of it, occasions permanent blindness and immobility of the pupil, and temporary muscular weakness, on the opposite side. This temporary disorder of the muscular system sometimes manifests itself in a tendency to move on the axis, as if the animal were giddy; and sometimes in irregular convulsive movements.—Here, then, we have proof of the necessity of the integrity of this ganglionic centre, for the possession of the sense of vision; and we have further proof, that the ganglion is connected with the muscular apparatus, by motor nerves issuing from it. The reason why the eye of the opposite side is affected, is to be found in the decussation of the optic nerves;—a point to be immediately adverted to (§910). The influence of the operation on the muscles of the opposite side of the body, is at once understood from the fact of the decussation of the motor fibres in the anterior pyramids (§ 890). 903. Thus we see, that the Optic ganglia receive the impressions brought from the eyes by the optic nerves,—convert them, as it were into sensations,—and also transmit motor impulses to the muscular system, in respondence to those sensations. Thus they have much analogy to the cephalic ganglia of the lower animals; the greater part of whose purpose seems to be, to guide the actions of the beings to which they belong, through the sensations which they receive (§ 860). But, with a function that is probably the same, there is this important difference, as to the purpose served by these parts, in the Encephalon of Man, and of the animals that approach nearest to him in the conformation of his nervous centres. The Consensual or In- stinctive movements, which make up nearly the whole of those actions in the Invertebrata that are not simply reflex, constitute a compara- tively small proportion of the actions of the higher Vertebrata; these being guided in a much greater degree by Intelligence, which reasons upon the sensations, and devises means to gratify the desires created by them. Consequently there is reason to think, that the direct ac- tion'of the sensory ganglia upon the muscles is comparatively seldom exercised, in the active condition of the Cerebrum. There are certain actions, however, which would seem to take place regularly through this channel. Thus the consensual movements of the eyes, which concur to direct their axis towards the same object, appear to depend upon the impressions made upon the retinae ; for we do not see these movements taking place with nearly the same exactness in the eyes of persons who have been born totally blind ; and in those who have completely lost their sight, after having enjoyed the power of vision, we may always perceive that, although the two eyes usually move consentaneously from habit, yet that their axes are parallel, instead of convergent; so that they do not seem to look at any object, but beyond it, into vacancy. 33 514 MUSCULAR SENSE.—CONSENSUAL ACTIONS. 904. The existence of a Sensation of some kind, in connection with a Muscular exertion, seems essential to the continuance of the latter. Our ordinary movements are guided by what is termed the Muscular Sense; that is, by a feeling of the condition of the muscle, that comes to us through its own sensory nerves. How necessary this is to the exercise of muscular power, may be best judged of from cases in which it has been lost. Thus a woman, who had suffered complete loss of sensation in one arm, but who retained its motor power, found that she could not support her infant upon it, without constantly looking at the child; and that, if she were to remove her eyes for a moment, the child would fall, in spite of her knowledge that her infant was resting upon her arm, and of her desire to sustain it. Here, the muscular sense being entirely deficient, the sense of vision supplied what was deficient, so long as it was exercised upon the object; but as soon as this guiding influence was withdrawn, the strongest will could not sustain the muscular contraction.—Again, in the production of vocal sounds, the nice adjustment of the muscles of the larynx, which is requisite to produce determinate tones, can only be effected in obedience to a mental conception of the tone to be uttered ; and this conception cannot be formed, unless the sense of hearing has previously brought similar tones to the mind. Hence it is, that persons who are born deaf, are also dumb. They may have no malformation of the organs of speech; but they are incapable of uttering distinct vocal sounds or musical tones, because they have not the guiding conception, or recalled sensation, of the nature of these. By long training, and by efforts directed by the muscular sense of the larynx itself, some persons thus circumstanced have acquired the power of speech; but the want of sufficiently definite control over the vocal muscles, is always very evident in their use of the organ. 905. Hence, although the proper consensual actions of Man and of the higher animals are comparatively few, (the wants which these are destined to supply in the lower, being in them provided for by the exercise of intelligence,) we see that not even the proper voluntary movements can be effected, without the influence of guiding sensa- tions, felt or conceived.—There are several actions, in regard to which it does not seem easy to say with certainty, whether they are of a simply reflex nature, or whether sensation is a necessary link in the series of changes which they involve. Such are, the act of vom- iting, produced by various causes that excite nausea,—for example, by tickling the fauces with a feather; or the acts of coughing and sneezing, excited by irritation in the air-passages. In regard to these last it may be observed, that although the ordinary movements of Respiration are undoubtedly of a purely reflex character, yet it seems uncertain, whether those of an extraordinary nature can be excited by an impression that is not felt. The act of sneezing is usually excited by an impression upon the 5th pair; but it may result from the action of a strong light upon the eyes, and cannot then be excited, unless CONSENSUAL ACTIONS IN MAN. 515 this produces the sensation of dazzling.—There are numerous cases, again, in which painful sensations appear to produce or to modify movement, in a manner that is altogether involuntary. Thus in cases of excessive irritation of the retina, rendering the eye most painfully sensitive to even a feeble amount of light, the eyelids are drawn together spasmodically; and, if they be forcibly opened, the pupil is frequently rolled beneath the upper lid, much farther than it could be carried by a voluntary effort. And in pleuritis, pericarditis, and other painful affections of the parietes of the chest, we may observe the usual movements of the ribs to be very much abridged ; and if the affection be confined to one side, there is a marked curtailment in its movements, whilst those of the other side may take place as usual,—a difference which cannot be imitated by a voluntary effort. 906. Various other facts might be adduced, to show that, in Man, certain movements are as intimately and necessarily connected with the excitement of sensations in the sensory ganglia, as others are with the production of impressions in the ganglia of reflex action. And it maybe further questioned, in the absence of any precise knowledge of the subject, whether the emotions, when so strongly excited as to act involuntarily on the body, do not operate through this group of ganglia and the fibres proceeding from them. There are many ana- logies between the purely emotional actions of Man, and the instinctive movements of the lower animals; each following closely upon sensa- tions, without any exercise of the reasoning faculty ; and each being performed, not merely without the mandate of the will, but often in direct opposition to it. That the Emotions, when they thus affect the body, do not operate through the same set of nervous fibres as those which convey the influence of the Will, seems proved by this fact,— that cases have occurred, in which muscles have been paralyzed to the Will, whilst they remained obedient to the Emotions; and vice versa. Thus, in one instance, the muscles of one side of the face were palsied in such a manner, that the individual could not volun- tarily close his eye, nor draw his mouth towards that side; yet when any ludicrous circumstance caused him to laugh, their usual play was manifested in the expression of his countenance. And in another case, the muscles were obedient to the will; but when the individual laughed or cried under the influence of an emotion, it was only on one side of his face. To these may be added another case, in which the right arm was completely palsied, so that the individual had not the least voluntary power over it; yet it was violently agitated, whenever he met a friend whom he desired to greet. 907. These and similar cases afford sufficient proof, that the direct influence of the Emotions on the Muscular System operates through a channel distinct from that, which conveys the influence of the Will; and when we consider how closely the Emotions are connected with the sensations which excite them, and their close analogy with the instincts of the lower animals, there seems a strong presumption in favour of the idea, that the motor nerves proceeding from the sensory 516 CONSENSUAL ACTIONS IN MAN. ganglia constitute their peculiar instrument of operation on the body. A very characteristic example of the immediate dependence of the actions of this class upon Sensation, is afforded by the peculiar move- ments which are excited by the act of tickling. No one can question the completely involuntary nature of these movements; on the other hand, they are not reflex, for they do not take place unless the irrita- tion is felt. They strictly belong, therefore, to the consensual group we are at present considering. Now the tickling may produce, not merely a variety of semi-convulsive movements, tending'to withdraw the body from the source of irritation, but also a tendency to laughter, and an emotional state connected with it. But it would appear that the semi-convulsive movements are immediately excited, not by the emotion, but by the sensation. For there is a great variation amongst different individuals, as to the results of the irritation; the action of laughter being excited in some, without any other effect; whilst in others, spasmodic movements of the extremities take place without any tendency to laughter, indeed with a feeling of extreme distress. 908. The influence of an excited state of the emotional system of nerves, is very strongly marked in various disordered states of the system; and particularly, as already remarked, in Hydrophobia and Hysteria (§§ 886, 887). In both these diseases, violent convulsive paroxysms are brought on, by causes that produce particular sensa- tions, or emotions consequent upon them. The tendency to imitation is a most powerful cause in Hysterical subjects; the mere sight of a paroxysm in one young female, being often sufficient to produce a similar attack in a whole room-full of her companions. And there are some persons who possess the power of commanding an hysterical paroxysm at will; not by voluntarily executing the convulsive actions themselves; but by " getting up" the particular emotional condition on which it depends. There can be no doubt that many of the pecu- liar movements exhibited by the subjects of Mesmeric phenomena, are the result of a condition of this nature. There appears to be, in such persons, an excessive activity of the consensual and emotional system; so that very slight impressions may produce very powerful effects ; especially when favoured by the strong desire, on the part of the patient, to exhibit any peculiar manifestation, that is known to be expected on the part of the bystanders,—a desire which keeps the emotional system in a state of tension, and renders it peculiarly respon- sive to any external influence. 909. Quitting now the functions of the Sensory ganglia, we have briefly to notice certain peculiarities in the characters of the nerves which issue from them. And of these peculiarities, there is one of a very remarkable nature, which is common to the three nerves of special sense,—namely, the Olfactive, Optic, and Auditory ;—that they are not in the least degree endowed with common sensibility ; so that they may be cut, stretched, pinched, &c, without producing the least pain. Consequently, the ordinary sensibility of the surfaces they supply is entirely due to the branches of the Fifth pair, which are distributed DECUSSATION OF THE OPTIC NERVES. 517 upon them ; and we may have a loss of either the general or the special sensibility' of any of the organs of sense, without the other being affected, save indirectly.—Again, we do not find that irritation of these nerves produces any other purely reflex movements, than such as are connected with the operations of the organs of sense, in which they respectively originate. Thus the Olfactory nerve cannot, by any irritation, be made to excite a reflex movement; the only reflex action that can be excited by irritating the Optic nerve, is contraction of the Pupil; and the regulation of the tension of the Membrana Tympani (if, as'is probable, this is effected by the motor power of the Facial nerve, excited by impressions made upon the organ of sense,) appears to be the only reflex action to which the Auditory nerve can mi- nister. 910. There is a further peculiarity, of a very marked kind, attend- ing the course of the Optic nerves ; this is the crossing or decussation which they undergo, more or less completely, whilst proceeding from their ganglia to the eyes. In some of the lower animals, in which the two eyes (from their lateral position) have entirely different spheres of vision, the decussation is complete ; the whole of the fibres from the right optic ganglion passing into the left eye, and vice versa. This is the case, for example, with most of the Osseous Fishes (as the cod, halibut, &c.) ; and also, in great part at least, with Birds. In the Human subject, however, and in animals which, like him, have the two eyes looking in the same direction, the decussation seems less complete ; but there is a very remarkable arrangement of the fibres, which seems destined to bring the two eyes into peculiarly consenta- neous action. The posterior border of the optic Chiasma is formed exclusively of commissural fibres, which pass from one optic ganglion to the other, without entering the real optic nerve. Again, the ante- rior border of the chiasma is composed of fibres, which seem, in like manner, to act as a commissure between the two retina ; passing from one to the other, without any connection with the optic ganglia. The tract which lies between the two borders, and occupies the middle of the chiasma, is the true optic nerve ; and in this it would appear that a portion of the fibres decussates, whilst another portion passes directly from each Optic ganglion into the'corresponding eye. The fibres which proceed from the ganglia to the retina?, and constitute the proper optic nerves, may be distinguished into an internal and an external tract. Of these, the external, on each side, passes directly onwards to the eye of that side ; whilst the internal crosses over to the eye of the opposite side. The distribution of these two sets of fibres in the retina of each eye respectively, is such that, according to M. Mayo, the fibres from either optic ganglion will be distributed to its' own side of both eyes;—the right optic ganglion being thus exclusively connected with the outer part of the retina of the right eye, and with the inner part of the retina of the left eye ; and the left optic ganglion being, in like manner, connected exclusively with the outer side of the left retina, and with the inner side of the right. Now 518 FUNCTIONS OF THE CEREBELLUM. as either side of the eye receives the images of objects, which are on the other side of its axis, it follows, if this account of their distribution be correct, that in Man, as in the lower animals, each ganglion receives the sensations of objects situated on the opposite sides of the body. The purpose of this decussation may be, to bring the visual impres- sions, which are so important in directing the movements of the body, into proper harmony with the motor apparatus ; so that, the decussa- tion of the motor fibres in the pyramids being accompanied by a decussation of the optic nerves, the same effect is produced as if neither decussated,—which last is the case with Invertebrated ani- mals in general. 6. Functions of the Cerebellum. 911. Much discussion has taken place, of late years, respecting the uses of the Cerebellum ; and many experiments have been made to determine them. That it is in some way connected with the powers of motion, might be inferred from its connection with the antero- lateral columns of the Spinal Cord, as well as with the posterior ; and the comparative size of the organ, in different orders of Vertebrated animals, gives us some indication of what the nature of its function may be. For we find its degree of development corresponding pretty closely with the variety and energy of the muscular movements which are habitually executed by the species; the organ being the largest in those animals, which require the combined effort of a great variety of muscles to maintain their usual position, or to execute their ordi- nary movements ; whilst it is the smallest in those, which require no muscular exertion for the one purpose, and little combination of dif- ferent actions for the other. Thus in animals that habitually rest and move upon four legs, there is comparatively little occasion for any organ to combine and harmonize the actions of their several muscles; and in these, the Cerebellum is usually small. But among the more active predaceous Fishes (as the Shark),—Birds of the most powerful and varied flight (as the Swallow),—and such Mammals as can main- tain the erect position, and can use their extremities for other purposes than support and motion,—we find the Cerebellum of much greater size, relatively to the remainder of the Encephalon. There is a marked advance in this respect, as we ascend through the series of Quadrumanous animals; from' the Baboons, which usually walk on all-fours, to the semi-erect Apes, which often stand and move on" their hind-legs only. The greatest development of the Cerebellum is found in Man; who surpasses all other animals in the number and variety of the combinations of muscular movement, which his ordi- nary actions involve, as well as of those which he is capable, by practice, of learning to execute. 912. From experiments upon all classes of Vertebrated animals, it has been found that, when the Cerebellum is removed, the power of walking, springing, flying, standing, or maintaining the equilibrium of the body, is destroyed. It does not seem that the animal has in REGULATION OF MOVEMENTS. 519 any degree lost the voluntary power over its individual muscles; but it cannot combine their actions for any general movements of the body. The reflex movements, such as those of respiration, remain unim- paired. When an animal thus mutilated, is laid on its back, it cannot recover its former posture ; but it moves its limbs, or flutters its wings, and evidently is not in a state of stupor. When placed in the erect position, it staggers and falls like a drunken man,—not, however, without making efforts to maintain its balance. Phrenologists, who attribute a different function to the Cerebellum, have attempted to put aside these results, on the ground that the severity of the operation is alone sufficient to produce them; but as we shall presently see, many animals may be subjected to a much more severe operation,— the removal of the Cerebral hemispheres,—without the loss of the power of combining and harmonizing the muscular actions, provided the Cerebellum be left uninjured.—Thus, then, the idea of the func- tions of the Cerebellum, which we derive from Comparative Anatomy, seems fully borne out by the results Of experiment; and it is also consistent with the indications, which may be drawn from the obser- vations of Pathological phenomena. When the Cerebellum is affected with chronic disease, the motor function is seldom destroyed; but the same kind of want of combining power shows itself, as when the organ has been purposely mutilated. Some kind of lesion of the motor function is invariably to be observed ;- whilst the mental powers may or may not be affected,—probably according to the influence of the disease in the Cerebellum upon other parts. The same absence of any direct connection with the Psychical powers, is shown in the fact, that inflammation of the membranes covering it, if confined to the Cerebellum, does not produce delirium. Sudden effusions of blood into its substance may produce apoplexy or paralysis; but this may occur as a consequence of effusions into any part of the Ence- phalon, and does not indicate, that the Cerebellum has anything to do with the mental functions, or with the power of the Will over the muscles. . 913. There is another doctrine, however, in regard to the tunctions of theCerebellum, first propounded by Gall; which ought not to be altogether passed by. According to the system of Phrenologists, the Cerebellum is the organ of the sexual instinct; and its connection with the motor function is limited to the performance of the movements, to which that instinct leads. This doctrine derives no support, however, from the facts supplied by Comparative Anatomy; for there is a com- plete want of correspondence between the size of the Cerebellum in different animals, and the power of their sexual instinct.—Again, although pathology has been appealed to, as showing a decided con- nection between disease of the Cerebellum and affection of the Genital organs, (manifesting itself in priapism, turgescence of the testes, seminal emissions, &c.,) yet it appears, on a careful examination of evidence that such a sympathy is comparatively rare, not being displayed in more than one out of every seventeen cases of Cerebellic disease. And 520 SEXUAL INSTINCT.—FUNCTIONS OF THE CEREBRUM. where it is manifested, it is explicable quite readily by the known fact that this kind of excitement of the genital organs may be pro- duced by excitement of the spinal cord and medulla oblongata.—Little or no light has been thrown on this question by experiment. It was asserted by Gall, that the Cerebellum is very small in castrated ani- mals; but this assertion has been met by the most positive counter- statements on the part of Leuret, who has shown that the average weight of the Cerebellum (both absolutely, and in proportion to the weight of the entire encephalon), is even greater in Geldings than in Stallions or Mares.—It is asserted, however, that the results of ob- servation in Man lead to a positive conclusion, that the size of the Cerebellum is a measure of the intensity of the sexual instinct in the individual. This assertion has been met by the counter-statement of others,—that no such relation exists. There are, of course, very great difficulties in regard to the collection of accurate information on this subject; and the question must be at present regarded as sub judice. 914. It may be added, that the idea of a special connection between the sexual instinct and the Cerebellum, is not inconsistent with the view of its function previously stated; and it would seem to derive some confirmation from the fact that an unusual amount of muscular exertion appears to have a peculiar tendency to depress the sexual passion even whilst it increases the general vigour of the system. If the Cerebellum be really connected with both kinds of functions, it does not seem unlikely that the excessive employment of it upon one, should diminish its energy in regard to the other. Further, it seems not improbable that the Lobes of the Cerebellum are the parts spe- cially concerned in the regulation of the muscular movements ; whilst the central portion (constituting the Vermiform process in Man, but forming the entire cerebellum of many of the lower Vertebrata, such as the Frog), may be the centre of the sexual sensations, and the instrument of the consensual actions to which they give rise. 7. Functions of the Cerebrum. 915. The view which has been taken of the Comparative structure of the Nervous system, in different animals, leads to the conclusion, that the Cerebral hemispheres are far from being the essential parts of the apparatus they were formerly imagined to be ; and that they are on the contrary, superadded organs, of which we find no distinct re- presentatives in the Invertebrata, and of which the first appearance (in the class of Fishes) exhibits them in the light of appendages, de- stined to perform some special function peculiar to Vertebrated animals. The results of the removal of the Cerebral Hemispheres, in animals to which the shock of the operation does not prove immediately fatal, fully confirm this view ; and must appear extraordinary to those, who have been accustomed to regard these organs as the centre of all en- ergy. Not only Reptiles, but Birds and Mammalia, if their physical RELATIVE DEVELOPMENT OF THE CEREBRUM. 521 wants be supplied, may survive the removal of the whole Cerebrum for weeks, or even months. If the entire mass be taken away at once, the operation is usually fatal; but if it be removed by successive slices, the shock is less severe, and the depression it produces in the organic functions is soon recovered from. It is difficult to substantiate the existence of actual sensation, in animals thus circumstanced ; but their movements appear to be of a higher kind than those resulting from mere reflex action. Thus they will eat food when it is put into their mouths; although they do not go to seek it. One of the most remarkable phenomena of such beings, is their power of maintaining their equilibrium; which could scarcely exist without consciousness. If a Rabbit, thus mutilated, be laid upon its back, it rises again; if pushed, it walks; if a Bird be thrown into the air, it flies; if a Frog be touched, it leaps. If violently aroused, the animal has all the manner of one waking from sleep; and it manifests about the same degree of consciousness as a sleeping Man, whose torpor is not too profound to prevent his suffering from an uneasy position, and who moves himself to amend it. In both cases, the movements are con- sensual only, and do not indicate any voluntary power; and we may well believe that, in the former case as in the latter, though felt, they are not remembered; an active state of the Cerebrum being essential to memory, though not to sensations, which simply excite certain actions. 916. As already stated, the relative amount of Intelligence in dif- ferent animals bears so close a correspondence with the relative size and development of the Cerebral Hemispheres, that it can scarcely be questioned that these constitute the organ of the Reasoning faculties, and issue the mandates by which the Will calls the muscles into action. It must be borne in mind, however, that size is not by any means the only indication of their comparative development. As we advance from the lower to the higher Vertebrata, we observe a marked advance in the complexity of the structure of the Cerebrum. Its surface becomes marked by convolutions, that greatly increase the area over which blood-vessels can enter it from the surrounding membranes; and in proportion to the increase in the number and depth of these, do we find an increase in the thickness of the layer of gray matter, which is the source of all the powers of the organ. The arrangement of the white or fibrous tissue, which forms the interior of the mass, also in- creases in complexity; and as we ascend even from the lower Mam- malia up to Man, we trace a marked increase in the number of the fibres, which establish communications between different parts of the organ. It is, in fact, not merely from the different parts of the gray matter, which forms the surface of the hemispheres, that these com- missural fibres arise; but also from those isolated portions of vesicular substance, which are found in different parts of their interior ; and an extremely complex system is thus formed, which is still but very im- perfectly understood. 917. The two hemispheres are united on the median line by several 522 COMMISSURES OF THE CEREBRAL HEMISPHERES. transverse Commissures; of which the Corpus Callosum is the most important. This consists of a mass of fibres very closely interlaced together; which may be traced into the substance of the hemispheres on each side, particularly at their lower part, where they are connected with the thalami optici and corpora striata. It is difficult, if not im- possible, to trace its fibres any further; but there can be little doubt that they radiate, with the fibres proceeding frora the bodies just named, to the different parts of the surface of the hemispheres. This commis- sure is altogether absent in Fish, Reptiles, and Birds; and it is par- tially or completely wanting in the Mammalia with least perfect brains,—as the Rodents and Marsupials.—The anterior commissure Fig. 145. The mesial surface of a longitudinal section of the brain. The incision has been carried along the middle line ; between the two hemispheres of the cerebrum, and through the middle of the cerebellum and medulla oblongata. 1. The inner surface of the left hemisphere. 2. The divided surface of the cerebellum, showing the arbor vitse. 3. The medulla oblongata. 4. The corpus callosum. curving downwards in front to terminate at the base of the brain; and rounded behind, to become continuous with 5, the fornix. 6. One of the crura of the fornix descending to 7. one of the corpora albicantia. 8. The septum lucidum. 9. The velum interpositum. communicating with the pia mater of the convolu- tions througli the fissure of Bichat. 10. Section of the middle commissure situated in the third ven- tricle. 11 Section of the anterior commissure. 12. Section of the posterior commissure; the commis- sure is somewhat above and to the left of the numeral. The interspace between 10 and 11 is the foramen commune anterius, in which Ihe crus of the fornix (6) is situated. The interspace between 10 and 12 is the foramen commune posterius. 13. The corpora quadrigemina, upon which is seen resting the pineal gland, 14. 15. The iter a tertio ad quartum ventriculum, or aqueduct of Sylvius. 16. The fourth ventricle. 17. The pons Varolii, through which are seen passing the diverging fibres of the corpora pyramidalia. 18. The crus cerebri of the left side, with the third nerve arising from it. 19. The tuber cinereum, from which projects the infundibulum, having the pituitary gland appended to its extremity. 20. One of the optic nerves. 21. The left olfactory nerve terminating anteriorly in a rounded bulb. particularly unites the corpora striata of the two sides; but many of its fibres pass through those organs, and radiate towards the convolu- tions of the hemispheres, especially those of the middle lobe. This commissure is particularly large in those Marsupials, in which the Corpus Callosum is deficient.—The posterior commissure is a band of fibres which connects the optic thalami; crossing over from the poste- rior extremity of one to that of the other.—Besides these, there are other groups of fibres, which seem to have similar commissural func- tions, but which are intermingled with vesicular substance. Such are the soft commissure, which also extends between the thalami; the Pons Tarini, which extends between the two crura or peduncles of the cerebrum; and the Tuber cinereum, which seems to unite the FUNCTIONS OF THE CEREBRUM. 523 optic tracts with the thalami, the corpus callosum, the fornix, &c, and to be a common point of meeting for several distinct groups of fibres. 918. The anterior and posterior parts of the hemispheres, more- over, are connected by longitudinal Commissures; of which some lie above, and some below, the corpus callosum. Above the transverse fibres of the corpus callosum, there is a longitudinal tract on each side of the median line, which serves to connect the convolutions of the anterior and posterior lobes of the brain.—And above this, again, is the superior longitudinal commissure, which is formed by the fibrous matter of the great convolution nearest the median plane on the up- per surface of the brain, and which connects the convolutions of the anterior and middle lobe with those of the posterior.—Beneath the great transverse commissure, we find the most extensive of all the longitudinal commissures, namely, the fornix. This is connected in front with the optic thalami, the mammillary bodies, the tuber cine- reum, &c; and behind, it spreads its fibres over the hippocampi (major and minor), which are nothing else than peculiar convolutions that project into the posterior and descending cornua of the lateral ventri- cles.—The fourth longitudinal commissure is the tceniasemicircularis, which forms part of the same system of fibres with the fornix ; con- necting the corpus mamrailare and thalamus opticus with the middle lobe of the cerebral hemisphere.—If, as Dr. Todd has remarked, we could take away the corpus callosum, the gray matter of the internal convolution, and the ventricular prominence of the optic thalami, then all these commissures fall together, and become united as one and the same series of longitudinal fibres. 919. Besides these, it is probable that the different convolutions have their own commissural fibres uniting them with each other, as well as their radiating fibres connecting them with the thalami optici and corpora striata ; but these have not been certainly demonstrated. It is curious that there should be no direct communication between the Cerebral hemispheres and the Cerebellum ; the only commissural band between them being the processus a cerebello ad testes, which passes onwards, through the tubercula quadrigemina, to the thalamus opticus on each side. This would seem to confirm the idea of the complete distinctness of their functions. 920. Very little light can be thrown by experiment upon the func- tions of the several parts of the Cerebral hemispheres, or of the gan- glionic masses with wThich they are so intimately connected. In the experiments already referred to, in which the hemispheres were en- tirely removed, slice by slice, it was noticed that injuries of these organs neither occasion any signs of pain, nor give rise to convulsive movements. Even the thalami and corpora striata may be wounded, without the excitement of convulsions; whilst, if the incisions involve the tubercula quadrigemina, convulsions uniformly occur. It has been often observed in Man, that, when it has been necessary to separate protruded portions of the brain from the healthy part, no sensation 524 SLEEP; COMA. was produced, even though the mind was perfectly clear at the time. Hence it would appear that neither is the Cerebrum itself the centre of sensation, nor is it so connected with that centre, as to be able to convey to it sensory impressions of an ordinary kind. This is analo- gous to the condition of the nerves of special sense, as already re- marked. That no irritation of the cerebral substance should excite convulsive movements, is a very remarkable circumstance; and it seems to indicate, that the changes which mental operations produce in the cerebral fibres, cannot be imitated, as changes in other motor fibres may be, by physical impressions. 921. There are various conditions, some of them natural, others morbid, in which the distinctness of the functions of the Cerebral He- mispheres is well marked. Thus in profound sleep, they seem to be entirely dormant; the Spinal system, by which the necessary reflex actions are carried on, being alone in a state of activity. In this con- dition, the Sensory ganglia also appear to be in a torpid state ; but in less profound sleep, actions are often performed, which may be referred to the consensual group,—being such as the sensation would imme- diately prompt, without any reflection, and not being remembered in the waking state. Thus we turn in our beds, under the influence of an uneasy sensation ; or we give some sign of recognition when our names are called. The first of these appears to be a purely consen- sual movement, being as automatic as if it were a reflex action ; the other seems to have become as automatic, by the influence of habit, and to belong to that class, in which the mind was at first involved, but in which, after very frequent performance, the sensation suggests the action so immediately and invariably, that the action seems to take place without any concern on the part of the will. Of these secondary automatic actions, as they are termed, we have many examples in that condition, in which the mind, though active, is so completely absorbed by some train of thought, as to be in a state of revery, and to be in- sensible to external objects ; in such a condition, the individual may continue reading aloud, playing a piece of music, or performing any other action, in which the muscular movements are immediately directed by the sensations ; but he cannot carry on twTo distinct and independent trains of thought. 922. In the Coma of Apoplexy, Narcotic Poisoning, &c, we wit- ness the same gradations as in ordinary sleep. When it is least pro- found, it seems to affect the Cerebral hemispheres alone ; the Sensory Ganglia being still, in some degree, open to the reception of impres- sions. When complete, however, none but reflex actions can be excited ; and if it advance to a fatal termination, it does so by the supervention of the same state of torpidity in the Medulla Oblongata, whereby the respiratory movements are brought to a close. These movements do not cease until the power of deglutition has been lost, and until the eye ceases to close when the edge of the lid is irritated ; but when this is the case, a fatal termination may be apprehended, as SOMNAMBULISM; MEMORY. 525 it is thus shown that the torpor is extending to the Spinal system of nerves. 923. In the condition of Dreaming, it would seem as if the Cere- brum were partially active; a train of thought being suggested, fre- quently by sensations from without; which is carried on without any controlling or directing power on the part of the Mind ; and which is not corrected, or is only modified in a limited degree, by the know- ledge acquired by experience. This condition is still more remarkable in Somnambulism, or (as it has been better termed) Sleep-waking ; on which the dreams are not only acted, but may be often acted on with the utmost facility,—a suggestion conveyed through any of the senses excepting sight (which is usually in abeyance) being apprehended and followed up with the utmost readiness, and, in like manner, with little or no correction from experience. Between this condition, and that of ordinary dreaming, on the one hand, and that of complete insensi- bility on the other, there is every shade of variety; which is presented by different individuals, or by the same individuals at different times. The Cerebellum, in the Sleep-waking state, seems to be frequently in a condition of peculiar activity; remarkable power of balancing and combining the movements of the body, being often exhibited. 924. The faculty of Memory appears to be the exclusive attribute of the Cerebral hemispheres; no impressions made upon the Organs of Sense being ever remembered, unless they are at once registered (as it were) in this part of the nervous centres. This faculty is one of those first awakened in the opening mind of the Infant; and it is one of which we find traces in animals, that seem to be otherwise governed by pure Instinct. It obviously affords the first step towards the exer- cise of the reasoning powers; since no experience can be obtained without it; and the foundation of all intelligent adaptation of means to ends, lies in the application of the knowledge which has been acquired, and stored up in the mind. There is strong reason to believe, that no impression of this kind, once made upon the Brain, is ever entirely lost,—except through disease or accident, which will frequently destroy the memory altogether, or will annihilate the recol- lection of some particular class of objects or of words. All memory, however, seems to depend upon the principle of Association; one idea being linked with another, or with a particular sensation, in such a manner as to be called up by its recurrence; and a period of many years frequently intervening, without the combination of circumstances presenting itself, which is requisite to arouse the dormant impression of some early event. Sometimes this combination occurs in dreaming, delirium, or insanity; and ideas are recalled, of which the mind, in a state of healthy activity, has no remembrance. 925. Although there does not seem any improbability in the suppo- sition, that different faculties of the mind should have different parts of the Cerebral hemispheres as their special instruments, yet sufficient evidence of the correctness of the (so-called) Phrenological distribu- tion of organs, has not yet, in the Author's opinion, been adduced, to 526 FUNCTIONS OF THE SYMPATHETIC SYSTEM. justify its admission into an Elementary Treatise like the present; and the subject will therefore be passed by. 8. Functions of the Sympathetic System. 926. The Cerebro-Spinal apparatus, of which the several parts have now been described, is not the only system of ganglia and nerve- trunks, that is contained within the body of a Vertebrated animal. There is another system, having its own set of centres, and its own distribution of branches; characterized also by a peculiarity in the nature of the nervous fibres, of which its trunks are composed ; and communicating at numerous points with the preceding. It will be remembered that, in front of the vertebral column, there is a series of ganglia on each side ; communicating, on the one hand, with the spinal nerves, as they issue from the vertebral canal; and also con- necting themselves with the two large semilunar ganglia, which lie amidst the abdominal viscera ; as well as with a series of ganglia, that is found near the base of the heart. In the head, also, there are numerous scattered ganglia, which evidently belong to the same system; having numerous communications with the cephalic nerves; and being also connected with the chain of ganglia in the neck. The branches proceeding from this series of ganglia are distributed, not to the skin and muscles (like those of the cerebro-spinal system), but to the organs of digestion and secretion, to the heart and lungs, and particularly to the walls of the blood-vessels, on which they form a plexus, whose branches probably accompany their minutest ramifica- tions. The peculiar connection of this system of nerves with the organs of vegetative life, has caused it to receive the designation of the Nervous System of Organic Life; the Cerebro-Spinal system being termed the Nervous System of Animal Life. It is also not unfre- quently termed the ganglionic system; on account of the separation of its centres into scattered ganglia, which forms a striking contrast to the concentration that is so evident in the Cerebro-Spinal system. But this term is objectionable, as leading to a supposed analogy between this system and the general nervous system of Invertebrata, whose centres are equally scattered ;—an analogy which is completely erroneous, since, as we have seen, this last is chiefly the representative of the Cerebro-Spinal system of Vertebrated animals. The term Sympathetic is, perhaps, the best; although it must not be supposed that the system of nerves is the instrument of by any means all the sympathies, which manifest themselves between different organs. 927. The Sympathetic system contains two classes of nervous fibres;—the ordinary white tubular fibres, all of which are probably derived from the Cerebro-Spinal system ; and the gray or gelatinous fibres, which seem to belong exclusively to itself (§ 375). True it is, that some of these last are found in the spinal nerves; but they seem, even there, to form part of the Sympathetic system,—their centres being the ganglia on the posterior roots of the Spinal nerves FUNCTIONS OF THE SYMPATHETIC SYSTEM. 527 which communicate with the true Sympathetic ganglia, and which seem to form a part of the same series. Thus we may consider each system as intermingling itself with the other;—the Cerebro-spinal system transmitting some of its fibres, both motor and sensory, into the Sympathetic ;—whilst the Sympathetic is represented in the Cere- bro-Spinal system, by certain fibres and collections of vesicular mat- ter of its own. The trunks that proceed from the Semilunar ganglia, are almost entirely composed of gray or organic fibres; whence it is evident that these ganglia are to be regarded as the true centres of the Sympathetic system. On the other hand, the trunks which issue from the chain of spinal ganglia, contain a large admixture of white or tubular fibres. 928. The Sympathetic nerves possess a certain degree of power of exciting Muscular contractions, in the various parts to which they are distributed. Thus by irritating them, immediately after the death of an animal, contractions may be excited in any part of the aliment- ary canal, from the pharynx to the rectum, according to the trunks which are irritated,—in the heart, after its ordinary movements have ceased,—in the aorta, vena cava, and thoracic duct,—in the ductus choledochus, uterus, Fallopian tubes, vas deferens, and vesiculae senii- nales. But the very same contractions may be excited, by irritating the roots of the Spinal nerves, from which the Sympathetic trunks receive their white fibres; and there is, consequently, strong reason to believe that the motor power of the latter is entirely dependent upon the Cerebro-spinal system. Whatever sensory endowments the Sympathetic trunks possess, are probably to be referred to the same connection. In the ordinary condition of the body, these are not manifested. The parts exclusively supplied by Sympathetic trunks do not appear to be in the least degree sensible ; and no sign of pain is given, when the Sympathetic trunks themselves are irritated. But in certain diseased conditions of those organs, violent pains are felt in them; and these pains can only be produced, through the me- dium of fibres communicating with the sensorium through the spinal nerves. 929. It is difficult to speak with any precision, as to the functions of the Sympathetic system. There is much reason to believe, how- ever, that it constitutes the channel, through which the passions and emotions of the mind affect the Organic functions; and this especially through its power of regulating the calibre of the arteries. We have examples of the influence of these states upon the Circulation, in the palpitation of the heart which is produced by an agitated state of feeling; in the Syncope, or suspension of the heart's action, which sometimes comes on from a sudden shock; in the acts of blushing and turning pale, which consist in the dilatation or contraction of the small arteries ; in the sudden increase of the salivary, lachrymal, and mammary secretions, under the influence of particular states of mind, which increase is probably due to the temporary dilatation of the ar- teries that supply the glands, as in the act of blushing; and in many 52S SENSATION. other phenomena. It is probable that the Sympathetic system not only thus brings the Organic functions into relation with the Animal: but that it also tends to harmonize the former with each other, so as to bring the various acts of secretion, nutrition, &c, into mutual con- formity. Of the distinctive function of the gray or organic fibres, we have no knowledge whatever. Possibly they may have some direct influence upon the chemical processes, which are involved in these changes, and may thus affect the quality of the secretions ; whilst the office of the white fibres is rather to regulate the diameter of the blood- vessels supplying the glands, and thus to determine the quantity of their products. CHAPTER XIII. OF SENSATION, GENERAL AND SPECIAL. 1. Of Sensation in general. 930. All beings of a truly Animal Nature possess, there is good reason to believe, a consciousness of their own existence, first derived from a feeling of some of the corporeal changes taking place within themselves; and also a greater or less amount of sensibility to the condition of external things. This consciousness of what is taking place within and around the individual, is all derived from impres- sions made upon its afferent nervous fibres ; which, being conveyed by them to the central sensorium, are there felt (§ 390). Of the mode in which the impression, hitherto a change of a physical character, is there made to act upon the mind, we are absolutely ignorant; we only know the fact. Although we commonly refer our various sensations to the parts at which the impressions are made,—as, for instance, •when we say that we have a pain in the hand, or an ache in the leg, —we really use incorrect language ; for though we may refer our sensations to the parts where the impression is first made on the nerves, they are really felt in the brain. This is evident from two facts ;—first, that if the nervous communication between the part and the brain, be interrupted, no impressions, however violent, can make themselves felt; and second, that if the trunk of the nerve be irritated or pinched, anywhere in its course, the pain which is felt is referred, not to the point injured, but to the surface to which these nerves are distributed. Hence the well-known fact that, for some time after the amputation of a limb, the patient feels pains, which he refers to the fingers or toes that have been removed ; this continues, until the irritation of the cut extremities of the nervous trunks has subsided. SENSATION;—GENERAL AND SPECIAL. 529 931. It would seem probable that, among the lower tribes of Ani- mals, there exists no other kind of sensibility, than that termed gene- ral or common; which exists, in a greater or less degree, in every part of the bodies of the higher. It is by this, that we feel those impressions, made upon our bodies by the objects around us, which produce the various modifications of pain, the sense of contact or resistance, the sense of variations of temperature, and others of a similar character. From what wTas formerly stated (§ 403)-of the dependence of the impressibility of the sensory nerves, upon the activity of the circulation in the neighbourhood of their extremities, it is obvious that no parts destitute of blood-vessels can receive such impressions, or (in common language) can possess sensibility. Accordingly we find that the hair, nails, teeth, cartilages, and other parts that are altogether extra-vascular, are themselves destitute of sensibility; although certain parts connected with them, such as the bulb of the hair, or the vascular membrane lining the pulp-cavity of the tooth, may be acutely sensitive. Again, in tendons, ligaments, fibro-cartilages, bones, &c, whose substance contains very few ves- sels, there is but a very low amount of sensibility. On the other hand, the skin and other parts, which are peculiarly adapted to re- ceive such impressions, are extremely vascular: and it is interesting to observe, that some of the tissues just mentioned become acutely sensible, when new vessels form in them in consequence of diseased action. It does not necessarily follow, however, that parts should be sensible in a degree proportional to the araount of blood they may contain; for this blood may be sent to them for other purposes, and they may contain but a small number of sensory nerves. Thus, it is a condition necessary to the action of Muscles, that they should be copiously supplied with blood (§ 359); but they are by no means acutely sensible ; and, in like manner, Glands, which receive a large amount of blood for their peculiar purposes, are far from possessing a high degree of sensibility. 932. But besides the general, or common sensibility, which is dif- fused over the greater part of the body, in most animals, there are certain parts, which are endowed with the property of receiving im- pressions of a peculiar or special kind, such as sounds or odours, that would have no influence on the rest; and the sensations which these excite, being of a kind very different from those already mentioned, arouse ideas in our minds, which we should never have gained with- out them. Thus, although we can acquire a knowledge of the shape and position of objects by the touch, we could form no notion of their colour without sight, of their sounds without hearing, or of their odours without smell. The nerves which convey these special im- pressions, as already mentioned, are not able to receive those of a common kind ; thus the eye, however well fitted for seeing, would not feel the touch of the finger, if it were not supplied by branches from the 5th pair, as well as by the Optic. Nor can the different nerves of special sensation be affected by impressions, that are adapted to ope- 34 530 OF SENSATION IN GENERAL. rate on others ; thus the ear cannot distinguish the slightest difference between a luminous and a dark object; nor could the eye distinguish a sounding body from a silent one, except when the vibrations can be seen. But Electricity possesses the remarkable power, when trans- mitted along the several nerves of special sense, of exciting the sen- sations peculiar to each ; and thus, by proper management, this single agent may be made to produce flashes of light, distinct sounds, a phosphoric odour, a peculiar taste, and a pricking feeling, in the same individual, at one time. Each kind of sensation may also be excited, however, by mechanical irritation of the nerve which is subservient to it.—The feeling of pain may be induced by impressions made upon the nerves of special sense, as well as upon those of feeling; if these impressions be too violent or excessive. Thus the dazzling of the eye by a strong light, and still more, the action of a moderate light in an irritable state of the retina,—sudden loud sounds, or even sounds of moderate intensity but of peculiar harshness,—powerful odours, even such as are agreeable in moderation,—produce feelings of un- easiness, which may be properly called painful, even though they are different from those excited through the nerves of common sensation. 933. As a general rule, it may be stated, that the violent excite- ment of any sensation is disagreeable ; even when the same sensation, experienced in a moderate degree, may be a source of extreme plea- sure. But the question of degree is relative, rather than absolute; that is, a sensation may be felt as extremely violent by one individual; whilst another, who is more accustomed to sensations of the same kind, is not disagreeably affected by it. Thus, our sensations of heat and cold are entirely governed by the previous condition of the parts affected; as is shown by the well-known experiment, of putting one hand in hot water, the other in cold, and then transferring them both to tepid water,—which will seem cool to the one hand, and warm to the other. The same is the case in regard to light and sound, smell and taste. A person going out of a totally dark room, into one mode- rately bright, is for the time painfully impressed by the light, but soon becomes habituated to it; whilst another who enters it from a room brilliantly illuminated, will consider it dark and gloomy. 934. The intensity with which sensations are felt, therefore, de- pends upon the degree of change which they produce in the sensorium. The more frequent the recurrence of any particular sensation, the more does the system become adapted to it, and the less change does it produce. It is, therefore, perceived in a less and less degree, and at last it ceases to excite attention. The stoppage of a constantly-recur- ring sensation, however, will produce a change, which makes as strong an impression on the system as its first commencement; thus there are persons, who have become so habituated to the sound of a waterfall or even of a forge-hammer, that they cannot sleep anywhere but in its vicinity ; and it is well known that, when a person has gone to sleep under the influence of some continuous or frequently-recur- ring sound (such as the voice of a reader, the dropping of water, the EFFECTS OF ATTENTION. 531 tread of a sentinel, &c), the cessation of the sound will cause his awaking. 935. The acuteness of particular sensations is influenced in a re- markable degree, by the attention they receive from the mind. If the mind be entirely inactive, as in profound sleep, no sensation what- ever is produced by very feeble impressions; on the other hand, when the mind is from any cause strongly directed upon them, impressions very feeble in themselves produce sensations of even painful acute- ness. It is in this manner, that the habit of attending to sensations of any particular class, increases their vividness ; so that they are at once perceived by an individual on the watch for them, when they do not excite the observation of others. We may even, by a strong effort, direct the mind into one particular channel, so as to receive only those sensations which have reference to it, and to be uncon- scious quoad all others. Thus, the application of the mind to some particular train of thought may prevent our being conscious of any- thing that is going around or within us,—the conversation of friends, —the striking of the clock,—the calls of hunger, &c. This abstrac- tion may be altogether voluntary; and the possession of the power of thus withdrawing the mind at will from the influence of external dis- turbing causes, and of fixing it upon any particular train of ideas, is an extremely valuable one. But it may also be involuntary, and may be a source of inconvenience from its tendency to recur at improper times,—producing the habitual state which is known as absence of mind or revery. 936. It is desirable that we should make a distinction, between the sensations themselves, and the ideas which are the immediate results of those sensations, when they are perceived by the mind. These ideas relate to the cause of the sensation, or the object by which the impression is made. Thus, the formation of the picture of an object, upon the retina, produces a certain impression upon the optic nerve ; which being conveyed to the sensorium, excites a correspond- ing sensation, with which, in all ordinary cases, we immediately con- nect an idea of the nature of the object. So closely, indeed, is this idea usually related to the sensation, that we are not in the habit of making a distinction between them. Thus I may say at this moment, " I see a book on the table before me;" the fact being, that I am con- scious of a certain picture, which conveys to ray mind the ideas of a book and of a table, and of their relative positions ; these ideas being (in Man) the result of experience and association,—in fact, originating in the immediate application of the knowledge we have previously acquired, that a certain object, whose picture we see, is a book, an- other object a table, and so on. We are liable to be deceived in this assumption; as when, by a clever imitation, a picture on a plane surface is made to represent an object in relief, so perfectly as at once to excite the idea of the latter,—which may not be corrected, until we have ascertained by the touch the flatness of the real object. 937. This production of ideas, by the agency of sensations, is a 532 PERCEPTIONS, INTUITIVE AND ACQUIRED. process altogether mental, and dependent upon the laws of Mind. We find that some of these perceptions or elementary notions are intui- tive; that is, they are prior to all experience, and are necessarily con- nected with the sensation which produces them, as reflex movements are with the impression that excites them. This seems to be the case, for example, with regard to erect vision. There is no reason whatever to think, that either infants or any of the lower animals see objects in an inverted position, until they have corrected their notion by the touch; for there is no reason why the inverted picture on the retina should give rise to the idea of the inversion of the object. The picture is so received by the mind, as to convey to us an idea of the position of external objects, which harmonizes with the ideas we derive through the touch; and whilst we are in such complete igno- rance of the manner in which the mind becomes conscious of the sensation at all, we need not feel any difficulty about the mode in which this conformity is effected. But in Man, as already stated, the attaching definite ideas to certain groups of lines, colours, &c, with respect to the objects they represent, is a subsequent process, in which experience and memory are essentially concerned ; as we see particularly well, in cases presently to be referred to, in which the sense of sight has been acquired comparatively late in life, and in which the mode of using it, and of connecting the sensations received through it with those received through the touch, has had to be learned by a long-continued training. The elementary notions thus formed,—which may, by long habit, present themselves as immedi- ately and unquestionably, as if they were intuitive,—are termed ac- quired perceptions. 938. It is probable that, among the lower animals, the proportion of intuitive perceptions is much greater than in Man; whilst, on the other hand, his power of acquiring perceptions is much greater than theirs. So that, whilst the young of the lower animals very soon becomes possessed of all the knowledge, which is necessary for the acquirement of its food, the construction of its habitation, &c, its range is very limited, and it is incapable of attaching any ideas to a great variety of objects, of which the Human mind takes cognizance. This correspondence between the acquired perceptions of Man, and the intuitive perceptions of many of the lower animals, is strikingly evident in regard to the power of measuring distance. This is acquired very gradually by the Human infant, or by a person who has first obtained the faculty of sight later in life; but it is obviously pos- sessed by many of the lower aniraals, to whose maintenance it is essen- tial immediately upon their entrance into the world. Thus a Fly- catcher, immediately after its exit from the egg, has been known to peck at and capture an insect,—an action which requires a very exact appreciation of distance, as well as a power of precisely regulating the muscular movements in accordance with it. SENSE OF TOUCH.—CUTANEOUS PAPILLiE. 533 2. Of the Sense of Touch. 939. By the sense of Touch is usually understood that modification of the common sensibility of the body of which the surface of the Skin is the especial seat, but which exists also in some of its internal re- flexions. In some animals, as in Man, nearly the whole exterior of the body is endowed with it, in no inconsiderable degree ; whilst in others, as the greater number of Mammalia, most Birds, Reptiles, and Fishes, and a large proportion of the Invertebrata, the greater part of the body is so covered with hairs, scales, bony or horny plates, shells of various kinds, complete horny envelops, &c, as to be nearly insensible; and the faculty is restricted to particular portions of the surface, or to or- gans projecting from it, which often possess a peculiarly high degree of this endowment. Even in Man, the acuteness of the sensibility of the cutaneous surface varies greatly in different parts; being greatest at the extremities of the fingers, and in the lips; and least in the skin of the trunk, arm, and thigh. Thus the two points of a pair of com- passes (rendered blunt by bits of cork) can be separately distinguished by the point of the middle finger, when approximated so closely as one- third of a line ; whilst they require to be opened so widely as 30 lines from each other, to be separately distinguished, when pressed upon the skin over the spine, or upon that of the middle of the arm or thigh. 940. The impressions that produce the sense of Touch are received through the sensory papillce, with which the surface of the true Skin is beset,— more or less closely according to the part of it that is examined. These papillae are minute elevations, which enclose loops of capillary vessels (Fig. 146), and branches of the sensory nerves. With regard to the precise course of the latter, there is some uncertainty; but it is probable from ana- logy, that the representation given of them by Gerber (Fig. 147), is in the Capillary network at margin of lips. main correct; and that each loop of the Sen- sory nerve is surrounded by a small quantity of vesicular matter, on some change in which the formation of the sensory impression is immediately dependent. It is peculiar to the sense of Touch, and to that of taste (which is a modification of it) that the impression must be made by the contact of the object itself with the sensory surface and not through any intermediate agency. The only excep- tion to this is in regard to the sense of Temperature, which seems to be in many respects different from ordinary touch ; here the proximity of the warm or cold body is sufficient,—the impressions being made after the manner of those of odours, sounds, &c. ^ It is worth re- marking, with reference to the question of the special nature of the 534 CUTANEOUS PAPILLA.—SENSE OF RESISTANCE. sensory fibres, which are the channel of these impressions, that no mechanical irritation of the nerves of common sensation ever seems Fig. 147. Wm 1111 snip ItfSmKBBs MaSm Distribution of the tactile nerves at the extremity of the human thumb, as seen in a thin perpendicu- lar section of the skin. to excite sensations of heat or cold; these being apparently as distinct from the sense of contact, as they are from that of light or sound. 941. The only idea communicated to our minds, when this sense is exercised in its simplest form, is that of resistance; and we cannot acquire a notion of the size or shape of an object, or of the nature of its sur- face, through this sense alone, unless we move the object over our own sensory organ or pass the latter over the former. By the various de- grees of resistance which we then encounter, we form our estimate of the hardness or softness of the body. By the impressions made upon our sensory papillae, when they are passed over its surface, we form our idea of its smoothness or roughness. But it is through the mus- cular sense which renders us cognizant of the relative position of the fingers, the amount of movement the hand has performed in passing over the object, and of other impressions of like nature, that we ac- quire our notions of the size and figure of the object; and hence we perceive, that the sense of touch, without the power of giving motion to the tactile organ, would have been of comparatively little use It is chiefly in the variety of movements, of which the hand of Man is capable,—thus conducive as they are, not merely to his prehensile powers, but to the exercise of his sensory endowments,--that it is superior to that of every other animal; and it cannot be doubted, that »Z\tXt ^ V67 im?ortant, means of acquiring information in re- gard to the external world, and especially of correcting many vague f usefaW "n0^ ^ T S?0Uld derive fr0m the *ense ofSifht, ed« wnnlH\\« If °ther1hand'k must be evident that our know- medgium tnrnnT \ V^ ^ ranSe' if this Sense were the °nly deres? eviZ? ^ W* C°?ld aCC*uire ideas- 0f this we have the powers in 1 'V? "** imperfeCt development of the mental denr va'tion n^^Tu^ Persons w^o have suffered under the coSseauen?h r?? and ™h° ha™ been ZXZ L 0ffX°mthe m°st direct raeans «f Panting by the tui thfoLansTf bj ^^ ? OW"being^ through want of power use the organs of speech. It is only where sucg individual^f have SENSE OF TEMPERATURE.—NERVES OF TASTE. 535 fallen under the care of judicious and persevering instructors, that their mental powers have been called into their due activity, or that any ideas have been awakened, beyond those immediately connected with the gratification of the animal wants, or with painful or pleasur- able sensations. Thus a mind, quite capable of being aroused to activ- ity and enjoyment, may remain in a condition nearly allied to that of idiocy, simply for want of the sensations requisite to produce ideas of a higher and more abstract character than those derived through the senses of Touch, Taste, and Smell. 942. It is not by any means certain, whether the sense of Tempe- rature is not conveyed by a set of fibres, altogether distinct from those which minister to the proper sense of Touch or resistance. For many cases are on record, in which it has been lost, whilst the ordinary sense of touch remains; and it is sometimes preserved, when there is a complete loss of every other kind of sensibility. So again we find that the subjective sensations of temperature,—that is, sensations which originate from changes in the body itself, not from external impres- sions,—are frequently excited quite independently of the tactual sen- sations ; a person being sensible of heat or of chilliness in some part of his body, without any real alteration of its temperature, and without any corresponding affection of the tactual sensations.—It is curious that the intensity of the sensation of temperature should depend, not merely upon the relative degree of heat to which the part is exposed (§ 933), but also upon the extent of the surface over which it is applied; —a weaker impression made on a larger surface, seeming more power- ful than a stronger impression made on a small surface. Thus, if the forefinger of one hand be immersed in water at 104°, and the whole of the other hand be plunged in water at 102°, the cooler water will be thought the warmer; whence the well-known fact, that water in which a finger can be held without discomfort, will produce a scald- ing sensation when the entire hand is immersed in it. 3. Of the Sense of Taste. 943. The sense of Taste, like that of Touch, is excited by the direct contact of particular substances with certain parts of the body: but it is of a much more refined nature than touch; inasmuch as it commu- nicates to us a knowledge of properties, which that sense would not reveal to us. All substances, however, do not make an impression on the organ of Taste. Some have a strong savour, others a slight one, and others are altogether insipid. The cause of these differences is not altogether understood ; but it may be remarked that, in general, bodies which cannot be dissolved in water, alcohol, &c, and which thus cannot be presented to the gustative papillae in a state of solution, have no taste. This sense has for its chief purpose, to direct animals in their choice of food ; hence its organ is always placed at the en- trance to the digestive canal. In higher animals, the tongue is the principal seat of it; but other parts of the mouth are also capable of 536 NERVES OF TASTE—COMPOUND NATURE OF THE SENSE. receiving the impression of certain savours. The mucous merabrane which covers the tongue is copiously supplied with papillae, of various forms and sizes. Those of simplest structure closely resemble the cutaneous papillae; but there are others, Fi?148- which resemble clusters of such papillae, each being composed of a fasciculus of looped capillaries (Fig. 148), and probably containing a similar fasciculus of nervous loops, lying in the midst of vesicular mat- ter. No difference of function has yet been ascertained to exist among the several forms of lingual papillae. When the pa- pillae are called into action by the contact paS'ofVerngul0' fungif°rm of substances having a strong savour, they not unfrequently become very turgid, by a distension of their vessels analogous to that which occurs in erection; and they rise up from the surface of the mucous membrane, so as to produce a decided roughness of its surface. 944. There has been much discrepancy of opinion as to the nerve which is specially concerned in the sense of Taste. The tongue is supplied by two sensory nerves; the lingual branch of the 5th pair; and the glosso-pharyngeal. The former chiefly supplies the upper surface of the front of the tongue, and is copiously distributed to the papillae near the tip. The latter is mostly distributed upon the mu- cous surface of the fauces, and upon the back of the tongue; but it sends a branch forwards, beneath the lateral margin on each side, which supplies the edges and inferior surface of the tip of the tongue, and inosculates with the preceding. There is reason to believe, from experiment, that the gustative sensibility of the tongue is not destroyed by section of either of these nerves; though it is impaired by the total or partial loss of sensibility over certain parts of the surface. There seems good reason to conclude, that the lingual branch of the 5th pair is the nerve through which the sense of Taste, as well as that of Touch, is exercised, in the parts of the tongue to which it is specially distributed,—which are those that possess both senses in the most acute degree ; and that the Glosso-pharyngeal is subservient to the same functions in the parts supplied by it, being probably the exclusive channel, also, through which the impressions made by disagreeable substances taken into the mouth are propagated to the Medulla Oblongata, so as to produce nausea and excite efforts to vomit. The latter nerve is also, as we have seen, the principal chan- nel of the impressions, that give rise to the reflex act of swallowing; with which the 5th pair is concerned in a much inferior degree (§ 897). 945. A considerable part of the impression produced by many sub- stances taken into the mouth, is received through the sense of Smell, rather than through that of Taste. Of this, any one may easily satisfy himself, by closing the nostrils, and breathing through the mouth only, CONDITIONS OF THE SENSE OF SMELL. 537 whilst holding in his mouth, or even rubbing between his tongue and his palate, some aromatic substance ; its taste is then scarcely recog- nized, although it is immediately perceived when the nasal passages are reopened, and its effluvia are drawn into them. There are many substances, however, which have no aromatic or volatile character; and whose taste, though not in the least dependent upon the action of the nose, is nevertheless of a powerful character; but these for the most part produce, by irritating the mucous membrane, a sense of pungency, allied to that which the same substances (acids, for instance, pepper, or mustard), wrill produce, when applied to the skin for a suffi- cient length of time, especially if the Epidermis have been removed. Such sensations, therefore, are evidently of the same kind with those of Touch ; differing from them only in the degree of sensibility of the organ, through which they are received. The sense of Taste, then, in its ordinary acceptation, may be regarded as a compound of those of Smell and Touch. 4. Of the Sense of Smell. 946. Certain bodies possess the property of exciting sensations of a peculiar nature, wdiich cannot be perceived by the organs of taste or touch, but which seem to depend upon the diffusion of the particles of the substance through the surrounding air, in a state of extreme minuteness. As the so- lubility of a substance in liquid seems a neces- sary condition of its ex- citing the sense of Taste, so does its volatility, or tendency to a vaporous state, appear requisite for its having Odorous properties. Most vola- tile substances are more or less odorous; whilst those which do not readily transform them- selves into vapour, usu- ally possess little or no fragrance in the liquid or solid state, but ac- quire strong odorous properties, as soon as they are converted into vapour,—by the aid of heat, for example. There are some solid substances, which possess very strong odorous properties, without losing weight in any appre- ciable degree by the diffusion of their particles through the air. This Fig. 149. A view of the First pair, or Olfactory Nerves, with the Nasal Branches of the Fifth pair; 1, frontal sinus; 2, sphenoidal sinus; 3, hard palate; 4, bulb of olfactory nerve; 5, branches of the olfactory nerve on the superior and middle turbinated bones; 6, spheno-palatine nerves from the second branch of the fifth pair; 7, internal nasal nerve from the first branch of the fifth; 8, branches of 7 to the Schneiderian membrane ; 9, ganglion of Cloquet in the foramen incisivum; 10, anastomosis of the branches of the fifth pair on the inferior turbinated bone. 538 CONDITIONS OF THE SENSE OF SMELL. is the case, for example, with Musk; a grain of which has been kept freely exposed to the air of a room, whose door and windows were constantly open, for a period often years ; during which time the air, thus continually changed, was completely impregnated with the odour of musk; and yet, at the end of that time, the particle was not found to have perceptibly diminished in weight. We can only attribute this result to the extreme minuteness of the division of the odorous particles of this substance. There are other odorous solids, such as Camphor, which rapidly lose weight by the loss of particles from their surface, when freely exposed to the air. 947. The conditions of the sense of Smell are very simple. The Olfactory nerve is minutely distributed over the Schneiderian mem- brane, which is itself highly vascular. The arrangement of the ulti- mate fibres of this nerve has not been ascertained. The Schneiderian membrane is kept constantly but moderately moist, by a mucous se- cretion from its surface ; and this condition is essential to the acute perception of odours. If the mucous surface be too dry, as happens when the 5th pair is paralyzed, the sensation is blunted or even de- stroyed; and the same effect is produced by the presence of too copious a secretion,—as when we are suffering under an ordinary cold.—The highest part of the nasal fossae appears to be that in which there is the most acute sensibility to odours; and hence it is that, when we Snuff the air, so as to direct it into this portion of the cavity, we per- ceive delicate odours, which would otherwise have escaped us. The acuteness of the sense of Smell depends, in no small degree, upon the extent of surface exposed by the membrane lining the nasal cavi- ty ; and in this respect, Man is far surpassed by many of the lower Mammalia, especially the Ruminants, which are warned by its means of the proximity of their enemies. The habit of attention to sensory impressions of this class, however, very much heightens their acute- ness ; hence in those who suffer under blindness and deafness con- jointly, it is usually the principal means by which individuals are distinguished, and the presence of strangers recognized ; and there are cases, in which individuals in a state of Somnambulism have ex- hibited a degree of acuteness of smell, quite comparable to that which is characteristic of Deer, Antelopes, &c. 948. Besides ministering to the sense of Smell, by stimulating the secreting powers of its surface, the 5th pair has another very import- ant function,—that of endowing the interior of the nose with com- mon sensibility, and thus receiving the impression produced by acrid or pungent substances, which act upon it in the same way as they do upon the tongue. Such substances are felt, by the irritation they produce, rather than smelt; and the sensation they occasion gives rise to the consensual act of sneezing, by which a violent blast of air is di- rected through the nasal passages, in such a manner as to clear them of the irritating matter, whether solid (as snuff), fluid, or gaseous. Hence this action may be excited by the contact of an irritant with the Schneiderian membrane, after the olfactory nerve has been di- SENSE OF HEARING.—ACOUSTIC PRINCIPLES. 539 vided, if the branches of the 5th pair be entire; whilst it does not take place when the 5th pair is paralyzed, even though the sense of smell is retained. 5. Of the Sense of Hearing. 949. By this sense we become acquainted with the sounds pro- duced by bodies in a certain state of vibration; the vibrations being propagated through the surrounding medium, by the corresponding waves or undulations which they produce in it. Although air is the usual medium through which sound is propagated, yet liquids or solids may answer the same purpose. On the other hand, no sound can be propagated through a perfect vacuum.—It is a fact of much importance, in regard to the action of the Organ of Hearing, that so- norous vibrations which have been excited, and are being transmitted, in a medium of one kind, are not imparted with the same readiness to others. The following conclusions have been drawn from experi- mental inquiries on this subject. i. Vibrations excited in solid bodies, may be transmitted to water without much loss of their intensity ; although not with the same rea- diness that they would be communicated to another solid. n. On the other hand, vibrations excited in wrater lose something of their intensity in being propagated to solids; but they are returned, as it were, by these solids to the liquid, so that the sound is more loudly heard in the neighbourhood of these bodies, than it would otherwise have been. in. The sonorous vibrations are much more weakened in the trans- mission of solids to air ; and those of air make but little impression on solids. iv. Sonorous vibrations in water are transmitted but feebly to air; and those which are taking place in air are with difficulty communi- cated to water ; but the communication is rendered more easy, by the intervention of a membrane extended between them. The application of these conclusions, in the Physiology of Hearing, will be presently apparent. 950. It is on the Auditory nerve (commonly termed the Portio Mollis of the 7th pair), that the sonorous undulations make their im- pression; but we invariably find, that this impression is made through the medium of a liquid, contained in a cavity, on the walls of which the ultimate branches of this nerve are distributed. The simplest form of the organ of Hearing, such as we find in some Crustacea and certain Fishes, consists merely of a cavity excavated in the solid framework of the head; which cavity is filled with liquid, and lined by a membrane on which the auditory nerve is distributed. These animals are inhabitants of the water; and the sonorous vibrations excited in this medium, being communicated to the solid parts of the head will be by them again transmitted to the contained fluid, with- out much diminution of their intensity; according to principles i. and 540 SIMPLEST FORMS OF THE ORGAN—TYMPANUM. n.—In those Crustacea, however, which chiefly inhabit air, as well as in the greater number of the class of Fishes, we find the auditory cavity or vestibule no longer entirely closed ; but having an aperture on its external side, which is covered in by a membrane. Here the vibrations of the liquid within the cavity, will be more directly excited by those of the surrounding medium ; for if this be water, it will pro- pagate its undulations into the cavity, with little interruption from the membrane stretched across its mouth ; whilst, if it be air, the interposition of this very membrane will greatly assist in the trans- mission of the vibrations to the water of the auditory cavity, accord- ing to principle iv. In most animals, which have the organ of hearing constructed upon this simple plan, the force of the vibrations of the fluid within the cavity is increased by several minute stony concretions (termed otolithes), which are suspended in it. These act according to principle n. Some traces of them are found in the higher aniraals ; in which they are for the most part superseded, how- ever, by an apparatus better adapted to augment the intensity of the sonorous vibrations. 951. This apparatus consists, in all Vertebrated animals above the inferior Reptiles, of the tympanum or drum, with its membrane and chain of bones; together with, in the Mammalia, the external ear; which is adapted to direct itself, more or less completely, towards the point from which the sonorous vibrations proceed, and to give them a degree of preliminary concentration. The tympanic apparatus is inter- posed between the external ear and the merabrane covering the fora- men ovale, which is the entrance to the real auditory cavity; and its purpose is evidently to receive the sonorous vibrations from the air, and to transmit them to that membrane, in such a manner that the vibrations thus excited in the latter may be much more powerful than they would be if the air acted immediately upon it, as in the lower Vertebrata. The usual condition of the Membrana Tympani appears to be rather lax; and, when in this condition, it vibrates in accord- ance with grave or deep tones. By the action of the tensor tympani it may be tightened, so as to vibrate in accordance with sharper or higher tones ; but it will then be less able to receive the impressions of deeper sounds. This state we may easily induce artificially, by holding the breath, and forcing air from the throat into the Eustachian tube, so as to make the membrane bulge out by pressure from within ; or by exhausting the cavity by an effort at inspiration, with the mouth and nostrils closed, which will cause the membrane to be pressed inwards by the external air. In either case, the hearing is imme- diately found to be imperfect; but the deficiency relates only to grave sounds, acute ones being heard even more plainly than before. There is a different limit to the acuteness of the sounds, of which the ear can naturally take cognizance, in different persons. If the sound be so high in pitch, that the membrana tympani cannot vibrate in unison with it, the individual will not hear it, although it may be loud ; and it has been noticed that certain individuals cannot hear the very shrill SEMICIRCULAR CANALS ; COCHLEA.—SENSE OF HEARING. 541 tones produced by particular Insects, or even Birds, which are dis- tinctly audible to others. 952. Not only do we find the tympanic apparatus superadded, in the higher forms of the organ of Hearing, but also the Semicircular Canals, and the Cochlea.—The former exist in all Vertebrata, save the lowest Fishes; and in nearly every case, they are three in num- ber, and lie in three different planes. Hence it has been supposed, with some probability, that they assist in producing the idea of the direction of sounds. The Cochlea does not exist at all in Fishes ; and in Reptiles its condition is quite rudimentary. In Birds, this cavity is more completely formed, though the passage is nearly straight instead of spiral; of its real character, however, there can be no doubt, from its being divided, like the Cochlea of Man, by a membranous partition, on which the ramifications of the auditory nerve are spread out. This appendage has been supposed to be the organ, that enables us to judge of the pitch of sounds; an idea which derives some con- firmation from the correspondence between the development of the cochlea in different animals, and the variety in the pitch (or length of the scale) of the sounds, which it is important that they should hear distinctly, especially the voices of their own kind.—That the Vesti- bule, with the passages proceeding frora it, constitutes the true organ of hearing, even in Man, is evident from the fact, that when (as not unfrequently happens) the tympanic apparatus has been entirely de- stroyed by disease, so as to reduce the organ to the condition of that in which no such apparatus exists, the faculty of Hearing is by no means abolished, though it is deadened. 953. The faculty of Hearing, like other senses, may be very much increased in acuteness by cultivation ; but this improvement depends rather upon the habit of attention to the faintest impressions made upon the organ, than upon any change in the organ itself. This habit may be cultivated in regard to sounds of some one particular class; all others being heard as by an ordinary person. Thus, the watchful North American Indian recognizes footsteps, and can even distinguish between the tread of friends and foes ; whilst his white companion, who has lived among the busy hum of cities, is unconscious of the slightest sound. Yet the latter may be a musician, capable of dis- tinguishing the tones of all the different instruments in a large orches- tra, of following any one of them through the part which it performs, and of detecting the least discord in the blended effects of the whole, —effects which would be to the unsophisticated Indian but an indis- tinct mass of sound. In the same manner, a person who has lived much in the country, is able to distinguish the note of every species of bird that lends its voice to the general chorus of nature; whilst the inhabitant of a town hears only a confused assemblage of shrill sounds, which may impart to him a disagreeable rather than a pleasurable sensation. 954. In all continued sounds or tones, there are several points to be attended to. In the first place, we take cognizance of their pitch; 542 OF THE SENSE OF SIGHT. which depends upon the number of vibrations in a given time,—the high notes being produced by the most rapid vibrations, and the low notes by the slowest. The" ear can appreciate tones produced by 24,000 impulses per second ; the pitch of which is about four octaves above the highest F of the piano-forte. On the other hand, no sequence of vibrations fewer than 7 or 8 in a second, can produce a continuous tone; because the impression left by each impulse has passed away, before the next succeeds ; and there is consequently no- thing more than a succession of distinct beats.—The strength or loud- ness of musical tones depends (other things being equal) on the force and extent of the vibrations, communicated by the sounding body to the medium which propagates them. This will diminish, however, wTith distance; which softens loud tones by lowering the intensity of the undulations, as a consequence of their more extensive diffusion. The cause of the differences, in the timbre, or quality of musical tones, —such, for instance, as those which exist between the tones of a flute, a violin, a trumpet, and a human voice, all sounding a note of the same pitch,—are unknown: but they probably depend upon differ- ences of form in the undulations.—Our ideas of the direction and distance of sounds, are for the most part formed by habit. Of the former we probably judge in great degree, by the relative intensity of the impressions received by the two ears; though we may form some notion of it by a single ear, if the idea just stated as to the use of the semicircular canals (§ 952), be correct. Of the distance of the sound- ing body, we judge by the intensity of the sound, comparing it with that which we know the same body to produce when nearer to us. The Ear may be deceived in this respect, as well as the eye ; thus the effect of a full band at a distance may be given by the subdued tones of a concealed orchestra close by us ; and the Ventriloquist produces his deception, by imitating as closely as possible, not the sounds themselves, but the manner in which they would strike our ears. 6. Of the Sense of Sight. 955. By the faculty of Sight, we are made acquainted, in the first place, with the existence of Light; and by the medium of that agent, we take cognizance of the form, size, colour, position, &c, of bodies that transmit or reflect it. As to the mode in which luminous im- pressions are propagated through space, philosophers are at present undetermined ; and the question is of no physiological importance, since all are agreed as to the laws which regulate their transmission. These laws, which will be found at large in any Treatise on Natural Philosophy,* may be briefly stated as follows. i. Light travels in straight lines, so long as the medium through which it passes is of uniform density. n. When the rays of light pass from a rarer medium into a denser * See Dr. Golding Bird's Manual, Chap. XXII. LAWS OF TRANSMISSION OF LIGHT. 543 one, they are refracted towards a line drawn perpendicularly to the surface they are entering. in. When the rays of light pass from a denser medium into a rarer one, they are refractedyrom the perpendicular. iv. When rays proceeding from the several points of a luminous object, at a distance, fall upon a double convex lens, they are brought to a focus upon the other side of it; in such a man- Fig.wo. ner that an inverted pic- ture of the object is formed upon a screen, placed in the proper position to receive it. 6 Thus in Fig. 150, A B is the object, and E F the lens; the rays issuing from the two extremities and the centre of the object, are brought to a corresponding focus at a less distance on the other side of it, so as to form a distinct picture ; but as the rays from a are brought to a focus at d, and those from b at c, the picture will be inverted. v. The further the object is removed from the lens, the nearer will the picture be brought to it, and the smaller will it be. vi. If the screen be not held precisely in the focus of the lens, but a little nearer, or further off, the picture will be indistinct ; for the rays which form it will either not have met, or they will have crossed each other. 956. The Eye, in its most perfect form—such as it possesses in Man and the higher animals,—is an optical instrument of wonderful completeness; designed to form an exact picture of surrounding objects, upon the Retina or expanded surface of the Optic nerve, by which the impression is conveyed to the brain. The rays of light, which diverge from the several points of any object, and fall upon the front of the cornea, are refracted by its convex surface, whilst passing through it into the eye, and are made to converge slightly. They are brought more close'ly together by the crystaline lens, which they reach after passing through the pupil; and its refracting influence, together with that produced by the vitreous humour, is such as to cause the rays, that issued from each point, to meet in a focus on the retina. In this manner, a complete inverted image is formed, as shown in Fig. 151 ; which represents a vertical section of the eye, and the general course of the rays in its interior. As in the preceding figure, the rays which issue from the point a are brought to a focus at d ; whilst those diverging frora b are made to converge upon the retina at c. The Retina, which is itself so thin as to be nearly trans- parent, is spread over the layer of black pigment, which lines the choroid coat. The purpose of this is evidently to absorb the rays of light that form the picture, immediately after they have passed through the retina; in this manner, they are prevented from being reflected 544 OF THE EYE AS AN OPTICAL INSTRUMENT. from one part of the interior of the globe to another; which would cause great confusion and indistinctness in the picture. Hence it is that, in those albino individuals Fig-isi. (both of the Human race, and among the lower aniraals), in whose eyes this pigment is de- ficient, vision is extremely im- perfect, except in a very feeble light; for the vascularity of the choroid and iris is such, as to give to these membranes a bright red hue, which enables them powerfully to reflect the light that reaches the interior of the eye, when they are not prevented from doing so by the interposition of the pigmentary layer. 957. The Eye is so constructed as to avoid certain errors and defects, to which all ordinary optical instruments are liable. One of these imperfections, termed spherical aberration, results from the fact, that the rays of light, passing through a convex lens whose curvature is circular, are not all brought to their proper foci; those which have passed through the exterior of the lens being made to converge sooner than those which have traversed its central portion. The result of this imperfection is, that the image is deficient in clearness, unless only the central part of the lens be employed.—The other source of imperfection is what is termed chromatic aberration; and it results from the unequal degree in which the differently coloured rays are refracted, so that they are brought to a focus at different points. The violet rays, being the most refrangible, are soonest brought to a focus; and the red being the least refrangible, have their focus at the greatest distance from the lens. Hence it is impossible to obtain an image by an ordinary lens, in which the colours of the object are accurately represented ; for the foci of its differently coloured portions will be different; and its white rays will be decomposed, so that the outlines will be surrounded by coloured fringes.—The Optican is enabled to correct the effects of these aberrations, by combining lenses of differ- ent densities and curvatures; so arranged as to correct each others' errors, without neutralizing the refractive power. This is precisely the plan adopted in the construction of the Eye ; which, when per- fectly formed, and in a healthy state, forms an accurate picture of the object upon the retina, free from either spherical or chromatic aberration. This is effected by the combination of humors of differ- ent densities, having curvatures precisely adapted to the required purpose. 958. There are certain variations, however, in the conformation of the Eye, which diminish the perfection of its result. Thus the Cornea may be too convex, and the whole retractive power too great; so that the image of an object at a moderate distance is formed in front of the retina, instead of upon it. When this is the case, a distinct image can only be formed, by bringing the object nearer to the eye; the OF THE EYE AS AN OPTICAL INSTRUMENT. 545 effect of which will be, to throw the picture further back. Such an eye is said to be myopic, or short-sighted ; and its imperfection may be corrected by placing a concave lens in front of the cornea, of a curvature adapted to neutralize what is superfluous in the convexity of the latter.—On the other hand, if the cornea be too flat, and the refractive power of the humors be too low, the convergent rays pro- ceeding from an object at a moderate distance will not meet upon the retina, but behind it (if they were allowed to pass on); consequently the picture is indistinct; and it can only be made clear, either by withdrawing the object to a greater distance, which will bring the focus of the eye nearer to its front, or by interposing a convex lens to increase the refractive power of the eye. Such a condition is termed presbyopic (from its being common in aged persons), or long- sighted. It may proceed to such an extent, that not even the removal of the object to any distance can permit the formation of a distinct picture; so that the assistance of a convex lens must be obtained, even to see remote objects clearly; though a less degree of convexity will be required than for the clear vision of nearer objects. This state is particularly well-marked after the operation for cataract; for the removal of the crystaline lens so greatly diminishes the refractive power of the eye, as to render necessary the assistance of convex lenses of high curvature. 959. The power, by which a healthy well-formed eye can accom- modate itself to the distinct vision of objects at varying distances, is a very remarkable one; and its rationale is not yet properly under- stood. According to the laws already stated (§ 955, V. and VI.), the picture of a near object can only be distinct when formed more remotely from the lens than the picture of a distant object. Conse- quently when the eye, that has been looking at a distant object, and has seen it clearly, is turned to a near object, a distant picture of the latter cannot be formed without some alteration, either in the distance between the cornea and the retina, or in the curvature of its refractive surfaces. Of the mode in which this adjustment is made, however, nothing is certainly known ; the minuteness of the requisite araount of alteration being such as to prevent its precise seat from being de- termined. 960. The various humors and containing membranes of the Eye, thus answer the purpose of a most delicate and self-adjusting Optical instrument; the sole part, which is immediately concerned in the act of sensation, being the Retina, or net-like expansion of the Optic nerve, which lies between the black pigment and the vitreous humor. It is in this structure that the presence of cells at the peripheral as well as the central extremities of the afferent nerves (§ 381), may be most clearly demonstrated. They can scarcely be distinguished, in many animals, from the cells of the vesicular matter of the brain ; and, like the latter, they lie in the midst of a plexus of capillary blood- vessels, which supplies the materials requisite for their growth and activity. For the maintenance of the due nutrition of this organ, it is 35 VISUAL PERCEPTIONS. occasionally called into use. If its func- tional power be destroyed, by opacity of the anterior portion of the eye, the nutri- tion of the retina and optic nerve suffers to such a degree that these parts cease after a time to exhibit their characteristic structure ;—thus showing that the general rules already stated (Chap. VII.) in re- gard to the connection between the func- tional activity, and the due nutrition of tissues and organs, hold good with re- spect to the Nervous structure.—The mode in which the vesicular layer of the retina comes into relation with the net- work of nerve-fibres, of which it chiefly consists, has not yet been clearly ascer- tained. 961. The picture of external objects, which is formed upon the Retina, closely resembles that which we see in a Camera Obscura. It represents the outlines, colours, lights and shades, and relative posi- tions, of the objects before us ; but these do not necessarily convey to the mind the knowledge of their real forms, characters, or distances. The perception of the latter, as already remarked (§ 936), is a mental process; and it may be intuitive, or acquired,—the latter, it would seem, being the general condition of the function in Man, the former in the lower animals. The Infant is educating his perceptive powers, long before any indications present themselves of the exercise of higher mental faculties. By the combination, especially of the sen- sations of sight and touch, he is learning to judge of the surfaces of objects as they feel, by the appearance they present,—to form an idea of their distance, by the mode in w7hich his eyes are directed towards them,—and to estimate their size, by combining the notions obtained through the picture on the retina, with those he acquires by the move- ment of his hands over their different parts.—A simple illustration will show how closely the ideas excited by the two sets of sensations, are blended in our minds. The idea of smoothness is one which has reference to the touch ; and yet it constantly occurs to us, on looking at a surface which reflects light in a particular manner. On the other hand, the idea of polish is essentially visual, having reference to the reflection of light from the surface of the object; and yet it would occur to us from the sensation conveyed through the touch, even in the dark. 962. That this sort of combination is not intuitive in Man, but is the result of experience, is evident from the numerous observations made upon those, who had acquired the sense of Sight for the first time, after long familiarity with the characters of objects as perceived through the Touch. Thus a boy of four years old, upon whom the operation for congenital cataract had been very successfully performed, requisite that it should be Fig. 152. Distribution of Capillaries in vascu- lar layer of Retina. ERECT AND SINGLE VISION. 547 continued to find his way about his father's house, rather by feeling with his hands, as he had been formerly accustomed to do, than by his newly-acquired sense of Sight; being evidently perplexed, rather than assisted, by the sensations which he derived through it. But when learning a new locality, he employed his sight, and evidently perceived the increase of facility which he derived from it. Among the many interesting particulars recorded of the youth, on whom Cheselden operated with equal success, it is mentioned that, although perfectly familiar with a dog and a cat by feeling them, and quite able to distinguish between them by his sight, he was long before he associated his visual with his tactual sensations, so as to be able to name either animal by sight alone.—The question was put by the celebrated Locke, whether a person born blind, who was able by his touch to distinguish a cube from a sphere, would, on suddenly obtaining his sight, be able to recognize each by the latter sense ; the reply was given in the negative; and the experience of the cases just referred to, as well as of many others, fully justifies such an answer. 963. Still there are, even in Man, certain intuitive perceptions, which afford great assistance in the formation of ideas regarding ex- ternal objects, through the visual sense. And the first of these is the power by which we recognize their erect position, notwithstanding the inversion of the image upon the retina. This is certainly not a matter of experience ; nor is it capable of explanation (as some have thought) by a reference to the direction in which the rays fall upon the retina. It is the mind which rectifies the inversion ; and, as already remarked, it is just as difficult to understand, how the inverted image on the retina should be taken cognizance of by the mind at all, as it is to comprehend how it should be thus rectified. In fact, there is no real connection whatever, between the inversion of the image upon the retina, and that wrong perception of external objects, which some have thought to be its necessary consequence. Any distortion of the picture, giving a wrong view of the relative positions of the objects represented, would be attended with a different result.—The same may be said of the cause of the singleness of the sensation per- ceived by the mind, although an image is formed upon the retina of each eye,—of those objects, at least, which lie in the field of vision that is common to both. This blending of the pictures formed upon the two retinae into a single perception, appears to be, in part at least, the effect of habit. For when the images do not fall upon the parts of the two retina?, which are accustomed to act together, double vision is the result. Thus if, when looking steadily at an object, we press one of the eyeballs sideways with the finger, we see two representations of the object; and the same thing frequently occurs as a result of an affection of the nerves or muscles of one or both eyes, (as in ordinary strabismus or squinting,) or from some change in the nervous centres, as in various disorders of the Encephalon, and in intoxication. If this condition should be permanent, however, we usually find that the in- 548 COMBINATION OF PICTURES IN TWO EYES. dividual becomes accustomed to the double images, or rather ceases to perceive that they are double ; probably because the mind becomes habituated to receive the impressions from the two parts of the retina, which now act together. And if, after the double vision has passed away, the conformity of the two eyes be restored (as by the operation for the cure of squinting) there is double vision for some little time, although the two parts of the retina, which originally acted together, are now brought into their pristine position. 964. But the images thus combined are far from being identical; and one of the most remarkable of all our intuitive perceptions is that by which they are reconciled and combined, and are caused to give rise to an idea, that differs essentially from either image. No near object can be seen by the two eyes in the same manner; of this the reader may easily convince himself, by holding up a thin book in such a manner that its back shall be in a line with the nose, and at a mo- derate distance from it; and by looking at the book first with one eye, and then with the other. He will find that he gains a different view of the object with each eye, when used separately; so that if he were to represent it as he actually sees it under these circumstances he would have two perspective delineations differing from one another, because drawn from different points. But on looking at the object with the two eyes conjointly, there is no confusion between these pic- tures; nor does the mind dwell upon either of them singly; but the union of the two intuitively gives us the idea of a solid projecting body,—such an idea as we could only have otherwise acquired by the exercise of the sense of touch. That this is really the case, has been proved by experiments with a very ingenious instrument, the Stereo- scope, invented by Prof. Wheatstone; which is so contrived as to bring to the two eyes, by reflection from mirrors, two different pic- tures, such as would be accurate representations of a solid object, as seen by the two eyes respectively. When the arrangement is such, as to bring the images of these pictures to those parts of the retinae which would have been occupied by the images of the solid (supposing that to have been before the eyes), the mind will perceive, not one or other of the single representations of the object, nor a confused union of the two, but a body projecting in relief, the exact counterpart of that from which the drawings were made.—Thus the combination of the two pictures and the perception of an object different from either of them, is effected by a mental process of an instinctive kind ; of the nature of which we know nothing further. 965. When two pictures, representing dissimilar objects, are pro- jected upon the retinae of the two eyes by means of the Stereoscope, the result is a curious one. The mind perceives only one of them, the other being completely excluded for a time; but it commonly happens that, after one has been seen for a short period, the other begins to attract attention and takes its place, the first entirely dis- appearing; so that there is no confusion or intermingling of images except at the moment of change. The Will may determine, to a ESTIMATION OF DISTANCE OF OBJECTS. 549 certain extent, which object shall be seen; but not entirely; for if one picture be more illuminated than the other, it will be seen during a larger proportion of the time.—An interesting variation of this ex- periment may be made, without the aid of the Stereoscope, by holding a piece of blue glass before one eye, and a piece of yellow glass before the other. The result will be, not that everything will be seen of a green colour, but that the surrounding objects will be seen alter- nately blue and yellow;—or sometimes the field of vision will be blue, spotted with yellow ; alternating with yellow spotted with blue. Thus, when we have two dissimilar objects before the eyes, our at- tention cannot be kept upon either, to the exclusion of the other, but is alternately and involuntarily directed, either in part or completely, to one and the other. 966. Our idea of the distance of near objects is evidently acquired from experience; and is suggested by the muscular sensations, which are produced by the contraction of the adductor muscles of the eyes. When we direct our eyes towards a near object, a certain degree of convergence takes place between their axes; the degree increasing as the distance between the object and the eyes diminishes; and vice versa. We instinctively interpret the sensations thus produced, in such a manner as to be able to compare, with great accuracy, the rela- tive distances of two objects, that are not remote from the eyes. This intuition, however, is evidently one of the acquired kind; as may be seen by watching the actions of an infant, or of a person who has recently become possessed of Vision. When an object is held before the eyes and an attempt is made to grasp it, the manner in which the attempt is made clearly sh.ows, that there is no power of forming a precise idea of its situation, such as that which exists in many of the lower animals from their first entrance into the world (§ 938). The impressions made upon the eyes have to be corrected by those received through the touch, before the power of judging of distance is accom- plished. How much this power depends upon the conjoint use of both eyes, is evident from the difficulty with which any actions, that require an exact appreciation of distance, are performed by those who have lost the sight of one eye, until they have acquired new modes of judging of it. 967. In regard to remote objects, we have not the same guide ; since the convergence of the eyes, in viewing them, is so slight that the axes are virtually parallel. Our judgment of their distance is chiefly founded upon their apparent size, if their actual size be known to us ; and also upon the extent of ground, which we see to intervene between ourselves and the object. But if we do not know their actual size, and are so situated that we cannot estimate the interven- ing space, we form our judgment chiefly from the greater or less dis- tinctness of their colour and outline. Hence our idea of it will be very much affected by varying states of the atmosphere; a slight haziness increasing the apparent distance ; whilst a peculiarly clear state of the air will seem to cause remote objects to approach much 550 CHANGES IN SIZE OF PUPIL. more closely. This want of convergence between the axes of the two eyes, has the further effect of causing the pictures upon the two retinae to be nearly identical; and consequently the idea of projection is not so strongly excited; nor are we able to distinguish with the same certainty between a well-painted picture, in which the lights and shades are preserved, and the objects themselves in relief. 968. Our notion of the size of an object is closely connected with that of its distance. It is founded upon the dimensions of the picture projected on the retina; and the dimensions of this picture will vary, according to the laws of optics, (§ 955,) inversely as the distance,— being, for example, twice as great when the object is viewed at the distance of one foot, as when it is carried to the distance of two feet. Where we know the relative distances of two objects, the estimation of their real comparative sizes from their apparent sizes, is easily effected by a simple process of mind; but this is not the case, when we only guess at their distances: and our estimate of the size of objects, even moderately remote, is as much affected by states of the atmosphere as that of their distance,—the one being, in fact, propor- tional to the other. Thus a slight mist, which gives the idea of in- creased distance, will also augment the apparent size; because, in order that an object two miles off should produce a picture upon the retina of the same extent with that made by an object one mile off, it must have double the dimensions. It is evident that our perception of the sizes of objects must be acquired by experience, in the same manner as that of their distance has been shown to be. 969. We have now to consider briefly some other phenomena of Vision, in which the acts of Mind, that have been just alluded to, do not participate.—The contraction of the Pupil, under the stimulus of light, seems to be affected by a sphincter muscle, which surrounds the orifice, and which is put in action by a branch of the third pair of nerves. This is an action with which the will has nothing to do ; and it takes place entirely without our consciousness. Although it is due to the stimulus of light, yet there is reason to believe that the con- sciousness of the presence of light is not requisite ; and that it is, therefore, a purely reflex action. The optic nerve seems to be the channel through which the impression is conveyed to the nervous centres ; and the third pair is that through which the motor impulse is conveyed to the iris. But there is some ground for the idea, that the fifth pair may in some degree convey the requisite stimulus, when the optic nerve has been divided. How far the dilatation of the pupil is a muscular action, or merely one which results from the elas- ticity of the tissue of the iris, when the sphincter is relaxed, has not been clearly ascertained ; the latter is probably the case, a perma- nently dilated state of the pupil being usually seen in cases where, from any cause, it is not affected by the stimulus of light." The con- traction of the pupil is evidently destined to exclude from the interior of the eye, such an amount of light as would be injurious to it; whilst its dilatation in opposite circumstances admits the greatest possi- PERCEPTION OF COLOURS. 551 ble number of rays. There is a contraction of the pupils, however, which takes place without any change in the amount of light. This occurs when the two eyes are made to converge strongly upon any object brought very near them ; and its purpose appears to be, to prevent the rays from entering the eye at such a wide angle as would render it impossible for them to be all brought to their proper foci, and would thus produce an indistinct image. 970. In the use of the Eye, like that of the Ear, there is a ten- dency to blend into one continuous image a succession of luminous impressions made at short intervals ; upon which fact depend a num- ber of curious optical illusions. The length of the greatest interval that can elapse without an interruption of the presence of the image, (in other words the duration of the visual impression,) may be mea- sured by causing a luminous object to whirl round, and by ascertain- ing the longest period that may be allowed for each revolution, consistently with the completeness of the circle of light thus formed. By experiments of this kind, the time has been found to vary, in dif- ferent individuals, or in different states of the same individual, from about l-4th to l-10th of a second : that is, the impression must be repeated from four to ten times in each second, to insure the continu- ousness of the image. 971. The impressions of variety of colour, are produced by the differently coloured rays, which objects reflect or transmit to the eye. It is curious that some persons, whose sight is perfectly good for forms, distances, &c., are unable to discriminate colours. This curious affec- tion has received the name of Daltonism, from the circumstance that the celebrated Dalton was an example of it. There are numerous modifications of it; the want of power to discriminate colour being total in some, whilst in others it extends only to certain shades of colour, or to the complementary colours. 972. When the retina has been exposed for some time to a strong impression of some particular kind, it seems less susceptible of feebler impressions of the same kind; thus if we look at any brightly luminous object, and then turn our eyes upon a sheet of paper, we shall per- ceive a dark spot upon it,—the portion of the retina, which had receiv- ed the brighter image, not being affected by the fainter one.—Again, when the eyes have received a strong impression from a coloured object, the spot which is seen when the eyes are directed upon a white surface exhibits the complementary colour; for the retina has been so strongly affected in the part that originally received the image, by its vivid hue, that it does not perceive the fainter hue of the same kind in the object to which it is then turned, and it is impressed only by the remaining rays forming the complementary colours. This explanation applies to the phenomena of the coloured shadows which are often seen at sunset, and of those which may be seen in a room whose light enters through coloured glass or drapery. For if the prevailing light be of one colour,—orange or red for instance,—the eye will not take cognizance of that colour in the faint light of the 552 OF THE VOICE AND SPEECH. shadows; and will see only its complement, blue or green. If the shadow be viewed through a tube, in such a manner that the general coloured ground is excluded, it presents the ordinary tint. CHAPTER XIV. OF THE VOICE, AND SPEECH. 973. There is one particular application of Muscular power in Man, which deserves special consideration, as being that by which he effects his most complete and intimate communication with his fellows;— that, namely, by which his organ of Voice is put into action. In all air-breathing Vertebrata, the production of sound depends upon the passage of air through a certain portion of the respiratory tubes; which is so constructed as to set it in vibration, as it passes forth from the lungs.—In Reptiles, the vibrating apparatus is situated at the point, where the trachea opens into the front of the pharynx ; it is of very simple construction, however, being only composed of a slit bounded by two contractile lips; and few of the animals of this class can pro- duce any other sound than a hiss, which, owing to the great capacity of their lungs, is often very much prolonged.—In Birds, the situation of the vocal organ is very different. The trachea opens into the front of the pharynx, as in Reptiles, by a mere slit; the borders of which have no other movement than that of approaching one another, so as to close the aperture when necessary. This appears to be the instru- ment for regulating the ingress and egress of air, in conformity with the wants of the respiratory function. The vocal larynx of Birds is situated at the lower extremity of the trachea, just where it subdivides into the bronchial tubes; and it is of very complex construction, espe- cially in the singing birds. In Mammalia, on the other hand, the vocal organ and the regulator of the respiration are united in the larynx, which is situated at the top of the trachea. There are few, if any, of this class, which have not some vocal sound ; but the variety and expressiveness which can be given to it, differ considerably in the several orders; being by far the greatest in Man, who alone, there is reason to believe, has the power of producing articulate sounds, or proper language. 974. The Larynx is built up, as it were, upon the Cricoid cartilage (Fig. 153, xw r u), which surmounts the trachea, and which might be considered as its highest ring, modified in form, its depth from above downwards being much greater posteriorly than anteriorly. This is embraced, as it were, by the Thyroid cartilage (g e h); which is articulated to the sides of the Cricoid by its lower horns, round the extremities of which it may be considered to rotate, as on a pivot. In STRUCTURE OF THE LARYNX. 553 this manner, the front of the Thyroid cartilage may be lifted up, or de- pressed, by the muscles which act upon it; whilst the position of its posterior part is but little changed. Upon the upper surface of the back of the Cricoid cartilage, are seated the two small Arytenoid car- tilages (n f) ; these are so tied to the cricoid by a bun- dle of strong ligaments (b b), as to have a sort of rota- tion upon an articulating surface, which enables them to be approximated or sepa- rated from each other,— their inner edges being nearly parallel in the first case, but slanting away from each other in the second. To the summit of these car- tilages are attached the Chordce vocales, or vocal ligaments (t u) composed of yellow fibrous or elastic tis- sue. These stretch across to the front of the Thyroid cartilage; and it is upon their condition and relative situation, that the absence or the production of vocal tones, and all their modifications of pitch, depend. They are rendered tense by the depression of the front of the Thyroid carti- lage, and relaxed by its elevation ; by which action the pitch of the tones is regulated. But for the production of any vocal tones what- ever, they must be brought into a nearly parallel condition, by the mutual approximation of the points of the arytenoid cartilages to which they are attached ; whilst in the intervals of vocalization, these are separated, and the rima glottidis, or fissure between the chordae vocales, assumes the form of a narrow V, with its point directed backwards. 975. Thus there are two sets of movements concerned in the act of vocalization :—the regulation of the relative position of the Vocal Cords, which is effected by the movements of the Arytenoid carti- lages;—and the regulation of their tension, which is determined by the movements of the Thyroid cartilage. The Arytenoid cartilages are made to diverge from one another by means of the Crico-arytenoidei postici of the two sides, (n /, n I,) which proceed from their outer cor- ners and turn somewhat round the edge of the Cricoid, to be attached to the lower part of its back; their action is to draw the outer corners of the Arytenoid cartilages outwards and downwards, so that the Fig. 153. Bird's-eye view of larynx from above, after Willis: —g e h, the thyroid cartilage, embracing the ring of the cricoid ruxw, and turning upon the axis x z, which passes through the lower horns ; iu.hf, the arytenoid cartilages, connected by the arytenoideus transversus; t v, t v, the vocal ligaments; n x, the right crico- arytenoideus lateralis (the left being re- moved) ; v kf, the left thyro-arytenoideus (the right being removed); n I, n I, the crico-arytenoidei pos- tici ; b b, the crico-arytenoid ligaments. 554 REGULATION OF THE APERTURE OF THE GLOTTIS. points to which the vocal ligaments are attached are separated from one another, and the rima glottidis is thrown open. The action of these muscles is antagonized by that of the Arytenoideus transversus, which draws together the Arytenoid cartilages ; and by that of the Crico-arytenoidei laterales of the two sides, (n x,) which run forwards and downwards frora the outer corners of the Arytenoid cartilages, and tend by their contraction to bring together their anterior points, to which the Vocal ligaments are attached.—The depression of the front of the Thyroid cartilage, and the consequent tension of the Vocal ligaments, is occasioned by the conjoint action of the Crico-thyroidei of the two sides, which occasions the Thyroid and Cricoid cartilages to rotate, the one upon the other, at the articulation formed by the inferior cornua of the former; and this action will be assisted by the Sterno-thyroidei, which tend to depress the front of the Thyroid car- tilage, by pulling from a fixed point below. On the other hand, the elevation of the front of the Thyroid cartilage, and the relaxation of the Vocal ligaments, are effected by the contraction of the Thyro- arytenoidei of the two sides (v kf), whose attachments are the same as those of the Vocal ligaments themselves ; and this is aided by the Thyro-hyoidei, which will tend to draw up the front of the Thyroid cartilage, acting from a fixed point above. 976. The muscles which govern the aperture of the glottis,—those namely, which separate and bring together the arytenoid cartilages, and thus widen or contract the space between the posterior extremi- ties of the vocal ligaments,—have important functions in connection with the Respiratory actions in general, and stand as guards, so to speak, at the entrance to the lungs. We can entirely close the glottis, through their means, by an effort of the Will, either during inspiration or expiration ; and it is a spasmodic movement of this sort, which is concerned in the acts of Coughing and Sneezing, the purpose of which is to expel, by a sudden and powerful blast of air, any irri- tating substances, whether solid, liquid, or gaseous, which have found their way info the air-passages. These muscles appear to be under the sole direction of the inferior or recurrent laryngeal nerve ; which seems to possess exclusively motor endowments. When this nerve is divided, on each side, or when the par vagum is divided above its origin, the muscles of the larynx (with the exception of the crico- thyroid) are paralyzed ; and the aperture of the glottis may remain open, or may be entirely closed, according to the manner in which its lips are affected by the currents of air in egress or ingress. It is found that, under such circumstances, tranquil respiration may be carried on ; but that any violent ingress or egress of air will tend to drive the lips of the glottis (these being in a state of complete relaxa- tion) into apposition with each other, so as completely to close the aperture. The character of the superior laryngeal nerve appears to be almost exclusively afferent; no muscle, except the crico-thyroid, being thrown into contraction when it is irritated; whilst, on the other hand, if it be divided, neither the act of coughing, nor any PRODUCTION OF VOCAL SOUNDS. 555 reflex respiratory movement whatever, can be excited, by irritating the lining membrane of the larynx. 977. During the ordinary acts of inspiration and expiration, the Chordae vocales appear to be widely separated from each other, and to be in a state of the freest possible relaxation. In order to produce a vocal sound, they must be made to approach one another, and their inner faces must be brought into parallelism ; both of which ends are accomplished by the rotation of the Arytenoid cartilages; whilst, at the same time, they must be put into a certain degree of tension, by the depression of the Thyroid cartilage. Both of these movements take place consentaneously, and are mutually adapted to each other; the vocal ligaments being approximated, and the rima glottidis con- sequently narrowed, at the same time that their tension is increased. There is a certain aperture, which is favourable to the production of each tone, although the pitch itself is governed by the tension of the Vocal Cords; and it is, perhaps, to a want of consent between the two, that the peculiarly discordant nature of some voices, which appear incapable of producing a distinct musical tone, is due. 978. It has been fully proved, by the researches of Willis, Miiller, and others, that the action of the Vocal ligaments, in the production of sound, bears no resemblance to that of vibrating strings; and that it is not comparable to that of the mouth-piece of the flute-pipes of the Organ: but that it is, in all essential particulars, the same with that of the reeds of the Hautboy or Clarionet, or the tongues of the Accor- dion or Concertina. All the phenomena attending the production of Musical tones are fully explicable on this hypothesis; except the pro- duction of falsetto notes, which has not yet been clearly accounted for.—The power which the Will possesses, of determining, with the most perfect precision, the exact degree of tension wrhich these liga- ments shall receive, is extremely remarkable. Their average length in the Male, in the state of repose, is estimated by Miiller at about 73-l00ths of an inch; whilst, in the state of greatest tension, it is about 93-100ths; the whole difference, therefore, is not above 20- lOOths, or one-fifth of an inch. In the female glottis, their average dimensions are about 51-100ths and 63-100ths, respectively; so that the difference is here only 12-100ths, or less than one-eighth of an inch. Now the natural compass of the voice, in most persons who have cultivated the vocal organ, may be stated at about two octaves, or 24 semitones. Within each semitone, a singer of ordinary capa- bility could produce at least ten distinct intervals ; so that for the total number of intervals, 240 is a very moderate estimate. There must, therefore, be at least 240 different states of tension of the vocal cords, every one of which can be at once determined by the will, when a dis- tinct conception exists of the tone to be produced (§ 904); and, as the whole variation in their length is not more than one-fifth of an inch, even in Man, the variation required, to pass from one interval to an- other will not be more than l-1200th of an inch.—And yet this esti- mate is much below that, which might be truly made frora the per- 556 REGULATION OF THE PITCH OF VOCAL SOUNDS. formance of a practiced vocalist. The celebrated Madame Mara is said to have been able to sound 50 different intervals between each semitone ; the compass of her voice was at least 40 semitones, so that the total number of intervals was 2000. The extreme variation in the length of the vocal cords, even taking the larger scale of the Male larynx, not being more than the fifth of an inch, it may be said that she was able to determine the contractions of her vocal muscles to the ten-thousandth of an inch. 979. It is on account of the greater length of the Vocal cords, that the pitch of the voice is much lower in Man than in Woman : but this difference does not arise until the end of the period of childhood,— the size of the larynx being about the same in the Boy and Girl, up to the age of 14 or 15 years, but then undergoing a rapid increase in the former, whilst it remains nearly stationary in the latter. Hence it is that Boys, as well as Girls and Women, sing treble; whilst Men sing tenor, which is about an octave lower than the treble; or bass, which is several notes lower still.—The cause of the variations in the timbre or quality in different voices, is not certainly known ; but it appears to be due, in part, to differences in the degree of flexibility and smoothness in the cartilages of the larynx. In women and children, these cartilages are usually soft and flexible, and the voice is clear and smooth ; whilst in men, and in women whose voices have a masculine roughness, the cartilages are harder, and are sometimes almost completely ossified. The loudness of the voice depends in part upon the force with which the air is expelled from the lungs ; but the variations in this respect, which exist among different indi- viduals, seem partly due to the degree in which its resonance is increased by the vibration of the other parts of the larynx, and of the neighbouring cavities. In the Howling Monkeys of America, there are several pouches opening from the larynx, which seem destined to increase the volume of tone that issues from it; one of these is exca- vated in the substance of the hyoid bone itself. Although these Monkeys are of inconsiderable size, yet their voices are louder than the roaring of lions, and are distinctly audible at the distance of two miles; and when a number of them are congregated together, the effect is terrific. 980. The vocal sounds produced by the action of the larynx are of very different characters ; and may be distinguished into the cry, the song, and the ordinary or acquired voice. The cry is generally a sharp sound, having little modulation or accuracy of pitch, and being usually disagreeable in its timbre or quality. It is that by which ani- mals express their unpleasing emotions, especially pain or terror; and the Human infant, like many of the lower animals, can utter no other sound.—In song, by the regulation of the vocal cords, definite and sustained musical tones are produced, which can be changed or mo- dulated at the will of the individual. Different species of Birds have their respective songs; which are partly instinctive, and partly ac- quired by education. In Man, the power of song is entirely acquired ; VARIOUS KINDS OF VOCAL SOUNDS. 557 but some individuals possess a much greater facility in acquiring it than others,—this superiority appearing to depend upon their more precise conception of the tones to be sounded, as well as their more ready imitation,—besides differences in the construction of the larynx itself. The larynx of an accomplished vocalist, obedient to the ex- pression of the emotions, as well as to the dictates of the will, may be said to be the most perfect musical instrument ever constructed.— The voice is a sound more resembling the cry, in regard to the absence of any sustained musical tone ; but it differs from the cry, both in the quality of its tone, and in the modulation of which it is capable by the will. In ordinary conversation, the voice passes through a great variety of musical tones, in the course of a single sentence, or even a single word,—sliding imperceptibly from one to another; and it is when we attempt to fix it definitely to a certain pitch, that we change it from the speaking to the singing tone. 981. The power of producing articulate sounds, from the combina- tion of which Speech results, is altogether independent of the Larynx; being due to the action of the muscles of the mouth, tongue, and pa- late. Distinctly articulate sounds maybe produced without any vocal or laryngeal tone, as when we whisper; and it has been experiment- ally shown, that the only condition necessary for this mode of speech, is the propulsion of a current of air through the mouth, frora back to front. On the other hand, we may have the most perfect laryngeal tone without any articulation ; as in the production of musical sounds, not connected with words. But in ordinary speech, the laryngeal tone is modified by the various organs, which intervene between the larynx and the os externum. The simplest of these modifications is that by which the Vowel sounds are produced : these sounds being continuous tones modified by the form of the aperture through which they pass out. Thus, let the reader open his mouth to the widest dimensions, depress the tongue, and raise the velum palati, so as to make the exit of air as free as possible ; on then making a vocal sound, he will find that this has the character of the vowel a in ah. On the other hand, if he draw together the lips, still keeping the tongue depressed, he will pass to the sound represented in the Eng- lish language by oo, in the Continental languages by u. By atten- tion to the production of other vowel sounds, it will be found that they are capable of being formed by similar modifications in the form of the buccal cavity and the size of the buccal orifice; and that they are capable of being sustained for any length of time. There is an exception, however, in regard to the sound of the English i, as in fine; which is, in reality, a diphthongal sound, produced in the act of transition frora a peculiar indefinite murmur to the sound of the long e, which takes its place when we attempt to continue it. The short vowel sounds, moreover,—such as a in fat, e in met, o in pot, &c, are not capable of being perfectly prolonged ; as they require, for their true enunciation, to be immediately followed by a consonant. __A tolerably good artificial imitation of Vowel sounds has been 558 VOWELS AND CONSONANTS.—STAMMERING. effected, by means of a reed-pipe representing the larynx, surmounted by an India-rubber ball, with an orifice, representing the cavity and orifice of the mouth. By modifying the form of the ball, the different vowels could be sounded during the action of the reed. 982. In the production of the sounds termed Consonants, the breath suffers a more or less complete interruption, in its passage through the parts anterior to the larynx. The most natural primary division of these sounds is into those which require a total stoppage of the breath at the moment previous to their being pronounced, and which, therefore, cannot be prolonged ; and those in pronouncing which the interruption is partial, and which can, like the vowel sounds, be pro- longed ad libitum. The former have received the designation of explosive consonants; the latter are termed continuous.—In pronoun- cing any consonants of the explosive class, the posterior nares are com- pletely closed ; and the whole current of air is directed through the mouth. This may be checked by the approximation of the lips, as in pronouncing b and p; by the approximation of the point of the tongue to the front of the palate, as in pronouncing d and t; or by the approximation of the middle of the tongue to the arch of the palate, as in pronouncing the hard g or k. The difference between b, d, and g, on the one hand, and p, t, and k, on the other, depends simply upon the greater extent of the meeting surfaces in the former case than in the latter.—In sounding some of the continuous conso- nants, the air is not allowed to pass through the nose ; but the inter- ruption in the mouth is incomplete ; this is the case with v andy, s and z. In others, the posterior nares are not closed, and the air has a nearly free passage, either through the nose alone, as in m and n, or through the nose and mouth conjointly as in I and r. The sound of h is a mere aspiration, caused by an increased force of breath; and that of the guttural ch, as it exists in Welsh, Gaelic, and most Continental languages, is an aspiration modified by the elevation of the tongue, which causes a slight obstruction to the air, and an in- creased resonance in the back of the mouth. 983. The study of the mode in which the different Consonants are produced, is of particular importance to those who labour under defective speech, especially that difficulty which is known as Stam- mering. This very annoying impediment is occasioned by a want of proper control over the muscles concerned in Articulation ; which, instead of obeying the Will, are sometimes affected with an involun- tary or spasmodic action, that interrupts the pronunciation of particular words,—just as, in Chorea, the muscles of the limbs are interrupted by spasmodic twitchings, in the performance of any voluntary move- ment. In fact, persons affected with general Chorea, frequently stammer; showing that ordinary Stammering may be considered as a kind of local Chorea. The analogy between the two states is further indicated by the corresponding influence of excited Emotions in aggravating both.—It is in the pronunciation of the consonants of the explosive class, that the stammerer usually experiences the greatest STAMMERING. 559 difficulty; for the total interruption to the breath, which they occasion, is frequently continued involuntarily;* so that either the expiration is entirely checked, the whole frame being frequently thrown into the most distressing semi-convulsive movements, r/r the sound comes out in jerks. Sometimes, however, the spasmodic action occurs in the pronunciation of vowels and continuous consonants; the stammerer prolonging his expiration, without being able to check it. 984. The best method of curing this defect (where there is no mal- formation of the organs of speech, but merely a want of power to use them aright), is to study the particular difficulty under which the individual labours; and then to cause him to practise systematically the various movements concerned in the production of the sounds in question, at first separately, and afterwards in combination,—until he feels that his voluntary control over the muscles is complete. The patient would at first do well to practise sentences, from which the explosive consonants are omitted ; his chief difficulty, arising from the spasmodic suspension of the expiratory movement, being thus avoided. Having mastered these, he may pass on to others, in which the difficult letters are sparingly introduced; and may finally accustom himself to the use of ordinary language. One of the chief points to be aimed at, is to make the patient feel that he has command over his muscles of articulation; and this is best done, by gradually leading him from that which he can do, to that which he fears to attempt. * The interruption of the expiratory movement in Stammering, is usually stated to take place in the glottis,- but the Author is satisfied that, in all ordinary cases at least, it is in that condition of the mouth which is preparatory to the pronunciation of one of the explosive consonants. INDEX. N. B. The Numbers refer to the Paragraphs. Aberration, corrected in eye, 957. Absorbent Cells, 241-244. " Vessels, 489. Absorption, from alimentary canal, 489,490; by lacteals, 241-244; by blood- vessels, 491-493. " from general and pulmonary surfaces, 522, 523. " interstitial, by lymphatics, 502, 503; by blood-vessels, 502, 503. Acalepha, circulation in, 550. Adipose tissue, 259-263, 423-425. Air-cells of lungs, 676-679. Albumen, 173-175. " conversion of into fibrin, 519. Albuminuria, 533, 728. Aliment, sources of demand for, 406-415. " effect of variations in supply of, 416-426. " relative value of different kinds of, 427, 441. " necessity for mixture in, 437. Allantois, formation of, 817. Amnion, formation of, 816. Amphioxus, see Lancelot. Anterior Pyramids, 890. Apoplexy, 688, 922. Area pellucida, 808. Areolar tissue, 194-196, 205. Arteries, movement of Blood in, 582-588. " elasticity of, 583; tonicity of, 584; contractility of, 585; pulsation of, ' 583, 584. " anastomosis of, 588. Articulata, circulation in, 552, 553; respi- ration in, 657-660; nervous system in, 657 -660. Articulate speech, 981, 982. Asphyxia, 628, 703-709. Assimilating cells, 212, 514, 519. Assimilation, 519. Asthma, 678. Atrophy, 619, 620. Attention, effects of, 935. Auditory ganglia, 900. " nerve, 949. 36 Azotized Compounds in Plants, 169,428,429. « " in Animals, 428,429. " " destination of in food, 429, 431, 433. B Basement membrane, 206-209. Batrachia, respiration of, 670, 671. Bile, composition and properties of, 724-726. " uses of, in digestion, 476-479. Birds, circulation in, 564, 565; respiration in, 672-674; lymphatic system in, 500; nervous centres of, 872; heat of, 761. Blood, composition of, 525-529; uses of seve- ral constituents of, 529,530; changes of, in disease, 531-534. " corpuscles of, white, 212; red, 215- 223. " coagulation of, 535. " buffy coat of, 536, 537. " rate of movement of, 577. " influence of respiration on, 609-702. Blushing, 603. Bone, structure and composition of, 277-289. Brunner's glands, 450. Buffy coat of blood, 536, 537. Butyric acid, 430. Cancelli of Bone, 281. Cancer-cells, 248. Capillaries, movement of Blood in, 589-604; variations in its rate, 597. " variations in size of, 595, 603; influence of nerves upon, 603, 604. " independent force generated in, 598-600. Carbonic acid, necessity for excretion of, 641; sources of, in Animal bo- dies, 642-648. " " mode of its extrication, 649- 652; amount set free, 691- 698. Cartilage, 264-273. 562 IND EX. Cartilage, ossification of, 300-303. Casein, 176, 832. Catamenia, 798, 799. Cell, Vegetable, general history of, 30-45. " Animal, general history of, 211-214. " isolated, various forms of, 210-216. " the immediate agents in Organic func- tions, 245, 246. " union of, 247-254. " coalescence of, 255-257. " changes of form in, 258. Cellular cartilage, 267, 268. Cementum, 319. Centipede, experiments on, 858. Cerebellum, 867, 869. « functions of, 911-914. Cerebric acid, 383. Cerebrum, 867, 868. " functions of, 915-925. Chlorosis, state of blood in, 219, 533, 537. Cholesterine, 724. Chondrine, 264, 265. Chorda dorsalis, 254, 812. Chorda? vocales, 974-979. Chorea, 983. Chorion, 795, 809, 818. Chyle, composition and properties of, 515- 519. " corpuscles of, 212, 518, 519. Chyme, 472, 476. Cicatricula, 808. Cilia, 234, 235. Cineritious substance, 379. Circulation, 538,539; in Plants, 540-548; in lowest Animals, 549, 550; in Echinodermata, 552; in Articulata, 552, 553; in Mol- lusca, 555-557; in Fishes, 558-561; in Reptiles, 562, 563; in Birds and Mammals, 564, 565. " in early embryo, 551,554, 566; in foetus at birth, 823, 824. Coagulation of Albumen, 173, 174. " of Blood, 535. " of Casein, 176. " of Fibrin, 180-187. Cochlea, 952. Caecum, secondary digestion in, 481. Cold, degree of, sustainable by Plants, 110, 111. " degree of, sustainable by Animals, 136. Colostrum, 835. Colours, perception of, 971. Commissures of brain, 917-919. Complementary colours, 972. Conchifera, nervous system of, 852, 853. Concussion, 581. Congestion, 601, 602. " venous, 609, 610. Consensual actions, 903-905. Consonants, 982. Contractility of Muscle, 347. " Vegetable tissues, 345, 346. Convulsive actions, 885. Cornea, 274. Corpora Malpighiana, 728. " Quadrigemina, 873, 900, 902. " Striata, 901. Corpora Wolffiana, 727. Corpuscles of Blood; red, 215-223; white, 212. " of Chyle and Lymph, 212. Cortical substance*of brain, 380. Cranium, circulation in, 611. Crura cerebri, 901. Crustacea, geographical distribution of, 130; respiration of, 65S. Crusta petrosa, 319. Crystaline lens, 275. Cuttle-fish, nervous cords in arms of, 854. D Daltonism, 971. Death, somatic, 65, 68, 69, 628, 629. " molecular, 66, 67. Decidua, 810, 811. Defecation, 462, 463. Deglutition, 453, 454, 897. Dentine, 311-316. Determination of blood, 601. Development of Embryo, 805 et seq. Diffusion, mutual, of gases, 650. Digestion, organs of, 442-450. " nature of the process, 472. Disintegration of tissues, 617. " of Muscular tissue, 361. " of Nervous tissue, 384. Distances, estimate of, 966, 967. Doris, gills of, 651, 656. Double vision, 963. Draper, Prof., his views on the capillary cir culation, 545-548, 598, 599. Dreaming, 923. Duration of pregnancy, 825, 826. " of impressions on Ear, 956. " of impressions on Eye, 970. Dytiscus, experiments on, 859. E Ear, structure of, 956-960. Echinodermata, circulation in, 552. Electricity, development of in Animals, 771- 777; in Torpedo and Gymno- tus, 771-775; in Muscles, 775; in Frog, 776; in higher ani- mals, 777. " influence of, on organic func- tions, 142, 146; effects of shocks of, 145-147. influence of, on Muscles, 351; on sensory nerves, 932. Embryo, early development of, 805—808; formation of vertebral column in, 812; formation of vessels in, 813; formation of heart in, 814; forma- tion of digestive cavity in, 815; circulation in, 551, 554, 556. Emotional movements, 906-908. Emotions, influence of on hunger, 483; on salivary secretion, 467; on heart's action, 580; on capillary circulation, 603; on mam- mary secretion, 836, 837. Enamel, 317, 318. INDI Endosmose, 491, 492. Entozoa, circulation in, 549, 550. Epidermis, 224-228. Epilepsy, 886. Epithelium, 231-239. Erect vision, 963. Exhalation of water, from lungs, 701; from cutaneous surface, 743-746. " of organic matter, 702. Excreting processes, general review of, 757- 759. Eye, structure of, 956-960. F Facial nerve, 888. Fat, 259-263, 423-425. Fecundation of Ovum, 803, 804. Ferments, action of, on blood, 534. Fertilization of ovum, 803, 804. Fibre, white, 189, 190. « yellow, 189, 192. Fibrillation, 183, 213., Fibrin, coagulation of, 180-187. " composition of, 178, 179. " production of, 519. Fibrous membranes, 188. Fibrous tissues, simple, 188-193. Fibro-Cartilage, 188, 269, 272. Fifth Pair, 686, 888. Fishes, lymphatic system in, 499; circulation in, 558-561; respiration in, 663-667; heat of, 761; electricity of, 771-774; nervous centres in, 869, 870. Fcetus, circulation in, 822-824. Follicles of glands, 238, 714-719. Follicles of Lieberkiihn, 449. Food, see Aliment. G 563 H Haematine, 221,222. Hair, 328-330. Haversian canals, 282. Hearing, sense of, 949-954. Heart, action of, 568-570; sounds of, 571- 575; propulsive power of, 576-578; frequency of contractions of, 579. " power of, independent of nervous agency, 580; influenced by mental emotions, 580; by state of nervous system, 581. " first development of, in embryo, 814. Heat, amount developed in Insects, 123; in Fishes, 760; in Birds, 761; in Mam- mals, 761; in Plants, 762. " development of, chiefly dependent on production of carbonic acid, 764; but partly on other oxidizing processes, 765; inferior in young animals, 766. " of body, kept down by perspiration, 745, 768. " its influence upon vital activity in gene- ral, 97, 98; upon Vegetation, 99-111; upon Animal life, 112-141. " degree of, sustainable by Animals, 138 -141; by Plants, 108. Hippuric acid, 734. Hunger, sense of, 483, 485. Hybernation, 120, 121. Hydra, stomach of, 443. Hydrophobia, 886, 908. Hypertrophy, 617, 618. Hypoglossal nerve, 888. Hysteria, 741, 887, 908. I Incubation, heat supplied in, 125-127. Inflammation, nature of the process, 631,632; state of the blood in, 531, 536. Inorganic substances in food, 438-441. Insalivation, 446, 451., 452. Insects, circulation in, 552; respiration in, 659, 660; nervous system of, 856-864; re- flex actions of, 859, 860; instinctive actions of, 860, 861; heat of, 123. Instinctive actions of Man, 906. Intelligence, 916. Intercellular substance, 247, 253. Intestinal canal, structure of, 447-450; move- ments of, 460, 461. Iris, movements of, 969. Irritability of Muscles, 348-363. K Kidneys, structure of, 727, 728; action of, 729-741. L Lacteals, 484, 496, 499. Lactic acid, 735. Lacuna; of Bone, 279. Gall-bladder, 481. Ganglion, 380. Gangrene, 633, 634. Gases, mutual diffusion of, 650. Gastric fluid, properties and actions of, 468- 472. " " conditions of its secretion, 474, 475. Gastric follicles, 470. Gelatin, 190, 191, 429. Geographical distribution of Animals, 130. « distribution of Plants, 102-106. Germinal membrane, 806. Gestation, duration of, 825, 826. Gills, structure of, 651, 655, 656, 663. Glands, essential parts of, 238, 714-719. Globuline, 221. Glosso-pharyngeal nerve, 888, 897.^ Glottis, regulation of aperture of, 976. Glycerine, 261. Gout, 422, 615. Graafian Vesicle, 796. Granulation, 636. Gravity, influence of on venous circulation, 609, 610. Gray matter of nerves, 379. Gymnotus, 771, 774. 564 INDEX. Lancelot, 251, 561,869. Laryngeal nerves, 976. Larynx, structure and actions of, 974, 979. Lead-palsy, 614. Light, laws of transmission and refraction of, 955. " influence of on Vegetation, 79-92; on growth and development of animals, 93-96. " emission of, by animals, 770; by man, 771. Lime, in Animal body, 438, 440, 441. Lithic acid diathesis, 422, 732, 733. Liver, structure of, 720-723; actions of, 477- 479, 724-726. Luminousness, animal, 770, 771. Lungs, structure of, 676-679. Lymph, composition and properties of, 515, 520. Lymphatics, 498-503. M Male, action of in reproduction, 785-790. Malignant diseases, 640. Malpighian bodies, of Kidney, 728; of Spleen, 506. Mammalia, lymphatic system in, 500; circu- lation in, 564, 565; respiration in, 674, 675; heat evolved in, 761; nervous centres of, 873; ovisac of, 795. Mammary glands, 830, 831. Margarine, 261. Mastication, act of, 451, 452, 896. Medulla Oblongata, structure of, 889-893; functions of, 894-899. Memory, 924. Menstrual secretion, 798, 799. Mesenteric glands, 496. Metamorphosis of animals, 407, 791, 792. Milk, 436; composition and properties of, 832-834. " circumstances influencing secretion of, 836, 839. Mineral ingredients of food, 438-441. Moisture, proportion of, 'in different parts of the body, 149-152; influence of, on growth of Plants and Animals, 153-157; effects of withdrawal of, 158-161. Mollusca, circulating system of, 555-557; respiratory organs of, 654-656; nervous system of, 850-855. Mucous membrane, 198-204. Mucus, 237, 464. Muscles, contractility of, 345-371; irritability of, 348-363; tonicity of, 364-366; rigor mortis of, 367-369; peculiar force of, 370; heat evolved by, 371; electricity evolved by, 775. " energy of, dependent on supply of blood, 358-361. " disintegration of, 361. Muscular fibre, striated, 332, 334-339; non- striated, 333, 337. Muscular sense, 904. " tissue, 340-344. Myopia, 958. N Nail, 226. Nervous System, general view of actions of, 840-847. Nervous System, in Radiata, 849; in Tunic- ata, 850, 851; in Bivalve Mollusca, 852, 853; in higher Mollusca, 853, 854; in Ar- ticulata, 855-863; in Insects, 856, 857, 861; in Vertebrata, 867, 868; in Fishes, 869, 870; in Reptiles, 871; in Birds, 872; in Mammals, 873. Nervous tissue, 372-405; fibrous, 373-377; vesicular, 378, 379. " " activity of, dependent on supply of arterial blood, 398-404. Nucleolus, 250. Nucleus, 249. Nutrition, 612-615. " activity of, dependent on func- tional activity of parts, 616- 627. 0 (Esophagus, passage of food along, 455,898. Oily compounds in food, 430, 432, 435. Oleine, 261. Oleo-phosphoric acid, 383. Olfactive lobes, 869-873, 900. Olfactory nerve, 946, 947. Olivary bodies, 891. Optic lobes, 869-873, 900. Optic nerves, 910. Orbit, motor nerves of the, 888. Osseous tissue, 277-289, 299-309. Ossification, 300-303. Otolithes, 950. Ova of animals, 791-794. Ovarium, 793-796. Ovisac, 793, 796. Oxygen, necessity for, in animal body, 649; mode of introduction of, 650-652. Pancreatic secretion, 480. Papillae, sensory, of skin, 381,940; of tongue, Parturition, 827-829. Par Vagum, 459, 487, 580, 685, 686, 888, 895, 897-899. ' Pedal ganglia in Mollusca, 852, 853. " " in Articulata, 857, 862. Pepsine, 470, 471. Perception, nature of, 936, 937. Perceptions, tactual, 941; visual, 961, 968. Peristaltic movement, 460. Perspiration, 743-746. Peyer's glands, 450. Phosphate of lime, in food, 438-441. Phosphatic deposits, 386, 738-740. Phosphorus, in animal body, 438, 439. Pigment-cells, 229, 230. Placenta, structure of, 819, 820. Placental tufts, 244. INDEX. 565 Plants, heat of, 762; circulation in, 540-548; respiration in, 84, 641, 642; reproduction in, 781-784. Pneumogastric nerve, see Par Vagum. Polypes, digestive process in, 443-445. Posterior Pyramids, 893. Pregnancy, duration of, 825, 826. Prehension of food, 896. Presbyopia, 958. Primary membrane, 206-209. Proteine, 168-173. Puberty, in male, 788; in female, 798. Pulp of hair, 328, 330. Pulp of teeth, 310, 313. Pulsations of heart, 579. Pulse, in arteries, 583, 584; respiratory, in veins, 607. Pupil, changes in diameter of, 969. Pus, 632, 636, 637. Pyramids, anterior, 890. " posterior, 893. R Radiata, Nervous System of, 849. Receptaculum Chyli, 497. Reduction of food, provisions for, 445. Reflex actions, nature of, 392-396. " " of Articulata, 858—860; of Mollusca, 851, 854; of Ver- tebrata, 875-879. Reproduction, general nature of the pro- cess, 778, 780. « in Plants, 781-784. Reproductive cells, 240. Reptiles, circulation in, 562, 563; lymphatic system in, 500; respiration in, 668-671; nervous centres in, 871. Respiration, nature of the process, 641-642. " organs of, in lowest animals, 653; in Mollusca, 654-656; in Annelida, 657; in Crusta- cea, 658; in Insects, 659- 660; in Spiders, 661; in Fish- es, 663-667; in Reptiles, 668 -671; in Birds, 672-674; in Mammalia and Man, 675- 688. " chemical phenomena of, 689- 702. " insufficient, effects of, 703-709. Respiratory movements, 680-682; frequency of, 683. Respiratory nerves, in Insects, 862; in Mol- lusks, 850-853; in Vertebrata, 684-688, 895. Respiratory palse, 607. Restiform bodies, 892. Retina, general structure of, 960. Rigor Mortis, 367-369.^ Ruminating stomach, 457. S Saccharine compounds in food, 430-434,493. Salivary glands, 465. " secretion, 446, 466, 467. Satiety, sense of, 486. Sebaceous follicles, 747, 748. Secreting cells, 238, 239, 712-714. Secretion, nature of the process, 710-713. " effects of suppression of, 711. Selecting power of individual parts, 612-615. Semicircular canals, 952. Sensation, 389, 390; nerves of, 389, 391,900, 901; general and special, 932. Sensations, regulation of muscular movement by, 904. Sensorium, 390. Sensory Ganglia, 900,901; functions of, 902- 908. Sensory nerves, 909. Serous Membranes, 197. Shell, of Echinodermata, 290, 291; of Mol- lusca, 292-295; of Articulata, 296-298. Sight, sense of, 955-972. Single vision with two eyes, 963. Size of objects, estimate of, 968. Skin, 198, 204, 742-748, 940. Sleep, 921. Smell, sense of, 946-948. Sneezing, 948. Solen, nervous system of, 852, 853. Somnambulism, 923. Sounds, propagation of, 949; qualities of, 954 Sounds of heart, 571-575. Speech, 981, 982. Spermatic fluid, 786, 787; emission of, 790. Spermatozoa, 240, 787; use of, in fecunda- tion, 804. Sphinx ligustri, nervous system of, 856, 857. Spiders, respiratory organs of, 661. Spinal Cord, 867, 868; structure of, 880-883; reflex actions of, 874-879, 884; disordered states of, 885-887. Spinal accessory nerve, 580, 888. Spinal nerves, origin of, 880, 882. " " peculiar, 888. Spiracles of Insects, 659. Spleen, structure of, 506; uses of, 507-509. Stammering, 983, 984. Starchy compounds in food, 430-432. Star-fish, nervous system of, 849. Stearine, 261. Stereoscope, 964, 965. Stomach, 447, 448; movements of, 456-459. " in Ruminants, 457. Stomato-gastric nerves of Invertebrata, 863. " " of Vertebrata, 896. Strabismus, 963. Suction, act of, 896. Sudoriparous glandulae, 743,' 744. Supra-renal capsules, 510. Sympathetic System, in Man, functions of, 926-929. " " traces of, among In- vertebrata, 864. Syncope, 581,628. Synovial membranes, 197. T Tadpole, respiration of, 670; metamorphosis of, 670. Taste, nerves of, 944. 566 INDEX. Taste, sense of, 94.3. Teeth, structure and development of, 310-327 Temperature, sense of, 933, 942. Testis, structure of, 7S5, 786. Tetanus, 886. Thalami Optici, 901. Thirst, 488. Thoracic duct, 497. Thymus Gland, 511. Thyroid Gland, 513. Tickling, 907. Tongue, papillae of, 943. Tonicity of arteries, 365, 584; of muscle, 364-3"66. Torpedo, electricity of, 771-774. Torpidity, induced by cold, 136; by want of moisture, 158-161. Touch, sense of, 939-942. Tubercula quadrigemina, 873, 900. Tubercular diathesis, 626, 638, 639. Tunicata, nervous system of, 850, 851. Tympanum, 951. Ulceration, 635. Urea, 730, 731. Uric acid, 732, 733. Urine, composition and properties of, 729- 741; effects of suppression of, 741. Vascular area, 551, 813, 814. Vegetation, influence of light upon, 79-92; influence of Heat upon, 98-107. Veins, movement of blood in, 605-610. Venous congestion, 609, 610. Villi of mucous membrane, 241, 242. Vitreous body of eye, 276. Vocal ligaments, 974-979. Voice, production of, 973-979. Vowel sounds, production of, 981. W Waste or disintegration of tissues, 361, 384, 617. ' White fibrous tissue, 189, 190. Worm tribes, circulation in, 552; respiration in, 657. y Yellow fibrous tissue, 189, 192. Zona pellucida, 802. THE END. LEA AND BLANCHARD Publish the following Valuable Works BY DR, CARPENTER, ELEMENTS OF PHYSIOLOGY, INCLUDING HYSIOLOGICAL ANATOMY, FOR THE USE OF THE MEDICAL STUDENT. With One Hundred and Eighty Illustrations. By W. B. CARPENTER, M.D., F.R.S., Lecturer on Physiology to the Bristol Medical School, &c. &c. In one very neat Octavo Volume. POPULAR TREATISE ON VEGETABLE PHYSIOLOGY. By W. B. CARPENTER, M. D., F.R.S., &c. Published under the Auspices of the Society for the Promotion of Popular Instruction. In one neat duodecimo Volume, Extra Cloth, WITH NUMEROUS ILLUSTRATIONS. Lea and Blanchard will publish the POPULAR CYCLOP/EDIA OF NATURAL SCIENCE, BY DR. CARPENTER, OF WHICH THE VEGETABLE PHYSIOLOGY FORMS A PART. (In press.) PRINCIPLES OF GENERAL AND COMPARATIVE PHYSIOLOGY, Intended as an Introduction to the Study of Human Physiology and as a Guide to the Philosophical Pursuit of Natural History. BY W. B. CARPENTER, M. D., F. R. S., &c. &c. FROM THE SECOND LONDON EDITION. With Alterations and Further Improvements by the Author. IN ONE VERY NEAT OCTAVO VOLUME. With Numerous Illustrations, Beautifully Executed on Stone. « This is an admirable work, and will give Dr. Carpenter a high rank among the cultiva- tors of Natural Philosophy. We. strongly recommend it to all who have leisure for the deliehtful subject of which it treats.''—Medical Gazette. "There are very few persons in Great Britain who could have undertaken, with any prospect of success, the execution of a plan requiring such varied and, at the same time, such accurate knowledge. It is difficult to decide in what department Dr. Carpenter dis- plays most research, for in each he appears as though that alone had engaged his exclusive attention. In fact, a work displaying so much erudition and so much knowledge derived from actual observation, we have not met with in any young English writer of the present dT The volume contains just so much of each subject as it actually deserves, considered not by itself in an insulated manner, but as a part of a great and systemat.c whole."- Dublin Medical and Surgical Journal. A NEW EDITION OF CARPENTER'S HUMAN PHYSIOLOGY, REVISED AND MUCH IMPROVED, PRINCIPLES OF HUMAN PHYSIOLOGY, WITH THEIR CHIEF APPLICATIONS TO PATHOLOGY, HYGIENE, AND FORENSIC MEDICINE. BY WILLIAM B. CARPENTER, M.D., F.R.S., &c SECOND AMERICAN, FROM A NEW AND REVISED LONDON EDITION. WITH NOTES AND ADDITIONS, BY MEREDITH CLYMER, M.D., &c. With Two Hundred and Sixteen Wood-cut and other Illustrations. In one octavo volume, of about 650 closely and beautifully printed pages. The very rapid sale of a large impression of the first edition is an evidence of the merits of this valuable work, and that it has been duly appreciate4 by the. profession of this country. The publish- ers hope that the present edition will be found still more worthy of approbation, not only from the ad- 2™°. SLi aTv. a"u editor' butaIso from its superior execution, and the abundance of its illustra- artded X^f^ht^^hfty-fiye>vood-cutsa"d another lithographic plate will be found to have been aw a/l0rdlnS the most material assistance to the student. on n^in?nV™T?LSatlSfa?ti0n in decla/inS our oP^on that this work is the best systematic treatise M&CKrZguTj^^***' and the b6St adapt6d fOTthe Student «ittin8 in a^ lan»««45e.»- hibitthiSr\mlrn3^H,ld-8 I? the 6nll Treatise.°n Physiology in the English language which ex- " Few indivf, fn?^nnM h ' ? comprehensive y,ew of the present condition of that science. t ew individuals could have been found so well qua Red as Dr. Carpenter for actin* as the histo- ^loffl'itt^mTTi'-t- He is„?nd™ed witli great PerwZrBn£SZi&£^MA KniXT, «te„tV«fwp™ ^f S6e dJTctl7 the saHent P°i"ts of the more abstruse and disputed doc- KuStt In exffinSZ ~npS^£ra.,,-Za,i0n; 1nd Can eXpreSS his ,houShts m lucid *nd correct Ian- portun.tj of ffi „g'outhow SeWotoScaffitf ™d ^T "'"^ •"! •B0 Pr°ne- He 0mitS n° °P" in furnishing more scientitl^^^0?S& d?B1a^» tilT/Jo T™?8*8,n W be employed of Medical Sciences «««,u!> oi treating aisease."— £he London^ Edinburgh Monthly Journal b-ntnTby^^^ ' Human Physiology,' many of its detaTare ™ 'h^i lucldne/!J .of arrangement. Although entitled development of tissuedTwhfch man nosiesse^n ™ gY ^ ^°S^Y^ or the minute anatomy and however, who are fond™ such SrVeSon. and who u' the re"?f the animated creation. They, the transcendental as well „ThemoKwvie™ nf™« "e Wh° 1S not„BS,n. or less so> wiU filld Library. me more souer views of modern inquiries well depicted."—Am. Medical degree of success in making abstruse Xec^etan^TP °-^ Powersof analysis, an uncommon life which he so well underitands whir-h win if, ' . i?rcAbly ""Pressing upon others the laws of insensible to praise. All who can Ifford ZZlTa ^t0 Dr- CarPe™er's reputation when he will be those who are able to keeiTpacewkh the orolr4 8°od»ystem of physiology'should possess this; and pages, large sized octavo, on S D nerS»l "f •h°U,ld n°l be without k' There are 618 necessity seems to exist fo'r extfacTing from hs naVZP^ aS dlstlnct'y mad« as it can be executed. No or portions of the volume, because it is Meaumedtf™ °ar)1commentln8: especially on any particular parts bly this improved edition does not cost more n^n «„ fl■ ?h? Can W1" avail *emaelvei of it. Proba- it is superior in very many re»pectt.-'^SMtonA&rf *<£ / Pr'Ce asked for il in England, and yet " It would be a dereliction of'our biblio^mhwSl h£hTg- Journ,al- work of Dr. Carpenter on the Principlesof h,™*!, pP" nof sPeclaHy to mention the highly meritorious published of equal value in the dSZuLf^tch^}0^' a T°rk to which there has been none of facts and modern discoveries in anatornv Md ,u.w* ' *mbodylnS' as " does, an immense store Retrospective Address. ' aualomy and physiology down to the present time."—Dr. Black's "It is a clear compendious re'sume of the p*Utin«,,.„. <• ™ .. ecuted ma genuine philosophical spirit, andnecuhl h™h ^yswlogical Science, conceived and ex- facts of physiology are presented ma we 11 ffitS1 I adapt«d l0 !he medical Student. All the received from them show close reasoning, and a very'impartial and lu,cld manner, and the deductions made CATALOGUE OP MEDICAL AND SURGICAL WORKS, PUBLISHED BY LEA & BLANCHARD, PHILADELPHIA. July, 1850. TO f HE MEDICAL PROFESSION. ££ w'hT Ue^ rio^hT.™ 5 M W*. Principal I^^-g^ the Union, fiom whom, or from the publishers particulars respecting price, &c, may be had on application. Philadelphia, May, lb50. Dictionaries and journals. American Journal of the Medical Sciences, quar- terly, at $5 a year. Cyclopedia of Practical Medicine, by Forbes, Tweedie, &c, edited by Dunglison, in 4 super royal volumes, 3154 double columned pages. Dunglison's Medical Dictionary, 7th ed., 1 vol. imp.8vo.,9l2 large pages, double columns. Hoblyn's Dictionary of Medical Terms, by Hays, 1 vol. large 12mo., 402 pages, double columns. Medical News and Library, monthly, at $ 1 a year. ANATOMY. Anatomical Atlas, by Smith and Horner, large imp. 8vo., 650 figures. New and cheaper ed. 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Cooper on Hernia, 1 vol. imp. 8vo., many plates. Cooper on the Testis and Thymus Gland, 1 vol. imperial 8vo., many plates. Cooper on the Anatomy and Diseases ofthe Breast, Surgical Papers, &c. &c, 1 vol. imp.Svo., pl'ts. Druitt's Principles and Practice of Modern Sur- gery, 1 vol. 8vo., 576 pages, 193 cuts, 4th ed. Dufton on Deafness and Disease ofthe Ear, 1 vol. liu2o., 120 pages. Durlacher on Corns, Bunions, &c, 12mo.,134 pp. LEA & BLANCHARD'S PUBLICATIONS —(Medical Works) 3 Fergusson's Practical Surgery, 1 vol. 8vo., 3d edition, 630 pages, 274 cuts. Guthrie on the Bladder, 8vo., 150 pages. Jones' Ophthalmic Medicine and Surgery, by Hays, 1 vol. 12mo., 529 pp., cuts and plates. Liston's Lectures on Surgery, by Mutter, 1 vol. 8vo., 566 pages, many cuts. Lawrence on the Eye, by Hays, new edition, much improved, 863 pages, many cuts St plates. Lawrence on Ruptures, 1 vol. 8vo., 480 pages. Miller's Principles of Surgery, 2d edition, 1 vol. 8vo.,538pp., 1848. 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Youatt on the Pig, a new work, with beautiful il- lustrations of all the different varieties, 12mo. Neill and Smith's Compend of the Medical Sciences, 1 vol. large 12mo., 900 pages. 350 cuts. MEDICAL BOOKS IN PRESS. A^oStelvor^ Malgaigne's Operative Surgery In one vol. bvo. (TuSine £ the Medical News and Library.) De La Beche's Geology, with many illustrations. A new workon PopularMenicine,one vol. i-vo. A Cyclopediaof Anatomy and Physiology, based on the large work £ Todd Graham's Chemist.y. by Bridges. 2d edition, much enlarged. One vol. 8vo, several hundred cuts. Mei« on some of the more important diseases of Infants, (nearly ready.) Longel't Treatise on Physiology, Rita 8VO*elated by Smith, (preparing). Together with other new and important work*. 4 LEA & BLANCHARD'S PUBLICATIONS. THE GREAT AMERSCAN MEDICAL, DICTIONARY, NEW AND ENLARGED EDITION.—Lately Issued. MEDICALLEXICON; A DICTIONARY OF MEDICAL SCIENCE, CONTAINING CONCISE EXPLANATIONS OF THE VARIOUS SUBJECTS AND TERMS, WITH THE FRENCH AND OTHER SYNONYMES; NOTICES OF CLIMATE AND OF CELEBRATED MINERAL WATERS; FORMULA FOR VARIOUS OFFICINAL AND EMPIRICAL PREPARATIONS, ETC. BY ROBLEY DUNGLISON, M. D., &C SEVENTH EDITION, CAREFULLY REVISED AND GREATLY ENLARGED. In One very large and beautifully printed Octavo Volume of over Nine Hundred Pages, closely printed in double columns. Strongly bound in leather, with raised bands. This edition is not a mere reprint ofthe last. To show the manner in which the author has la- bored to keep it up to the wants of the day, it may be stated to contain over SIX THOUSAND WORDS AND TERMS more than the fifth edition, embracing altogether satisfactory definitions of OVER FORTY-FIVE THOUSAND WORDS. Every means has been employed in the preparation of the present edition, to render its me- chanical execution and typographical accuracy in every way worthy its extended reputation and universal use. The size of the page has been enlarged, and the work itself increased more than a hundred pages; the press has been watched with great care; a new font of type has been used, procured for the purpose; and the whole printed on fine clear white paper, manufactured expressly for it. Notwithstanding this marked improvement over all former editions, the price is retained at the original low rate, placing it within the reach of all who may have occasion to refer to its pages, and enabling it to retain the position which it has so long occupied, as THE STANDARD AMERICAN MEDICAL DICTIONARY. We have examined Die Lexicon ("or a large number of words, including such terms as Anajsfhesia,Otiatria, Pyelitis. Mastms, and Stomatitis, which are not commonly met wilh in medical dictionaries, and on which medical readers occasionally require information; and we have found them with an explanation of their classical ongm. and the signification under which they are employed. Dr. Dunglison's Lexicon has the rare merit that it certainly has no rival in the English language for accuracy and extern of references. The terms generally include short physiological and pathological descriptions, so that, as the author justly observes the reader does not possess in this work a mere dictionary, but a book, which, while it in- structs htm in medical etymo.ogy. furnishes him with a large amount of useful information That Journal. age — ?i;rslo^tmcomnlT,rnledteai Y^™'1™? '° !he cu'lV10™ °f medical science.-Bo.ton Med Jour, W*»S™fCa' Lex.con-certa.nly one of the best works of the kind in the languag The most complete Medical Dictionary in the English language.- TTWem Lancet .,sr53r::rwy has notus ^^a^ ^^^^^.-8*. Loomed. cornSUt ffi^SL^^ the o„e before us the most ^or^!^ L^r-at-r^-- .Js£rX"°Jo"^ Dictionary now extant.-So«rtmi Medical looking for every new and strand word we con 1 .hin^r lCuU l° lma«'»e- We can only say that, after than a few of most recent introX°S Te haVe "0t been disappointed in regard to more P.£of.yenebr..-JMra Dr. Dunglison's masterpiece of literary labor.- N. Y. Journal of Medicine. a ™«r«,«„ HOBLYN'S MEDICAL DICTIONARY A DICTIONARY OP THE TERMS USSB m MEDICIZfE AND THE COLLATERAL SflFNirirQ BY RICHARD D. HOBLYN A M oV™ BY IRSEAACEHAYSTMNDUT°UTS 'T^^«"£^k™ EMTION, w!,. ' * '• In °De large r°?al 12rno' ™lu™ of 402 pages, double column* ^erj^KK^X^?thlS Sma" a"d Ch6ap V0l-e » *• "bmy of L'y student In* pra" LEA & BLANCHARD'S PUBLICATIONS.—(Surgery.) 5 LIBRARY OP SURGICAL KNOWLEDGE. A SYSTEM OF SURGERY. BY J. M. CHELIUS, Doctor of Medicine and Surgery, Public Professor of General and Ophthalmic Surgery, &c. &c. in the University of Heidelberg. TRANSLATED FROM THE GERMAN, AND ACCOMPANIED WITH ADDITIONAL NOTES AND REFERENCES, BY JOHN F. SOUTH, Surgeon to Saint Thomas' Hospital. Complete in three very large octavo volumes of nearly 2200 pages, strongly bound, with raised bands and double titles: or in seventeen numbers, at fifty cents each. . We now cordially congratulate him (Dr. South) on the completion of his task, and, as a whole, on the mode in which he has fulfilled it. Mr. South's ambition, however, was not limited to the production of a mere translation of his author, with the addition of such occasional notes as might render the handbook more acceptable to the English student; but he aimed at the higher object of supplying what he truly ob- serves has been a desideratum for some time past in English surgical literature, a complete "System of Sur- fery," suited to the wants ofthe practitioner, and worthy ofthe country and language in which it is written. n achieving this end, the character of the work as it appeared in German, and the purpose for which it was published, have been in a great degree sacrificed; and we scarcely anticipate that the present "System" will ever become the manual of the student, who, for the most part, prefers more succinct descriptions of disease, and a more dogmatic style. This, however, is not a subject of regret, for there was no lack of "Out- lines" and "Compendiums," which fully answer the purpose for which they were written; and we are there- fore better pleased to welcome the work in its existing form. It will be impracticable to give anything like a satisfactory analysis of the whole of Mr. South's copious work: we shall, therefore, content ourselves with laying before our readers an outline of the contents and arrangement of the volumes before us, selecting, in passing, certain divisions or chapters, as subject-matter for especial comment. To those who are familiar with the manual of Chelius in the original, the comparative bulk of Mr. Youth's translation will at once convey a just impression of the copious introduction of new matter into the English version. The plan adopted is this: The text of the original is printed in a larger type, with numerical head- ings to the paragraphs. The introduced matter consists of quotations, all of which have their appropriate references, and of the results of the translator's own experience. This is printed in a smaller type, and included between brackets, Mr. South's own opinions being further distinguished by the insertion ot hi- ini- tials at the close of the paragraphs containing them. There are, further, other occasional paragraphs, like- wise printed in a smaller type, which consiitute part of the original work, where they also appear under the same distinguished form. These, when they exist, immediately follow the principal text, ana are not con- tained between brackets. The arrangement of the work is based on scientific principles, and its contents are comprised under the following eight divisions: 1, Inflammation; 2. Diseases which result from the Disturbance of Physical Con- tinuity; 3. Diseases dependent on Unnatural Coherence; 4. Diseases dependent on the Presence of Foreign Bodies; 5, Diseases which consist in the Degeneration of Organic Parts, or in the Production of New Struc- tures; 6, Loss of Organic Parts; 7. Superfluity of Organic Parts; 8, The Elementary Proceedings of Surgical Operations. These are preceded by a brief introduction, a historical sketch of Surgery, and a copious lanie of its general literature. Of the arrangement of the work we cannot but approve, and think it judicious of Mr. South not to have interfered with the plan selected by his author. The annotations and additions evince infinite research and great judgment. The translator enters entirely into the spirit of his author, and we should.say that there is much that is congenial in their cast of mind; leaning;in surgical practice, rather to the cautious than to the heroic The tabular views of operations, especially of stone cases, are valuable and interesting in a statis- tical point of view. A very copious index is appended to the work, which of course greatly enhances the value of so voluminous a treatise. On the whole, we feel gratified with, and proud of, the work in its English garb; and we do not hesitate to pronounce it the best and most comprehensive system of modern surgery with which we are acquainted, and as such we earnestly recommend it to the student and practitioner — Medico-Chirurgical Review. ' . The fullest and ablest digest extant of all that relates to the present advanced state of burgical fathology.— American MedicaJ, Journal. . If we were confined to a single work on Surgery, that work should be Chelius's — St. Lows Med. Journal. As complete as any system ot" Surgery can well be.—Southern Medical and Surgical Journal. The most finished system of Surgery in the English language.- Western Lancet The most learned and complete systematic treatise now extant— Edinburgh Mediral Journal. No work in the English language comprises so large an amount of information relative to operative medi- cine and surgical pathology.—Mediral Gazette. . , ,., , c .u A comolete encyclopedia of surgical science-a very complete surgical lihrary-by far the most complete and Stific system ofsurgery in the English language.-JY. Y. Journal of Medicine One oftne most completeVreatises on Surgery in the English language -Monthly Journal of Med. Science. The most extensive and comprehensive account of the art and science of Surgery in our language.—iancrt. A TREATISE ON DISEASES OF THE BONES. BY EDWARD STANLEY, F. R. S., President ofthe Royal College of Surgeons of England, and Surgeon to St Bartholomew's Hospital. In one neat octavo volume, extra cloth, of 286 pages. A work which every medical or surgical pathologist must conmh.-London Medical Gazette^ PeTul'Lly wellldapted for practical reference as well as for closet meAaaUon.-Amencan Med. Journal. We scarcely know of a more useful and important work It should be in every surgeon's hands and the i ^Lo.iTioner of mediciiie would receive gratifying assistance from n.-BostonMed. and Surg. Journal. general P™e""0"?r°'S »ave bee" madet0 lhe medical profession, than this excellenttreat.se on1h^rseasesPof the bonef A philosophical monograph on this subject was needed, and we are thankful that the duty of W co&l.^ admirable work, at ,he very threshold of some of S most mpormrTt S 1° many points of view, we regard Mr. Stanley's » Treatise on the Diseases ofthe most imP°"""lb" / ,aan|e conlribu'ion to surgery we have seen for years, and we desire to make ourr'eadTrs as"ami?iar withitsrevelauons as a full anSly L can make them.-TAe WesUrn Journal ofMedi- cine and Surgery 6 LEA &, BLANCHARD'S PUBLICATIONS— (Surgery.) New and Improved Edition of THE STUDENT'S TEXT-BOOK. THE PRINCIPLES AND PRACTICE OF MODERN SURGERY. BY ROBERT DRUITT, Fellow of the Royal College of Surgeons. A New American, from the last and improved London Edition, Edited bt F. W. SARGENT, M.D., Author of "Minor Surgery," &c. ILLUSTRATED WITH ONE HUNDRED AND NINETY-THREE WOOD ENGRAVINGS. In one very handsomely printed octavo volume of 576 large pages. In preparing the new edition of this popular text-book, every care has been taken so to improve it in every respect as to raise it still higher in the estimation of the profession. The edition from which this is printed has large and important additions by the author; while the present editor Dr. Sargent, has added whatever appeared necessary to render the book a correct exponent of the' present state of surgical science in this country. The illustrations have been entirely remodeled- numerous new ones added by both author and editor; and many superior ones substituted for those rejected. The amount of these changes may be estimated from the fact, that ofthe 193 wood-cuts at present in this volume, more than one-half have appeared in no former American edition. In mechanical execution, also, the work will be found much improved; in clear type, white paper and handsome pr.ntmg, it will compare favorably, with the best executed works published in the country, while the price is still kept so low as to place it within the reach of all. t..- , ■ ., . , ,. From Professor Brainard, of Chicago, Illinois. I think it the best work of its size, on that subject, in the language. r i,,™ k». ■ , a ■ From.Prof •. From Professor May, of Washington D r I cannot withhold my approval oSKti!?^ of Baltimore. of the student. 1 sha/com^end it°to my'class,' a'dmak^i^X^.^ok." **"" SUhed * the ™m FERGUSSON'S OPERATIVE_SURGERY. NEW EDITION A SYSTEM OF PRACTICAL SURGERY BY WILLIAM FERGUSSOxY, F.KSE THIRD AMPPTP4Mrg:ry " K",g'S C°Ue^' L°»do"' * *°-'' with £%£££?£ ZZ^^sVNGUm EDITI0N- In one large and beautifullZin "d ocfa v„ ^ T™* by GUbert & Gihon" Itis with unfeigned satisfaction th t w'e" .HI,ae,til 7Z "X ""^ ",d ^ ™"- work, t richly deserves the reputatkm conceded^ U °A°f ,he Pr?fes^on in this country to this excellent 1 "pH lZiTr-~MedlalEmm^ ' g thC b6St PraCtlCal S^geryVxtant aUea t in rrotessor bergussons work, we feel nprsnoriori -n v. Sub£°c7in^^^ !t dese™8' ^ it combines as/was szss? The "'—> b«-^bs MILLER'S PRINCIPLES OF SURGERY Professor of Sunrerv in ih^ ti ■ >. °' -^-j Second American edition. I„ onIZZTvT™*??^"^ &c" octavojoW of five hundred and thirty-eight page*. BY THE SAME AUTHOR. THE PRACTICE OF siiRrcDv Second American edition. In one ont,„ , ° U K « t R Y, These two works are printed and bound to Latch T°lume °f five hu«dred pages. Taken together they form a very condensed Bnn ' "* t0Kvtbst a complete System of Surgery. y "ter since the days of Celsus.-iV. O. Med. and Surg. LEA & BLANCHARD'S PUBLICATIONS.—(Surgery.) 7 PART SECOND —WOW READY. SURCICA L~A N A T O M Y. BY JOSEPH MACLISE, Surgeon. To be complete in Four Parts, large imperial quarto, containing from Twelve to Sixteen Colored Plates. Price $2 00 each. Forming one large Imperial Quarto Volume, containing from Fifty to Sixty large Plates, many of the size of Life. Drawn in the best style, and beautifully colored. TOGETHER WITH ABOUT 150 PAGES OF LETTERPRESS. 85" PART III. is rapidly passing through the press, and will be issued at an early day. PLATES IN PART I. Plates 1 and 2.—Form ofthe Thoracic Cavity and Position of the. Lungs, Heart, and larger Blood- vessels. Plates 3 and 4.—Surgical Form of the Superficial Cervical and Facial Regions, and the Relative Positions ofthe principal Blood-vessels, Nerves, &c. Plates 5 and 6.—Surgical Form of the Deep Cervical and Facial Regions, and Relative Positions ofthe principal Blood-vessels, Nerves, &c. Plates 7 and 8.—Surgical Dissection ofthe Subclavian and Carotid Regions, and Relative Anatomy of their Contents. Plates 9 and 10.—Surgical Dissection of the Sterno-Clavicular or Tracheal Region, and Relative Position of its main Blood-vessels, Nerves, &c. Plates 11 and 12.—Surgical Dissection ofthe Axillary and Brachial Regions, displaying the relative order of their contained parts. Plates 13 and 14.—Surgical Form ofthe Male and Female Axillae compared. Plates 15 and 16.—Surgical Dissection of the Bend of the Elbow and the Forearm, showing the Relative Position ofthe Arteries, Veins, Nerves, &c. PLATES IN PART II. Plates 17, 18 and 19.—Surgical Dissections ofthe Wrist and Hand. Plates 20 and 21.—Relative Position of the Cranial, Nasal, Oral, and Pharyngeal Cavities, &c. Plate 22.—Relative Position ofthe Superficial Organs ofthe Thorax and Abdomen. Plate 23 —Relative Position ofthe Deeper Organs ofthe Thorax and those ofthe Abdomen. Plate 24.—Relations of the Principal Blood-vessels to the Viscera of the Thoracic Abdominal PlateTsf— Relations of the Principal Blood-vessels of the Thorax and Abdomen to the Osseous Skeleton, &c. , Plate 26.—Relation of the Internal Parts to the External Surface ofthe Body. Plate 27—Surgical Dissection ofthe Principal Blood-vessels, &c, of the Inguinofemoral Region. Plates 28 and 29.—Surgical Dissection of the First, Second, Third, and Fourth Layers of the Inguinal Region, in connection with those ofthe Thigh. From this brief summary of the plates in the two numbers now ready, some est.mate can be formed ofthe plan ofthe work, and ofthe manner in which its execut.on has been attempted. No complete work of the kind has as yet been published in the English Language, and it therefore will supply a want long felt in this country of an accurate and comprehens.ve Surgical Anatomy, o which the student and practitioner can at all timesrefer, to refresh the memory when called on to performoperations, in the absence of material for dissect.on. Notwithstanding the large stze beautvT and finish of the illustrations, it is put at a price so low as to place it w.th.n he reach of alT in expectation of a very extended circulation. During the short period wh.ch has elapsed since tie publication of the first number, it has met with the unanimous approbation of the profession, and from among a large number of commendatory notices, the publishers beg to submit a few. From Professor R. L.Howard, Columbus, Ohio. r n . .1,0 fl-. number is the bediming of a most excellent work, filling completely what might In al respects, the first number i» thbeg1.1 »'K ™ lf . behalf f the medical profession, I be considered hitherto a vacuum insure cal '^.ore ror y am fident ,hat it wil, ££ withTre^ IZ ^Sl ^hav^tkfnTitln the highest terms to my class and my profes- sional brethren. professor E. R. Peaslee, Brunswick, Me. _,.,.,. . , , j -,i .v-o c„,.„a,t nf IWarlise's Sureical Anatomy, and should the other numbers eJafu T^^^^^o^rA^^^^ Price, and all things else, which has ye, been published in the English Language. From Prof. C. B. Gibson, Richmond, Va. . v work on Surgical Anatomy with which I • i ,T„,, ma.iv thanks for the publication of this beautiful work—a work The profession, in my «P»"<>n; °™. °n, of ttcal Ana,omy?i. not surpassed by any work with which which, in the correctness.of ^J^ZmLiXZ-M ihe luho.raphic plates have been executed and loZ^Alt^ Cufho^and ,0 the arts in *• United States. From Prof. J. F. May, Washington, D. C. Having examined the work, I am °P?eaVd to add my testimony to iu correctness, and to ,U value as a work of reference by the surgeon. 8 LEA & BLANCHARD'S PUBLICATI0NS.-(3forg«ry.) MACLISE'S SURGICAL ANATOMY.—Continued. From Prof. Alden Marsh, Albany, N.Y. .... . ... From what I have seen of it. I think the design and execution of the work admirable, and, at the proper limeT mVcoarsS of lectures! I 'hall exhibit it to the class, and give it a recommendation worthy of its great From H. H. Smith, M. D, Philadelphia my < canti- to ray class as I have heretofore done. From Prof. D. Gilbert, Philadelphia. Allow me to say, gentlemen, that the thanks of the profession at large, in this country, are due to you for the republication of this admirable work of Maclise. The precise relationship of the organs in the regions their concise and accurate descriptions, win prove 01 iiiiume vaiuc. imc.->c h«»»^ cUFK.iv.» »T.»..,,,«. wliKh will enable them to refresh their knowledge ofthe important structures involved in their surgical cases, thus establishing their self confidence, and enabling them to undertake operative procedures with every assurance of success. And as all the practical departments in medicine rest upon the same basts, and are enriched from the same sources, I need hardly add that this work should be found in the library of every practitioner in the land. From Professor J. 31. Bush, of Lexington, Kij. I am delighted with both the plan and execution ofthe work, and shall lake all occasions to recommend it to my private pupils and public classes. The most accurately engraved and beautifully colored plates we have ever seen in an American book- one ofthe best and cheapest surgical works ever published.— Buffalo Medical Journal. It is very rare that so elegantly printed, so well illustrated, and so useful a work, is offered at so moderate a price.—Charleston Medical Journal. A work which cannot but please the most fastidious lover of surgical science, and we hesitate not to say lhal if the remaining three numbers of this work are in keeping with the present, it. cannot fail to give uni- versal satisfaction. In it, by a succession of plates, are brought to view the relative anatomy of the parts included in the important surgical divisionsof the human body, with that fidelity and neatness of touch which is scarcely excelled by nature herself. The part before us differs in many respects from anything ofthe kind which we have ever seen before. While we believe that nothing but an extensive circulation can compen- sate the publishers for the outlay in the production of this edition of the work—furnished as it is at a very moderate price, within the reach of all—we desire to see it have that circulation which the zeal and peculiar skill of the author (he being his own draughtsman), the utility of the work, and the neat style with which it ii executed should demand foi it in a liberal profession—N. Y. Journal nf Medicine. Tlii< is an admirable reprint of a deservedly popular London publication. Its English prototype, although not yet completed, has already won its way, amongst our British brethren, to a remarkable success. Its plates can boasi a superiority that places them almost beyond the reach of competition. And we feel too thankful to the Philadelphia publishers for their very handsome reproduction of the whole work, and at a rale within everybody's reach, not to urge all our medical friends to give it. for their own sakes, the cordial welcome it deserves, in a speedy and extensive circulation.— The Mediral Examiner. The plates are accompanied by references and explanations, and when the whole has been published it will be a complete and beautiful system of Surgical Anatomy, having an advantage which is important, and not possessed by colored plates generally, viz . its cheapness, which places it within the reach of every one who may teel disposed to possess the work. Every practitioner, we think, should have a work of this kind withm reach, as there are many operations requiring immediate performance in which a book of reference will prove most valuable.—Southern Medical and Surg. Journal. The work of Maclise on Surgical Anatomy is of the highest value. In some respects it is the best pub- lication oT us kind we have seen, and is worthy of a place in the library of any medical man, while the student coud scarcely make a better investment than this.— The Western Journal of Medicine and Surgery. No such lithographic illustrations of surgical regions have hitherto, we ihink, been given. While the operator is shown every vessel and nerve where an operation is contemplated, the exact anatomist is re- treslied by thOHS clear and distinct dissections which every one must appreciate who has a particle of enthusiasm The English medical press has quite exhausted the words of praise, in recommending this admirable treatise. I hose who have any curiosity to gratify, in reference to the perfectibility of the lith- -' complex mechanism ofthe human body, are invited surgeons and students to patronize a book of such ... hft » SllrVPV nf th.* Qr.icllnal clilll <» vU; u:... .1 :.. .1_____ Boston Med. and Surg. Journal ographic art in delineating the complex mechanism ofthe human body, are invited to examine our specimen copy, it anything will induce surgeons and students to patronize a book of such rare value, and every- ffiJ^^a^rJb^beaSUrvey °f lhearlisticalski11 exhlbilediuthese *-«»*" °f "alure-- M^'tJ^"^"^ accurac>' of the nla,te3 reflect the highest credit upon the anatomical knowledge of Mr. °J°*n,,| 'J11?"HHy reeomme.,d the descriptive commentaries to the perusal ofthe student both of 3_ULr^r> and medicine. These plates will form a valuable acquisition to nractitioners settled in the countrv acquisition to practitioners.settled in the country \\V »rt wpM u 1,11s. ^ t''",ethcaL or Se»eral practice -Edinburgh Medical and Surgical Journal ._..,are well assured that there are none of the cheaper, and but few ofthe more ^vnensiv* wo. per, and but few of the more expensive works on .*...!.....----------:.:_____ -. .i . m .. ___: . , --------,■■— "...»... ...we vcji.uus liiusirunons are ta Ken. an near to havp ^^^"we'kLow'no^6 rSt "e^Uy '-presented The stlrgfcal comrnen Heal. We know of no woik o.i surgical anatomy which can compete with it.-ic tary is pointed and prac- compete with it.—Lancet. A NE-W MINOR SURGERY. ON BAfMGiMO, km OTHER POINTS OF MINOR SURGERY. BY F. W. SARGENT, M. D. in one handsome royal 12mo. volume of nearly 400 pages, with 123 wood-cuts. TW-r-irin«r .V,» .m<,n a . •, From Profcssor Gilbert. Philadelphia. v-^m™o°cofte^ are Moated by very accurate engravings, the work .KS^:; Z?SBF^^*^^ on these subjects, and I fee. my- LEA & BLANCHARD'S PUBLICATIONS.—(Surgery.) 9 LISTOJST & MUTTER'S SUBG£B1T. LECTURES ON THE OPERATIONS OF SURGERY, AND ON DISEASES AND ACCIDENTS REQUIRING OPERATIONS. DELIVERED AT UNIVERSITY COLLEGE, LONDON, BY ROBERT LISTON, ESQ., F. R. S., &c. EDITED, WITH NUMEROUS ADDITIONS AMD ALTERATIONS, BY T. D MUTTER, M. D., Professor of Surgery in the Jefferson Medical College of Philadelphia. In one large and handsome octavo volume of 566 pages, with 216 wood-cuts. About two hundred and fifty pages in this work are contributed by Dr. Mutter, presenting the results of his great experience, and adapting the whole to the wants ofthe American physician and surgeon. Among the additions of Dr. M. will be found full and elaborate essays on Staphyloraphy, the various Plastic Operations, Club-foot, Affections of the Eye, Deformities arising from Burns, and many other important subjects not to be met with in so complete a form in perhaps any other accessible work. The great reputation of both the authors connected with the work, and the tho- rough manner in which it is illustrated, should command for it the universal attention of the pro- fession. " It is a compendium of the modern practice of Surgery as complete and accurate as any treatise of simi- lar dimensions in the English language."—Western Lancet. LIBRARY OF OPHTHALMIC MEDICINE AND SURGERY. A TREATISE ON THE "DISEASES OF THE EYE, BY W LAWRENCE, F. R. S., Surgeon Extraordinary to the Queen. Surgeon to St Bartholomew's Hospital, &c. &c. A NEW EDITION. With many Modifications and Additions, and the introduction of nearly two hundred Illustrations. BY ISAAC HAYS, M. D. In one very large 8vo. vol. of S60 pages, with twelve plates and many wood-cuts through the text. JON11S OBT THE BITE. THE PRINCIPLES-AND PRACTICE OF OPHTHALMIC MEDICINE AND SURGERY, BY T. WHARTON JONES, F. R. S., &c. &c. EDITED BY ISAAC HAYS, M. D., &c. In one very neat volume, large royal 12mo. of 529 pages, with four plates, plain or colored, and ninety-eight well executed wood-cuts. BRODIE'S SURGICAL LECTURES.—Clinical Lectures on Surgery. 1 vol. 8vo., cloth. 350 pp. BRODIE ON THE JOINTS —Pathological and Surgical Observations on the Diseases ofthe Joints. 1 vol. 8vo., cloth. 216 pp. BRODIE ON URINARY ORGANS.—Lectures on the Diseases of the Urinary Organs. 1 vol. 8vo., cloth. 214 DD * * These three works may be had neatly bound together, forming a large volume of " Brodie's * Surgical Works." 7S0 pp. RTCORD ON VFNEREAL A Practical Treatise on Venereal Diseases. With a Therapeutical Summary and Special Formulary. Translated by Sidney Doane,' M. D Fourth edition. 1 vol. 8vo. 340 pp COOPER (SIR ASTLEY) ON THE ANATOMY AND TREATMENT OF ABDOMINAL HERNIA. 1 lar°-e vol., imp. 8vo., with over 130 lithographic figures. COOPER ON THE STRUCTURE AND DISEASES OF THE TESTIS, AND ON THE THYMUS GLAND 1 vol., imp. Svo., with 177 figures on 29 plates. COOPFR ON THE ANATOMY AND DISEASES OF THE BREAST, WITH TWENTY-FIVE MISCELLANEOUS AND SURGICAL PAPERS. 1 large vol., imp. Svo.. with 252 figures on 36 plates. COOPER ON DISLOCATIONS AND FRACTURES OF THE JOINTS.—Edited by Bransby Cooper and J.C.Warren. 1 vol. 8vo., with 133 cuts. 500 pp. DURLACHER ON CORNS, BUNIONS. &c—A Treatise on Corns. Bunions, the Diseases of Nails, and the General Managementof the Feet In one 12ino. volume, cloih. 134 pp GUTHRIE ON THE BLADDER, &c—The Anatomy of the Bladder and Urethra, and the Treatment of the Obstructions to which those Passages are liable. In one vol. 8vo. 150 pp. LAWRENCE ON RUPTURES.—A Treatise on Ruptures, from the fifth London Edition. In one 8vo. vol. sheep. 480 pp. VTATIRV'S DFNTAL SURGERY.—A Treatise on the Deutal Art. founded on. Actual Experience. Illus- trated by 241 lithographic figures and 54 wood-cuts. Translated by J. B. Savier. In 1 8vo. vol, sheep. 2t6 pp ROBERTSON ON°THE TEETH.—A Practical Treatise on the Human Teeth, with Plates. One small volume, 8vo. 230 pp. DUFTON ON THE EAR— The Nature and Treatment of Deafness and Diseasesof the Ear; and the Treat- ment ofthe Deaf and Dumb. One small 12mo. volume. 120 pp. MALGAIGNE'S SURGERY-Operative Surgery, translated, with Notes, by Bnttan. With wood-cuts. ([Now publishing in the "Medical News and Library.") 10 LEA & BLANCHARD'S PUBLICATIONS— (Anatomy.) SHARPEY AND QUAIN'S ANATOMY. Just Issued. HUMAN ANATOMY. BY JONES QUAIN, M.D. FROM THE FIFTH LONDON EDITION. EDITED BY RICHARD QUAIN, F.R. S., AND WILLIAM SHARPEY, M.D., F.R.S., Professors of Anatomy and Physiology in University College, London. REVISED, WITH BfOTES AND ADDITIONS, BY JOSEPH LEIDY, M. D. Complete in Two large Octavo Volumes, of about Thirteen Hundred Pages. BEAUTIFULLY ILLUSTRATED "With over Five Hundred Engravings on "Wood. We have here one of the best expositions of the present state of anatomical science extant. There is not probably a work to be found in the English language which contains so complete an account ofthe progress and present state of general and special anatomy as this. The descriptions which it contains are remark- ably clear and explicit. The American editor has done his task with credit, and a glance at the typogra- phical execution of the work will show that its enterprising publishers have acted their part well in this respect; and in presenting to the American profession this edition, have placed us all under lasting obliga- tions. By the anatomist this work must be eagerly sought for, and no student's library can be complete without it.—The N. Y. Journal of Medicine. We know of no work which we would sooner see in the hands of every student of this branch of medical science than Sharpey and Quain's Anatomy.— The Western Journal of Medicine and Surgery. It may now be regarded as the most complete and best posted up work on anatomy in the language. It will be found particularly rich in general anatomy. The minute structure of bone is described and delineated most clearly and beautifully; in neither will this «?iiT««nI™0rar^ ^i *AnZ WOrk we havei and the delineations, for variety of detail, will be found to excel those even of Todd and Bowman. it P.h„e„«,b?Jii £°rk ,*owsIth«\?'n>oM care in preparation and most thorough knowledge of the subject, and nanicXrlv nn «,"™ , a Je,ad,ng P°S?°n &m°"S ana,omlcal works We would recommend it to students, lTlth%raZZVJic%Tunrn^anatomy or hlstology prefixed t0 lhe speciaI *»«**» °f each *y*: linYuished^f^i'rt^MpT'11° eK?rfS °V grueat indebtedness to Messrs. Lea & Blanchard, the most dis- frallv sunnheri(^ nn?iPUbllSher8'/°r the,m,any valuable works which they have so repeatedly and Quaint AnaTomv W?rX« e °C°aWOn dH W6 -' moJe ',hankful t0 lnem lhan in receiving Sharpey and is no work fuoer^r Wfwr ™ ,Z 1?™,™ Vh-e ?pLn,0n °f J?" who have examined these volumes, that there body WecSt^mmZSl^LhV rhKh lhey S0 ably d^cribe-the minute structure of he human catJournal ^mmenA it too highly to the patronage of the profession.-Sow/W Medical and Surgi- rufarhas beVnTnlLsLTn-r^wf the w?rk haS revea-,ed new beau,ies and °bi<>cts of worth; its pe- hh??di!™^ by preseming in a mas- anatomy and physiology.-The Ohio AJdicTaZsurltc^Journ^ ^ mUCh lhat " U8eful l° "8 8peClal JtVa^evt isTufd «£^Z^^££%>? "—d «»« handsome volumes as the best pimVSSt'. Sctwltha^wto *f£AZ&£TJ* ^ by placinS before ^ indent every de- parts been interwoven,'thano one who makeIrh^o^lfL0- Ti^ ""l"0 skUIf»'ly h*ve the different cuse for neglecting or undervaluinran^mnnrtnn,^ ,k .e basls of hls 8tudles wil1 hereafter have any ex- ^n/oWn^dcaTSSz^'.and SUrglCal anat°my'U COnlains a" lhe information which a student ean desiro- the^ghsh "«Sw!°.nndnthe Zw oSK'nWS ^ ^^ MJhe m0St COmp,ete on that 8ub^t ™ ward to the moft recent chsco^.l^ffi^^^ of knowledge for- oKd?^ for assistance in the prosecutio. Thermo worl i ZZT^™* '" T ^'^ ^^.-Edinburgh Medical Journal /0urna/o/nSc?n;nlneEngh8hlanSua^ t« be preferred to Dr. Quain's Elements of Anatomy.-!™*™ LEA & BLANCHARD'S PUBLICATIONS— (Anatomy.) 11 THE STUDENT'S TEXT-BOOK OF ANATOMY. NEW AND IMPROVED EDITION-JUST ISSUED. A SYSTEM OF HUMAN ANATOMY, GENERAL AND SPECIAL. BY ERASMUS WILSON, M. D. FOURTH AMERICAN FROM THE LAST ENGLISH EDITION. EDITED BY PAUL B. GODDARD, A. M., M. D. WITH TWO HUNDRED AND FIFTY ILLUSTRATIONS. Beautifully printed, in one large octavo volume of nearly six hundred pages. In many, if not all the Colleges ofthe Union, it has become a standard text-book. This, of itself, is sumcientry expressive of its value. A work very desirable to the student; one, the possession of which will greatly facilitate his progress in the study of Practical Anatomy.—New York Journal of Medicine. Its author ranks with the highest on Anatomy.— Southern Medical and Surgical Journal. It offers to the student all the assistance that can be expected from such a work —Medical Examiner._ The most complete and convenient manual for the student we possess.— American Journal of Med. Science. In every respect this work, as an anatomical guide for the student and practitioner, merits our warmest and most decided praise.—London Medical Gazette. HORNER'S ANATOMY. SPECIAL ANATOMY AND HISTOLOGY. BY WILLIAM E. HORNER, M. D., Professor of Anatomy in the University of Pennsylvania, &c. &c. SEVENTH EDITION. With many improvements and additions. In two 8vo. vols, of 1130 pages, with illustrations on wood. It is altogether unnecessary now to inquire into the particular merits of a work which has been so long be- fore the profession, and is so well known as the present one ; but in announcing a new edition, it is proper to state that it has undergone several modifications, and has been much extended, so as to place it on a level with the existing advanced state of anatomy. The histological portion has been remodelled and rewritten since the last edition ; numerous wood-cuts have been introduced, and specific references are made inrougn- out the work to die beautiful figures in the Anatomical Atlas, by Dr. H. H. Smith.-The American Medical .Journal. NEW AND CHEAPER EDITION OF SMITU Sf HOKJTEWS JUTJITOMICJIL. JlTLJlS. AN anatomYcal ATLAS, ILLUSTRATIVE OF THE STRUCTURE OF THE HUMAN BODY. BY HENRY H. SMITH, M. D., &c. UNDER THE SUPERVISION OF WILLIAM E. HORNER, M.D., Professor of Anatomy in the University of Pennsylvania. In one large octavo volume, with about six hundred and fifty beautiful figures. wt.v, ,iw v,pw of extending the sale of this beautifully executed and complete "Anatomical Atlas," the With ^^'^^"^'"l^edTtion, printed on both sides ofthe page, thus materially reducing its cost, PUHllShBehW?hem to nresentTatTprTceabout forty per cent, lower than former edit ons, while, at the same and e"8011"^™^/^" a e s iAno respect deteriorated, and not a single figure is omitted time, the execution ot eacn plateis 111 au re H com„|ete and accurate representation of that wonderful These figures are well eir «•**& LONGET'sTFh YSIOLOGY.- Preparing. A TREATISE ON PHYSIOLOGY. BYF. A. LONGETMD "^^^^n™^™*^™™*,'*, FRANCIS GURNEY SMITH, M. ft, u.i rnysioiogy in tne l ruladelphia Association for Medical Instruction, &x. The nrof.c • • u * W° large °CtaV° volumes. with numerous illustrations. *ivin!CTeVe^ «*• work of the celebrated teacher, M. Longet. as at the same time it presen° a full at d comulete treaiU aS T °f ,he hlghe8t a«'horities "» Physiology, while the most advanced state of science. Po,lUllSt'xteUi'lve subject, thoroughly brou|nt up to LEA & BLANCHARD'S PUBLICATIONS —(Physiology.) DUNGLISON'S HUMAN PHYSIOLOGY. HUMAN PHYSIOLOGY. WITH THREE HUNDRED AND SEVENTY ILLUSTRATIONS. BY ROBLEY DUNGLISON, M. D., PROFESSOR OF THE INSTITUTES OF MEDICINE IN THE JEFFERSON MEDICAL COLLEGE, PHILADELPHIA, ETC. ETC. SIXTH EDITIuN, GREATLY IMPROVED. In two large octavo volumes, containing nearly 1350 pages. Notwithstanding the numerous treatises which have appeared, during the last few yaars on this important subject, the work of Dr. Dunglison maintains its position as one of the fullest and most complete systems of physiology accessible to the profession in this country. As new editions are continually and frequently demanded, and as the author spares neither care nor exertion to keep each successive impression fully up to the most advanced state of science, the student may always rely upon being able t© procure an edition containing the most recent investigations and discoveries. It has long since taken rank as oneof the medical classics of our language. To say that it is by far the best text-book of physiology ever published in this country, is but echoing the general testimony ofthe protession. —N. Y. Journal of Medicine. The most full and complete system of Physiology in our language.— Western Lancet. The most complete and satisfactory system of Physiology in the English language.—Amer. Med Journal. The best work ofthe kind in the English language—Silliman^s Journal. ,< . , We have, on two former occasions. Drought this excellent work umlerthe notice of our readers, ana we ha.-e now only to say that, instead of failing behind in the rapid march of physiological science, each edition brings it nearer to the van.—British and Foreign Medical Review. A review of such a well-known work would be out of place at the present time We have lookedover it, and find, what we knew would be the case, that Dr. Dunglison has kept pace with the science to which lie has devoted so much study, and of which he is one ofthe living ornaments. We recommend the work to the med.cal student as a valuable text-book, and to all inquirers into Natural Science, as one which will well and delightfully repay perusaL— The New Orleans Medical and Surgical Journal. COMPENDIUM OP HIULLER'S PHYSIOIiOGY.-Lately Issued. A MANUAL OF PHYSIOLOGY, FOR THE USE OF STUDENTS. BY WILLIAM SENHOUSE KIRKES, M.D., Assisted by JAMES FAGET, Lecturer on General Anatomy and Physiology in St. Bartholomew's Hospital. In one handsome volume, royal 12mo., of 550 pages, with 118 wood-cuts. This is, certainly, a most able manual of Physiology. The «udem,wiirfind in %™**™l?££$™£ SOLLY ON THE BRAIN. THE HUMAN BRAIN; ITS STRUCTURE, PHYSIOLOGY, AND DISEASES. WITH A DESCRIPTION OF THE TYPICAL FORM OF THE BRAIN IN THE ANIMAL KINGDOM. BY SAMUEL SOLLY, F. R. S., &c, Senior Assistant Surgeon to the St. Thomas' Hospital, &c. From the Second and much Enlarged London Edition. In one octavo volurru, with 120 Wood-cuts. HARRISON ON THE NERVES.-An Essay towards a correet theory of the Nervous System. In one octavo volume, 292!P«*«"- ,NG^ _Lectureson the Physical Phenomena of Living Beings. Edited MATTEUCCI ON GIVING BEWGS^^cture^ ^ j^ ^_^ ^^ ROGET'rpaHYSIOLOGY.L0yA Treatise on Animal and Vegetable Physiology, with over 400 illustrations on wood. In two octavo volumes, ■cloth. Phrenology. In one octavo volume, cloth-516 pages. ^SSSS^^^^STSSSoOT mS .NTEL.SCTUAL SC.ENCE. .» o„« handsome wood-cuts. Th/^:'ou""* °' L„ ^ose who have the commencement will be enabled to procure conclusion may be expected this year, wneu m« the completion. 14 LEA & BLANCHARD'S PUBLICATIONS —(Pathology.) WILLIAMS' PRINCIPLES—JVew and Enlarged Edition. PRINCIPLES OF MEDICINE; Comprising General Pathology and Therapeutics, AND A BRIEF GENERAL VIEW OF ETIOLOGY, NOSOLOGY. SEIVIEIOLOGY. DIAGNOSIS, PROGNOSIS. AND HYGIENICS, BY CHARLES J. B. WILLIAMS, M. D., F. R. S.; Fellow ofthe Royal College of Physicians, &c. Edited, with Additions, BY MEKEDITH CLYMER, M. D., Consulting Physician to the Philadelphia Hospital, &c. &c. THIRD AMERICAN, FROM THE SECOND AND ENLARGED LONDON EDITION. In one volume, octavo, of 440 pages. imTle heASi eJP°si.tj?n in our 'anguage, or, we believe, in any language, of Rational Medicine, in its present improved and rapidly improving stale.-Britishand Foreign Medico°Chirurg. Review. ' P * *JZ„ Ttcom,.nend ?ve.rv Pa" °f Dr- Williams' excellent Principles of Pathology to the diligent perusal of ZfILf f/w^I7]l°w T farm'llar ^ *« acceMion8 which have been made to medical fcTence wUhfa Uie last tew years.— Western Journal of Medicine and Surgery. It fill* .1,^ „i„ f u- u • • From Fr°fessor Thayer, of Boston. It falls the place for which it was intended better than any other work. Tc.ol1, , , , , . From Prof 'essor S. H Dickson, of New York. pianShW $^£^£&%££*$£s.hands °f ™* ™^™ °f •« ?»*-»»■ ^ • MANUALS ON THE BLOOD AND URINE: T CONSISTING OF L^To^iAnT?^ * de!cJ?pti,°n °f the Genera1' Chemical, and Microscopical Cha- both"heir heahhvl11 Secre'lonsfofthe Human Body, as well asof their compounds, including inirrcdiM^ with the best method of separating and estimating their 11 dSliS^^.^wen'^ese^T YLh snd&tTe' a"'d °n the iient of Urin^ "alVrED MiRKWl'S110'1 °f the Uri"e'ln ^^th'and-disease, for the use of students. BY The whole forming one large royal 12*0. volume, of four hundred and sixty pages, WITH ABOUT ONE HUNDRED FIGURES ON FIVE PLATES. COPLAWDON^^ZjuTt Ready. OF THE CAUSES, NATURE, AND TREATMENT OF **,«.*. PAL'SY AND APOPLEXY BY JAMES COPLAND, M. D., F. R. S. &c. In one volume. THE PATHOLOGICAL ANATOMToF THE HUMAN BODY. BY JULIDS VOGEL, M. D &c Translated fro» the German, with Additions, BY GEORGE E. DAY M D ho In one neat octavo volume. ABERCROMBIE ON THF STOMAL o leases. In one neat Svo. ^S&^i^SS^&^^S^ "—*•»" ™.e»~ »r ,»e B,.i„ .* BtiKISTON ON- THE CHEST-T »««»«of,h. H.,„. I„0„,8to.vol,wilh color.,1 pl.i.n, BiEua,0pfRraciapLK- 'rn«'""""'^"""SSi" "" "'""" Di*""" °f "« Ck««. »»« » -i" hS'S AtW^ToIT W5* "' """ Di°^"<,,",' P"h0lOBr' "" TI"«'P»>«e« Indi- gnos.s ofthe Diseasesof the Lungs. In one 12mo. vol., pp. 310. LEA & BLANCHARD'S PUBLICATIONS —(Materia Medica, $c) 15 THE NEW UNIVERSAL FORMULARY.—Just Ready, 1850. A UNIVEHSAlT FOKMULAKT, CONTAINING THE METHODS OF PREPARING AND ADMINISTERING OFFICINAL AND OTHER MEDICINES, THE WHOLE ADAPTED TO PHYSICIANS AND PHARMACEUTISTS. BY R. EGLESFELD GRIFFITH, M. D., Author of " Medical Botany," &c. In one large octavo volume of 568 pagesj double columns. The delay which has taken place in the appearance of this work} has arisen from the great care exercised in its preparation, and the completeness with which formulae have been col- lected from the authors of all nations. The design of the work is to present a compendious collection of formula? and pharmaceutic processes, with such additional information as may render it useful to the physician and apothe- cary; and the principal aim has been to select materials most generally applicable, and of practical utility. The sources from which they have been derived are very numerous, as will be seen by a reference to the various authorities cited. The introduction contains tables and observations on the weights and measures employed for pharmaceutical purposes in the United States and in foreign countries, and an explanation, or vocabulary of the principal abbreviations and Latin terms used by physicians in writing prescrip- tions, followed by observations on the management of the sick-room, with rules for the adminis- tration ofthe different classes of medicines. The formulary is arranged alphabetically, according to the pharmaceutic names adopted in the United States Pharmacopeia; but in each formula, the English appellations for the articles com- posing it are used, and the quantities of these ingredients are expressed in words, and not in the usual pharmaceutic signs. These innovations may, and probably will, be objected to by many; but we feel convinced that a change has become requisite, and that fewer mistakes would be committed by physicians irf writing prescriptions, both in the names ofthe ingredients and in the quantities, were they given at length, and in common language, instead of in the abbreviated cabalistic terms now used. In France this change has been made, and it is full time that other countries should follow her example. In this portion of the work, the compiler has derived much important assistance from several of his friends, and is under great obligation to Mr. Win. Procter, Jr., for numerous formulas and many useful suggestions. He, also, is much indebted to Dr. Robert Bridges for his attentive revision ofthe sheets, as well as for some important corrections. The various tables which follow, it is hoped, will add to the value of the work. The observa- tions and directions on officinal preparations are given in as concise a form as possible; and are, for the most part, condensed from the excellent edition of Mohr and Redwood's Pharmacy, as edited by Mr. Wm. Procter. To this is added a short view ofthe action of poisons, with the best means of obviating their effects. To facilitate a reference to the contents of the work, copious indices have been added, not only ofthe formula?, but ofthe diseases for which they have been advised. To furnish an idea of the variety and value of the contents of this work, a con- densed summary will be found on the next page. Dr Griffith's Formulary is worthy of recommendation, not only on account of the care which has been bestowed on it by its estimable author, but for its general accuracy, and the richness of its details.—Medical Examiner, April, IS50. . , . . . ,. ,. , , . , . , To the more advanced practitioner, it affords occasional assistance in reminding him of combinations which have stood the test of time, and in which experience has shown some superiority of the associated means over their simple and unconnected application. The pharmaceutist will also find advantages in its posses- sion in the positions in which he is frequently placed, either in the demands of his occupation for judicious formula? or prescription of particular combinations under unusual or unfamiliar conventional names, in the extraction of various active principles, of vegetable origin, and in the production of those chemical com- pounds which, by choice or necessity, he may deem advisable to prepare for himself. The sources from which the formulae have been derived are appended to each formula?, and are very nu- „,"„, embracing names of high reputation in medical and pharmaceutical science, the former giving a ithoritv for the rational constitution of the formula? and their applicability to particular slates or stages of disease and the latter the eligibility of the processes and pharmaceutical preparations which they have recommended.-TAe American Journal of the Medical Sciences, April, 1850 The index of diseases and their remedies will be found exceedingly usefu in the selection of formula? for n»rtiVnlar diseases by young practitioners and others who consult the work with the same view. On the »ho e we view this work of Dr. Griffith's as a valuable acquisition to the medical practitioner, and as afford- ing a useful book of reference to the apothecary on numerous occasions —American Journal of Pharmacy, A Very rarely is a work combining so many advantages for the student and young practitioner, placed at thAhhou^hi.his is a simple compilation, it is really admirably executed, and will be found worthy of patron- age It is a volume of 567 octavo pages, not one of which but was required to give completeness to the Mn;ipru.Vin ™ACollection!f?r£r£EDICAL FORMULARY: To which is added an ArjDenHi «*s>icians of America and Europe. the whole accompanied wrrH1? "!we USUal Diet6tiC PrePa««ons and Antidotes for Poisons, BY c, iT^"™ PHARMACEU™ ^d medical observations. LEA & BLANCHARD'S PUBLICATIONS.—(Materia Medica, fa) 17 NEW AND IMPROVED EDITION, to 1850.—Just Ready. GENERAL THERAPEUTICS-AND MATERIA MEDICA; ADAPTED FOR A MEDICAL TEXT-BOOK, BY ROBLEY DUNGLISON, M.D., Professor of Institutes of Medicine, &c, in Jefferson Medical College; Late Professor of Materia Medica, &c. in the Universities of Maryland and Virginia, and in Jefferson Medical College. FOURTH EDITION, MUCH IMPROVED. With nearly Two Hundred Illustrations. In two large and handsomely printed octavo volumes. The steady demand which exists for this standard text-book having afforded an opportunity for the author to revise it, he has carefully embodied in it all the results of therapeutic science since the last edition was put to press. In thus bringing the work up to the present day, he has spared. no pains in chronicling the labors of Continental, English, and American Writers ; and the pub- lishers present the work as entirely worthy ofthe continued favor and confidence with which it has hitherto been met. In mechanical execution it is fully equal to the last edition, while the number of illustrations has been nearly doubled. At the same time, the price has been kept at the former very moderate rate. The most complete and satisfactory exponent of the existing state of Therapeutical Science, within the moderate limits of a text-book, of any hitherto published.—N. Y Journal of Medicine. Mfhica- Our junior brethren in America will find in these volumes of Professor Dunglison, a 'Thesaurus medica MlNUM," more valuable than a large purse of gold.-iondon Medico-Chirurgical Review. Foreisn No medical student on either side ofthe Atlantic should be without these volumes.-^ntoA and Foreign Medical Review. CHRISTISON & GRIFFITH'S DISPENSATORY.-Now Ready. A DISPENSATORY, OR COMMENTARY ON THE PHARMACOPCEIAS OF GREAT BRITAIN AND THE UNITED ' STATES: COMPRISING THE NATURAL HISTORY, DESCRIPTION, CHEMISTRY, PHARMACY, ACTIONS, USES, AND DOSES OF THE ARTICLES OF THE MATERIA MEDICA. BY ROBERT CHRISTISON, M. D., V. P. R. S. E.,_ President ofthe Royal College of Physicians of Edinburgh; Professor of Materia Medica in the University of Edinburgh, etc. Second Edition, Revised and Improved, WITH A SUPPLEMENT CONTAINING THE MOST IMPORTANT NEW REMEDIES. WITH COPIOUS ADDITIONS, AND TWO HUNDRED AND THIRTEEN LARGE WOOD ENGRAVINGS. BY R. EGLESFELD GRIFFITH, M. D., Author of " A Medical Botany," etc. In one very large and handsome octavo volume, of over one thousand closely printed pages, with In one very large ^^^ wood.cuts> beautifully printed on fine white paper. Presenting an immense quantity of matter at an unusually low price. It is enough to say that it appears to us as ^^^^^^%^^^AS^^x cal science, could be made. If it «™u *fj£99 fheomiS has escaped our scrutiny. We cordially ^omnllnVthisS EJSSfo'f 0^^^^ l^need of a Dispensary. They cannot make choice of a ben™-The Western Journal of Medicine and Surgery. . N y Annalist. There is not in any language a ™«W^KS^ Medical Journal As nearly complete as possible- a ™r* °fh^" ™^1» favor it deserves.-^. Jour, ofthe Med Sciences. One ofthe standards ofthe day, ^d as such rn^sl ^^3 °}alo Medical ^ iTlTdSp^ Surgical Journal. , la y proeure Christison 8c Griffith; and to those who do^ss Th^forme^,^1"^"ITJelZno^rZll *^*' as Soon aS oon.eni.nt-*. Louis Medical and Surgical Journal. ^^ profmor Rayburn, of St. Louis. The most valuable, in my opinion, of all the Dispensatories yet published. DUNGLISON ON NEW REMEDIES.-New Edition. NEW R EM EDIES, BY ROBLEY DUNGLISON, M.D., &C. &C. Fifth edition, with extensive additions. In one neat octavo volume. ... u ~„*u, .,nt suitable for either critical or analytical review. It is, so far as it goes, a A work like *"»<*>™£]?0Z^isgfven ofthe chemical and physical properties of all the articles recently dispensatory, in w ,ic.h,a". a„c;°"d"Vei? preparations, with a notice ofthe diseases for which they are pre- 18 LEA & BLANCHARD'S PUBLICATIONS—iMateria Medica, S-c.) MOHR, REDWOOD, AND PROCTER'S PHARMACY.- Just Issued. PRACTICAL "PHARMACY. COMPRISING THE ARRANGEMENTS, APPARATUS, AND MANIPULATIONS OF THE PHARMACEUTICAL SHOP AND LABORATORY. BY FRANCIS MOHR, Ph. D., Assessor Pharmaeiae ofthe Royal Prussian College of Medicine, Coblentz; AND THEOPHILUS REDWOOD, Professor of Pharmacy in the Pharmaceutical Society of Great Britain. EDITED, WITH EXTENSIVE ADDITIONS, BY PROFESSOR WILLIAM PROCTER, Ofthe Philadelphia College of Pharmacy. In one handsomely printed octavo volume, of 570 pages, with over 500 engravings on wood. In presenting the work of Mohr and Redwood to the American Pharmaceutical public, it is un- der the impression that the want of a treatise on the apparatus and manipulations of Practical Phar- macy has long been felt. The Practice of Pharmacy, as conducted in England and in the United States, is sufficiently alike to render this work appropriate as a handbook for the American Apothe- cary; and the eminence of the authors in their respective countries, is a guarantee of the value of the information it contains. In passing through the hands ofthe Editor, the book has been increased more than one-fourth in size, about one hundred wood-cuts have been added, the arrangement of the subjects materially changed, and the work divided into chapters, each of which includes either one distinct subject, or several that have a certain generic relation to each other. One object sought by the change of arrangement has been to fit the work as a text-book for the Editor's class in the Philadelphia College of Pharmacy, as far as its nature will admit, and some of the additions have been made with a view to the same object.—Editor's Preface. From Prof. Lewis C. Beck, ofthe Albany Medical College. It is a capital book, and ought to be in the hands of every apothecary in the country. I shall strongly recom- mend it to my class in the Albany Medical College. From Professor C. G. Page, of Washington. Truly a valuable work, and one which I have long desired to see. The authors have been so full and care- ful in the detail of their illustrations and descriptions, that a careful study ofthe work would be almost equi- valent to an apprenticeship in the laboratory. It will give me great pleasure to commend it to the profession. After a pretty thorough examination, we can recommend it as a highly useful book, which should be in the hands of every apothecary. Although no instruction of this kind will enable the beginner to acquire that practical skill and readiness which experience only can confer, we believe that this work will much facilitate their acquisition, by indicating means for the removal of difficulties as they occur, and sug- gesting methods of operation in conducting pharmaceutic processes which the experimenter would only hit upon after many unsuccessful trials; while there are few pharmaceutists, of however extensive expe- rience, who will not find in it valuable hints that they can turn to use in conducting the affairs of the shop and laboratory. The mechanical execution ofthe work is in a style of unusual excellence. It contains about five hundred and seventy large octavo pages, handsomely printed on good paper, and illustrated by over five hundred rrIIi?ruablyJv^e11 execuled wood-cuts of chemical and pharmaceutical apparatus. It comprises the whole of Mohr and Redwood's book, as published in London, rearranged and classified by the American editor, who has added much valuable new matter, which has increased the size of the book more than one-fourth, including about one hundred additional wood-cuts- The American Journal of Pharmacy. It is difficult to convey a satisfactory idea of a work of this kind, treating as it does, of many various sub- jects, in the limited space at our disposal, and we are conscious that in this hasty notice we have Riven a very inadequate impression of iis merits. It is a book, however, which will be in the hands of almost every one who is much interested m pharmaceutical operations, as we know of no other publication so well calculated to nil a void long felt in the absence of a practical work of this character.— The Medical Examiner. JVEW AJVO COMPLETE MEDICAL, BOTAJYY. Lately Published. MEDICAL" BOTANY; OR, A DESCRIPTION OF ALL THE MORE IMPORTANT PLANTS USED IN MEDICINE, AND OF THEIR PROPERTIES, USES, AND MODES OF ADMINISTRATION, BY R. EGLESFELD GRIFFITH, M. D., &c. &e. In one large 8vo. vol. of 704 pages, handsomely printed, with nearly 350 illustrations on wood. thfverv^arlE II should b? a11 mea»s be introduced at &lo&«ASK^^0i3r«dica, sch00ls'and occupy a p,ace Ll the library of every physicia*in the ™»« » ™* ~»ich P™-- ^-"-* We hopelhe^v t nm^T^7 af°sitive.d*n.ciency in our medical literature.- Western Lancet. college inthUnion'\n°'d,'sta"\when th's wojrk will not only be a lext-book in every medical school and conege in the Union, but find a place m the library of every private practitioner.- N. Y. Jour, of Medicine. CARPENTER ON ALCOHOLIC LIQUORS. Just Ready. ON THE USE OF ALCOHOLIC LIQUORS IOEALTH AND DISEASE. BY WILLIAM B. CARPENTER, M. D Author of" Principles of Human Physiology," &c. ' »__• - . In one 12mo. volume. awaked'.od"6 C^W"^^ ^e best ^«? °" *e above subject, that sum was Dr. W. A. Guy. Emanating from so d&SrfL - ?djud!ca!ors> Dr John Forbes, Dr. G. L. Roupell, and a subjectof Jch ^^^1^ «■ connected view of LEA & BLANCHARD'S PUBLICATIONS.—(Materia Medica, fa) 19 ROYLE'S MATERIA MEDICA. MATERIA MEDICA AND THERAPEUTICS; INCLUDING THE Preparations of the Pharmacopoeias of London, Edinburgh, Dublin, and of the United States, WITH MANY NEW MEDICINES. BY J. FORBES ROYLE, M. D., R R. S., Professor of Materia Medica and Therapeutics, King's College, London, &c. &c. EDITED BY JOSEPH CARSON, M. D., Professor of Materia Medica in the Philadelphia College of Pharmacy, &c. &c. WITH NINETY-EIGHT ILLUSTRATIONS. In one large octavo volume, of about seven hundred pages. Being one of the most beautiful Medical works published in this country. This work is, indeed, a most valuable one, and will fill up an important vacancy that existed between Dr. Pereira's most learned and complete system of Materia Medica, and the class of productions on the other ex- treme, wtiich are necessarily imperfect from their small extent.—British and Foreign Medical Review. Of the various works on the plan of the one before us, there is none more deserving of commendation. Every one who can afford it, should possess this excellent work.—Medical Examiner. We cannot too highly recommend this valuable work, both to the student and practitioner.—Southern Jour- nal of Medicine and Pharmacy. This work is ably done—the botanical part with great skill; and the chemical, natural history, and thera- peutic department most perfect and complete.—Edinburgh Medical Journal. The subject is well treated, the matter practical and well arranged, and we do not hesitate to recommend it as a most useful volume to the student and practitioner.—Medical Gazette. The wood engravings by which the crystals, the vegetable products, and the medicinal animals are illus- trated, are better than anything hitherto attempted in Materia Medica, and must prove a great assistance to the student, appealing as they do more powerfully to the mind than the most careful verbal descriptions taken alone could do.—Lancet. ILLUSTRATED ENCYCLOPEDIA OF MATERIA MEDICA. THE ELEMENTS OF MATERIA MEDICA AND THERAPEUTICS. COMPREHENDING THE NATURAL HISTORY, PREPARATION, PROPERTIES, COMPOSITION, EFFECTS, AND USES OF MEDICINES, BY JONATHAN PEREIRA, M. D., F. R. S. and L. S., Member ofthe Society of Pharmacy at Paris; Examiner in Materia Medica and Pharmacy in the University of London ; Lecturer on Materia Medica at the London Hospital, &c. &c. Second American Edition, Enlarged and Improved. WITH NOTES AND ADDITIONS, BY JOSEPH CARSON, M. D. In two volumes octavo, containing Fifteen Hundred very large pages, illustrated by 275 Wood-cuts. Notwithstanding the large size of this work, and the immense quantity of matter contained in its closely printed pages, it is offered at a price so low as to place it within the reach of all. An Encyclopaedia of knowledge in that department of medical science—by the common consent ofthe pro- fession the most elaborate and scientific Treatise on Materia Medica in our language.— Western Journal of Thfs^ncyclopae^Ta of Materia Medica, for such it may justly be entitled, gives the fullest arid most ample exposition of Materia Medica and its associate branches of any work heretofore published in the English lan- euaa-e.— N. Y. Journal of Medicine. ... , j- _,. i. j The work will be found an invaluable storehouse of information for the physician and medical teacher, and we congratulate the profession of this country that it is now placed within their reach.- Amer. Med. Journal. An authoritative and unerring pharmacological guide.—Medical Examiner. DISPENSATORY AND FORMULARY. A DISPENSATORY AND THERAPEUTICAL REMEMBRANCER. Comprising the entire lists of Materia Medica, with every Practical Formula contained in the three British Pharmacopoeias. With relative Tables subjoined, illustrating by upwards of six hundred and sixty examples, the Extemporaneous Forms and Combinations suitable for the different Medicines. BY JOHN MAYNE, M. D., L. R C. S., Edin., &c. &c. EDITED WITH THE ADDITION OF THE FORMULAE OF THE UNITED STATES PHARMACOPOEIA, BY R. EGLESFELD GRIFFITH, M. D. In one 12mo. volume, of over three hundred large pages. The neat typography, convenient size, and low price of this volume, recommend it especially to physicians, apothecaries, and students in want of a pocket manual. THE THREE KINDS OF COD-LIVER OIL, Comparatively considered, with their Chemical and Therapeutic Properties, BY L. J. DE JONGH, M. D. TRANSLATED, WITH AN APPENDIX AND CASES, BY EDWARD CAREY, M. D. To which is added an article on the subject from " Dunglison on New Remedies." In one small 12mo. volume, extra cloth. 20 LEA & BLANCHARD'S PUBLICATIONS.—(Practice of Medicine.') WATSON'S PRACTICE OF MEDICINE—New Edition. lectureTon the DELIVERED AT KING'S COLLEGE, LONDON, BY THOMAS WATSON, M. D., &c. &c. Third American, from the last London Edition. REVISED, WITH ADDITIONS, BY D. FRANCIS CONDIE, M. D. Author of " A Treatise on the Diseases of Children," &c. IN ONE OCTAVO TOLTJME, Of nearly ELEVEN HUNDRED LARGE PAGES, strongly bound with raised bands. To say that it is the very best work on the subject now extant, is but to echo the sentiment of the medical press throughout the country.— N. O. Medical Journal. Ofthe text-books recently republished Watson is very justly the principal favorite.—Holmes'1 Report to Nat. Med. Assoc. By universal consent the work ranks among the very best text-books in our language.— HI. and Ind. Med. Journal. Regarded on all hands as one of the very best, if not the very best, systematic treatise on practical medi- cine extant — St. Louis Med. Journal. Confessedly one ofthe very best works on the principles and practice of physic in the English or any other language.—Med. Examiner. As a text-book it has no equal; as a compendium of pathology and practice no superior.— IV". Y. Annalist. W e know of.no work better calculated for being placed in the hands of the student, and for a text book. Un every important point the author seems to have posted up his knowledge to the day.—Amer. Med. Journal. One of the most practically useful books that ever was presented to the student—indeed, a more admirable summary of general and special pathology, and of the application of therapeutics to diseases, we are free to say, has not appeared for very many years The lecturer proceeds through the whole classification of human ins, a rapitead calcem, showing at every step an extensive knowledge of his subject, with the ability of com- municating his precise ideas in a style remarkable for its clearness and simplicity.—N. Y. Journal of Medi- cine and Surgery. ■, J A careful examination of this volume has satisfied us that it merits all the commendation bestowed on it in in s country and at home. It is a work adapted to the wants of young practitioners, combining, as it does, sound principles and substantial practice It is not too much to say that it is a representative of the actual »™ \f mecllcine as taught and practised by the most eminent physicians of the present day, and as such we WU J. a(lv,seev,er>'0"e,.a^»« embarking in the practice of physic to provide himself with a copy of it.— vresietn journal of Medicine and Surgery. Mnditllf/6 ul !fV,er^' y.farS considered this one of the best works extant on the Principles and Practice of -Me Med. and Surg. fournalX° ° readers, and the views of the author are sound and practical. THE GREAT MEDICAL LIBRARY. THE CYCLOPEDIA OF "PRACTICAL MEDICINE; COMPRISING Tr-atises on the Nature and Treatment of Diseases, Materia Medica and Thera- peutics, Diseases of Women and Children, Medical Jurisprudence, &c. &c. EDITED BY JOHN FORBES, M. D., F. R. S., ALEXANDER TWEEDIE M D F R S AND JOHN CONNOLLY, M. D. Revised, with Additions, BY ROBLEY DUNGLISON, M. D. THTS WORK IS NOW COMPLETE, AND FORMS FOUR LARGE SUPER-ROYAL OCTAVO VOLUMES Containing Thirty-two Hundred and Fifty-four unusually large Pages in Double Columns, Printed on dood Paper, with a new and clear type. THE WHOLE WELL AND STRONOLY BOUND WITH RAISED BANDS AND DOUBLE TITLES. Or, to be had in Twenty-four Parts. This work contains no less than FOUR HUNDRED AND EIGHTEEX DISTINCT TREATISES By Sixty-eight distinguished Physicians. and^glla^rnal ™* °a PrMtieal Medici"e e*tant >' <"• « >-st, in our language.-Buffalo MedicaX Western Journal of Medicine and Surgery h<5 day-as a work of reference it is invaluable— 4"" ^f ™*»Wi. «SSSa inle^fa-dvaXot ffiy '^7 ^"^ °"e fa WWch animation, but after an intima?acquaintancederi-Jfmmfr ma,e ?f *' ***, "0t bePn fo™edTrom a has'V ex- ten years. The editors are practitioner" of e-taWUhed r?m,,7,nent °a°TU,'-,ation of jt durin* the Past nine °* ofthe most eminent professors Vn^Zheno^^V^m'unA ^- hsX °f contributors embraces many great merit of this work that ,he principal ™ Scl« have' he™ f^- ?",htn- and G,as?ow- II is ^deed, the devoted especial attention to the diseasesabout which fhifA furn,shed hJ Practitioners who have not only for an extensive practical ^LinT^e^Txh^^AlZl *""*?■ ^ have.a,s<> enjoyed °PP°"«nities competencyKistly to appreciate the opinions of oth"« -1-i.J V r"Pn,.R,!on carrie? the "Mnrance of .heir authority.-American Medical Journal ' e u 8tamps their own doctrines with high and just LEA & BLANCHARD'S PUBLICATIONS.—(Practice of Medicine.) 21 DUNGLISON'S PRACTICE OF MEDICINE. ENLARGED AND IMPROVED EDITION. THE ■PRACTICE"OF MEDICINE. A TREATISE ON SPECIAL PATHOLOGY AND THERAPEUTICS. THIRD EDITION. BY ROBLEY DUNGLISON, M. D., Professor ofthe Institutes of Medicine in the Jefferson Medical College ; Lecturer on Clinical Medicine, &c. In two large octavo volumes, of fifteen hundred pages. In Dr. Dunglison's volumes, there is a kind of pervading exactness on every page, that is at once recognized; and, in fact, the medical public has long since decided that implicit reliance may be placed on any work which he permits to appear with his name upon the title-page. A third edition of his treatise on Special Pathology and Therapeutics has just been published. It has pass- ed through so many careful examinations, and received so many improvements, under the vigilant eye ofthe indefatigable man who first gave it existence, that it would be an anomaly in medical literature if it had not grown better and better. The student of medicine will find, in these two elegant volumes, a mine of facts, a gathering of precepts and advice from the world of experience, that will nerve him with courage, and faithfully direct him in his efforts to relieve the physical suf- ferings ofthe race.—Boston Medical and Surgical Journal. Upon every topic embraced in the work the latest information will be found carefully posted up. Medical Examiner. It is certainly the most complete treatise of which we have any knowledge. There is scarcely a disease which the student will not find noticed.—Western Journal of Medicine and Surgery. One ofthe most elaborate treatises of the kind we have.—Southern Medical and Surg. Journal. Much Enlarged Edition, of BARTLETT ON FEVERS. THE HISTORY, DIAGNOSIS, AND TREATMENT OF THE FEVERS OF THE UNITED STATES. BY ELISHA BARTLETT, M.D., Professor of the Theory and Practice of Physic in the Medical Department of Transylvania University, &«. In one octavo volume of 550 pages, beautifully printed and strongly bound. CLYMER AND OTHERS ON FEVERS. FEVERS; THEIR DIAGNOSIS, PATHOLOGY, AND TREATMENT. PREPARED AND EDITED, WITH LARGE ADDITIONS, FROM THE ESSAYS ON FEVER IN TWEEDIE'S LIBRARY OF PRACTICAL MEDICINE, BY MEREDITH CLYMER, M. D. In one octavo volume of six hundred pages. BENEDICT'S CHAPMAN.—Compendium of Chapman's Lectures on the Practice of Medicine. One neat volume, 8vo., pp. 258. BUDD ON THE LIVER.—On Diseases ofthe Liver. In one very neat Svo. vol., with colored plates and wood-cuts. pp. 392. CHAPMAN'S LECTURES:—Lectures on Fevers, Dropsy, Gout, Rheumatism, &c. &c. In one neat 8vo. volume, pp. 450. FSOU1ROL ON INSANITY —Mental Maladies, considered in relation to Medicine, Hygiene,and Medical Jurisprudence. Translated by E. K. Hunt, M. D, &c. In one Svo. volume, pp. 496. THOMSON ON THE SICK ROOM.—Domestic management ofthe sick Room, necessary in aid of Medical Treatment for the cure of Diseases. Edited by R. E. Griffith, M. D. In one large royal 12mo. volume, with wood-cuts, pp. 360. HOPE ON THE HEART.—A Treatise on the Diseases ofthe Heart and Great Vessels. Edited by Pen- nock. In one volume, 8vo., with plates, pp. 572. LAILEMAND ON SPERMATORRHOEA.—The Causes, Symptoms, and Treatment of Spermatorrhoea. Translated and Edited by Henry J. McDougal. In one volume, Svo., pp. 320. PROUT ON THE STOMACH—On the Nature and Treatment of Stomach and Renal Diseases. In one volume, 8vo., with colored plates, pp. 4fi6. PHILIP ON INDIGESTION —A Treatise on Protracted Indigestion. In one volume, 8vo., pp. 240. PHILIPS ON SCROFULA.—Scrofula: its Nature, its Prevalence, its Causes, and the Principles of its Treatment. In one volume, Svo., with a plate, pp 350. wuitfhVAD ON ABORTION. &c—The Causes and Treatment of Abortion and Sterility ; being the rS ofian Extended Practical Inquiry into the Physiological and Morbid Conditions of the Uterus. In w^TrVA^ mr°'RFP<*PIRATORY ORGANS—A Practical Treatise on Diseases of the Respiratory Or- Win^ncln«^DUeSe^ of The Larynx, Trachea, Lungs, and Pleurae. With numerous Additions and Notes by M Clymer. M. D. With wood-cuts. In one octavo volume, pp. 508. WILSON ON THE SKIN—On Diseases of the Skin. Second American, from the second London edition SamTworkrwft'h^e'autifully colored plates. Also, the plates sold separately. 1 vol. bds. mVoNOID AGE-A Practical Treatise on the Domestic Management and more important Diseases of Advanced Life With an Appendix on a new and successful mode of treating Lumbago and other forma of Chronic Rheumatism. 1 vol. 8vo., pp. 226. 22 LEA & BLANCHARD'S PUBLICATIONS— (Diseasesof Women and Children.) NEW WORK BY DR. CHURCHILL. ON THE DISEASES OF INFANTS AND CHILDREN. BY FLEETWOOD CHURCHILL, M. D., M. R. I. A., Author of " Theory and Practice of Midwifery," '' Diseases of Females," &c. In one large and handsome octavo volume of over 600 pages. It is with much gratification that I acknowledge this volume to owe its existence to the solicita- tions of ray excellent American publishers. After making a considerable collection of works on Diseases of Children, I had laid them aside, hopeless of accomplishing the task of writing the work I had contemplated; but it was impossible to decline an invitation so flattering, from a country which had shown so much indulgence to my former works. I have, therefore, in such leisure as I have been able to command during the last three years, written this volume, not as the exponent of my own experience alone, but as embracing the infor- mation recorded by all the authors within my reach, of which I have freely availed myself; and, if it prove useful and acceptable to my American brethren, I shall be richly repaid.—Authok's Preface. The circumstances under which this work has been prepared are fully set forth in the Author's Preface. The American Publishers, therefore, have only to remark, that its progress through the press has been supervised by a competent member of the profession, who has added a complete and accurate Index of Diseases, as also a copious Bibliographical List of Authors and Works re- ferred to. From Dr. Churchill's known ability and industry, we were led to form high expectations of this work; nor were we deceived. Its learned author seems to have set no bounds to his researches in collecting informa- tion which, with his usual systematic address, he has disposed of in the most clear and concise manner, so as to lay before the reader every opinion of importance bearing upon the subject under consideration. we regard this volume as possessing more claims to completeness than any other of the kind with which we are acquainted. Most cordially and earnestly, therefore, do we commend it to our professional brethren, ana we feel assureu that the stamp of their approbation will in due time be impressed upon it. *vZ,. ™,f« altentlveu Perusal of its contents, we hesitate not to say, that it is one of the most comprehensive «ni™nJ !^ VP"!1 t.he.dlseases ,°f children, and that, for copiousness of reference, extent of research, and per- Journal 'S scaroe|y t0 be equalled, and not to be excelled in any language.—Dublin Quarterly rpJ,Iler™mefi'!.t,iVO',1.m-e,,wiI1J •U^a"1 the reputation acquired by the author from his previous works. The bu3 ctew !! f' £"d Jud'C'ous directions for the management of infants at birth, and a compendious, We ml 1 -?' 1 ^ dlseases to. which children are liable, and the most successful mode of treating them! ooliXri L1?Li? S n0Uce Wlth0ut calllnS attention to the author's style, which is perspicuous and wo k of Dr Chfrchi|W^ne,gret.t0 W,! Sen"ally characteristic of medical works. We^commend the the treatment a "th* \Z« <•'?■•& b°th l? 8tudents and Practitioners, as a valuable and reliable guide in me treatment of the diseases of children.—Am. Journ. ofthe Med. Sciences. ny^^thU^ATn^?^^-^^^^ notice, of Dr. Churchill's work, we shall conclude by sufl h&her the eZl »r,T fal'from «s copiousness, extensive research, and general accuracy, to exalt W find tha Dr KK^' aUrh,?r-'" .thls country. The American reader will be particularly pleased subject The nam, nf iw donf,,ful Ju?,tlce throughout his work, to the various American authors on this arewnstanVreferre\u\Zl fL ^ °°."dle' a",d ?te.wa.rt' occur °" near'y every PaSe' and these authors The Medkal Examiner Y m termS °f the h'gheSt praise> and wi,h the most liberal courtesy.- dice^a^iailmemnrn^,,011 this dePartme"t of Practical Medicine which presents so candid and unpreju- Its cla^msTo meri,Ph„,h g "P °f ?ur act«a'knowledge as this.-JV. Y. Journal of Medicine. P J not eleva,™ ttbove e'very o" e'r S Li PraCUCa' rrk' are of the highest order. Whilst we would to it, and none^er^^^ We ««■»* belie™ *« very few are equal MUCH ENLARGED JiJVD IMPROVED EDITTOJY-J\hw Ready. A PRACTICAL TREATISE ON INFLAMMATION OF THE UTERUS AND ITS APPENDAGES And on Ulceration and Induration of the Week of the Uterus. BY HENRY BENNETT, M. D. Obstetric Physician to the Western Dispensatory. Second Edition, much enlarged. In one neat octavo volume of 350 pages, with wood-cuts. feel\Tsrdk^^^^^ -und in doctrine: but such, we inculcates are all rigidly deduced from factEvervlS practical precepts wh.ch the author A TREATISE ON THE^MSEASES OF FEMALES BY W. P. DEWEES, M. D. NINTH EDITION. In one volume, octavo. 532 pages, with platea. LEA & BLANCHARD'S PUBLICATIONS.—(Diseases of Women and Children.) 23 JflEIGS OJT FEMALES. A SERIES OF LETTERS TO HIS CLASS. BY C. D. MEIGS, M. D., Professor of Midwifery and the Diseases of Women and Children in the Jefferson Medical College of Philadelphia, &c. &c. In one large and beautifully printed octavo volume of 670 pages. He has evidently seen almost every form and variety of female disease, and not only seen, but observed and reflected, and if we may judge by the innate evidence afforded by the volume itself, practised success- fully. His volume contains many practical hints aud suggestions which will repay perusal.— The Charleston Medical Journal and Review. The work is written in a free, animated conversational style, and is replete with sound practical instruc- tion.— The Western Lancet. We warmly commend the work of Professor Meigs as a highly interesting and instructive volume.—JV. Y. Journal of Medicine. The remaining affections ofthe womb, included in the volume before us, are treated of very learnedly, and much valuable instruction is communicated concerning them. Dr. Meigs' views as to the nature and causes of these affections are generally correct, while his long and extensive experience gives to his practical direc- tions no trifling weight. The work contains a very large fund of valuable matter, and will, in all probability, become a very popular one.—American Medical Journal. They are full of instruction. It would be difficult to point to a volume containing more valuable infor- mation relative to females and their diseases.— The Western Journal of Medicine and Surgery. ,We feel that in this hasty sketch we have given the reader scarcely an idea of the vast amount of useful information which the book contains, and ofthe pleasing style in which, generally, it is conveyed, and most conscientiously advise him to purchase and read it for himself.— The Annalist. ... Every chapter is replete with practical instruction, and bears the impress of being the composition of an acute and experienced mind. There is a terseness, and at the same time an accuracy, in his description of symptoms, and in the rules for diagnosis, which cannot fail to recommend the volume to the attention of the reader.—Ranking^ Abstract. MEIGS AND COLOMBAT ON" FEMALES. NEW AND IMPROVED EDITION, JUST ISSUED. A TREATISE ON THE DISEASES OF FEMALES, AND,ON THE SPECIAL HYGIENE OF THEIR SEX. WITH NUMEROUS WOOD-CUTS. BY COLOMBAT DE L'ISERE, M. D., Chevalier ofthe Legion of Honor ; late Surgeon to the Hospital of the Rue de Valois, devoted to the Diseases of Females, &c. &c. TRANSLATED, WITH MANY NOTES AND ADDITIONS, BY C. D. MEIGS, M. D., Professor of Obstetrics and Diseases of Women and Children in the Jefferson Medical College, &c. &c. SECOND EDITION, REVISED AND IMPROVED. In one large volume, octavo, of seven hundred and twenty pages. We are satisfied it is destined to take the front rank in this department of medical science; it is beyond all comparison, the most learned Treatise on the Diseases of Females that has ever been written, there being more than one thousand distinct authorities quoted and collected by the indefatigable author. It is in fact a complete exposition ofthe opinions and practical methods of all the celebrated practitioners of ancient and modern times The Editor and Translator has performed his part in a manner hardly to be surpassed, the translation is faithful to the original, and yet elegant. More than one hundred pages of original matter have been incorporated in the text, constituting a seventh part ofthe whole volume.-.New. York Jour, of Medicine. ASHWEL1 ON THE DISEASES OF FEMALES. A PRACTICAL TREATISE ON THE DISEASES PECULIAR TO WOMEN. ILLUSTRATED BY CASES DERIVED FROM HOSPITAL AND PRIVATE PRACTICE. BY SAMUEL ASHWELL, M. D. with additions, by PAUL BECK GODDARD, M. D. SECOND AMERICAN EDITION. In one octavo volume, of five hundred and twenty pages. fW ofthe verv best works ever issued from the press on the Diseases of Females.- Western Lancet. This invaluable work— Missouri Medical and Surgical Journal. ,.,,,,, i^™Zrr«Zmend Dr Ashwell's Treatise to our readers as a valuable book of reference, on an ex- tensive,SmfiiKSS^highly important class of di^s.-Edinburgh Monthly Journal of Med. Sciences. 24 LEA & BLANCHARD'S PUBLICATIONS.—(Diseases of Women and Children.) NEW AND IMPROVED EDITION.-Now Ready. (1850.) CHURCHILL «.V FEMALES. THE DISEASES OF FEMALES. INCLUDING THOSE OF PREGNANCY AND CHILDBED. BY FLEETWOOD CHURCHILL, M. D., M. R. I. A., Author of "Theory and Practice of Midwifery," " Diseases of Females," &c. A New American Edition (The Fifth), Revised by the Author. With the Notes of ROBERT M. HUSTON, M. D. In one large and handsome octavo volume of 632 pages, with wood-cuts. PREFACE TO THE FIFTH EDITION. The object of the following work was to combine the valuable information scattered through the various periodicals, or published as monographs, with that contained in the larger volumes, and to present the whole in a form equally suitable for the student and practitioner. With this view, as I mentioned in the preface to the first edition, I had arranged "that the text should con- tain an ample outline ofthe history, pathology, symptoms, and treatment of the diseases, without any detail of controversies or conflicting opinions, which,are given in full in the notes appended to each page; so that the student, by confining his attention to the text, may acquire elementary information, which may be subsequently extended by consulting the notes and references. In the notes, likewise, will be found extracts from various authors, wherein the support of their opinions seemed desirable. I have preferred giving their views in their own words, as being less liable to be mistaken. Where extracts were not deemed advisable, references are given, and considerable care has been taken to have them correct." That others felt the want of such a work as I had myself, and that, to a certain extent, my object was attained, I think I may assume from the number of editions which have been called for. For reasons which are now unimportant, the second part, on the Diseases of Pregnancy and Childbed, was published separately in England; but the two have been very properly combined in one volume by my American publishers, as they are both constructed on the same plan, and, together, complete the subject ofthe Diseases of Women. In the present edition, the valuable notes of my friend, Dr. Huston, have been carefully re- tained, and I have added considerably both to the text and notes, so that I trust it will be found an improvement upon its predecessors. Let me, in conclusion, express the gratitude I must ever feel to my American brethren for the kindness with which they have received this, and my other works, and my thankfulness that I have been spared to contribute something, however little, to the cultivation of our noble profession.— Dublin, December, 1849. By reference to the Author's Preface, it will be seen, that his thorough revision of the present edition has rendered unnecessary any material additions by the editor. The notes contained in former editions are retained in this, at the request of Dr. Churchill; and these, rather than any- thing omitted in the text, have suggested occasional, but brief remarks. Churchill's Monographs on Pemales.-Just Ready. ESSAYS ON THE PUERPERAL FEVER, AND OTHER DISEASES PECULIAR TO WOMEN. SELECTED FROM THE WRITINGS OF BRITISH AUTHORS PREVIOUS TO THE CLOSE OF THE EIGHTEENTH CENTURY. Edited by FLEETWOOD CHURCHILL, M. D., M. R. I. A., Author of "Treatise on the Diseases of Females,"&c. In one neat octavo volume, of about four hundred and fifty pages. I have thought it might be useful to prefix to these tracts a short historical sketch of the princi- pal epidemics. I have collected, with some labor, all the information within my reach, and I trust that the summary will be found tolerably complete. I should feel some apology necessary for my own share in this volume, which has been prepared under the pressure of many engagements, were it not for the great intrinsic value that such a collection of original writings, by men of great expe- rience, must possess.—Editor's Pbeface. A TREATISE ON THE PHYSICAL AND MEDICAL TREATMENT OP CHILDREN. BY W. P. DEWEES, M. D. NINTH EDITION. In one volume, octavo. 54S pages. MEIGS ON CERTAIN DISEASES OF INFANTS. In one octavo volume. Nearly ready, LEA & BLANCHARD'S PUBLICATIONS— (Diseases of Women and Children.) 25 New and Improved Edition—Just Issued, 1850. A PRACTICAL TREATISE ON THE ES OF CHILDREN. BY D. FRANCIS CONDIE, M. D., Fellow ofthe College of Physicians, &c. &c. Third edition, revised and augmented. In one large volume, 8vo., of over 700 pages. In the preparation of a third edition ofthe present treatise, every portion of it has been subjected to a careful revision. A new chapter has been added on Epidemic Meningitis, a disease which, although not confined to children, occurs far more frequently in them, than in adults. In the other chapters ofthe work, all the more important facts that have been developed since the appearance ofthe last edition, in reference to the nature, diagnosis, and treatment of the several diseases of which they treat, have been incorporated. The great object ofthe author has been to present, in each succeeding edition, as full and connected a view as possible of the actual state of the pa- thology and therapeutics of those affections which most usually occur between birth and puberty. To the present edition there is appended a list of the several works and essays quoted or referred to in the body ofthe work, or which have been consulted in its preparation or revision. We feel persuaded lhat the American Medical profession will soon regard it. not only as a very good, but as the very bust " Practical Treatise on the Diseases of Children."—American Medical Journal. We pronounced the first edition to be the best work on the Diseases of Children in the English language and, notwithstanding all that has been published, we still regard it in that light.—Medical Examiner. From Professor Win. P. Johnston, Washington, D. C. I make use of it as a text-book, aiwi place it invariably in the hands of my private pupils. From Professor D. Humphreys Storer, of Boston. I consider it to be the best work on the Diseases of Children we have access to, and as such recommend it to all who ever refer to the subject. From Professor M. M. Pollen, of St Louis. _ I consider it the best treatise on the Diseases of Children that we possess, and as such have been in the habit of recommending it to my classes. Dr Condie's scholarship, acumen, industry, and practical sense are manifested in this, as in all his nu- merous contributions to science.— Dr. Holmes's Report to the American Medical Association. Taken as a whole, in our judgment. Dr. Condie's Treatise is the one from the perusal of which the practi- tioner in this country will rise with the greatest satisfaction.- Western Journal of Medicine and Surgery. One ofthe best works upon the Diseases of Children in the English language.— Western Lancet. WEST OJT DISEASES OF CIMIEDREJT. NOW COMPLETE, AND SOLO SEPARATE, lectures" on the BY CHARLES WEST, M. D., Senior Physician to the Royal Infirmary for Children, &c. &c. In one volume, octavo; 'I This work was commenced in the "Medical News" for April, 1848, and completed in the number for December, 1849. It forms an octavo volume of nearly five hundred pages and contains thirtv-nine Lectures, embodying the results of the author's experience in the Children's Infirmary where upwards of 14,000 children have been brought under his notice, during the last nine years. ..'... j * v„prv portion of the=e lectures is marked by a general accuracy of description, and by the soundness of thf vie ws °et forO, in reu'on to the pathology ami therapeutics of the several malad.es treated of. The lec- •'Tih'd^eaVesof the respira ory apparatus, about one-third of the whole number, are particularly „,£ t formin*one of the fullest and, most able accounts of these affections, as they present themselves dm I fan e? a.!dSnood nhe English language. The history of the several forms of phthisis during ?„«eVer& »he« management, will be read by all with deep interest.-^ American ^n^Lfcfurefo^ *" the Lo"don Medical Gazette form a most valuable I he lecture!, o ut. we , „ med;£ine. For many years physician to the Children's Infirmary, his Rdnntnn.Uie9 fo obwrvin J their diseases have been most extensive, no less than 14,000 children having been ?PS der hi ^notiJe during the past nine years. These have evidently been studied with great care, b i ?h* relit h s been the production of the very best work in our language, so far as it goes, on toe dw- & I offh." clas* of our patients. The symptomatology and pathology of their diseases are especially "hlbifed most c"ariy; "icf we are convinced that no one can read with care these lectures without deriv- 'tr SeT^d Dr^sToTkTs o'ne'of great va.ue, as the most scientific treatise on the dis- ° ^^iMrenTn theDla.i"ua-e-not so full, however, on some points as we could have desired, but always Kctfvl^ and where so mucfis good we will not be too earnest in searching for faults.-ZT* Charleston Medical Journal and R*™™- careful perusal of Dr. West's work, we are convinced that it is 111 ^hclr^Vcaul^ children- Parts of il' ai'd esPecia»y the lectures one of the best pu bcauois ever i. , i ^ affeclions of the nervous sygleln are upon diseases of he' %fcPl^^fo7»patie„t research, happy descriptions of symptoms, accuracy, and plain deserving of the hwhc*' £iatrf»n > »£ fa is reeable and pleasing, and at the same and sensible direct'»»» ™, Jr?a™*"L h|gh de|ree. We recommend the work to our American brethren, as l£:S*to}2^™rt%^^toS™ and pn.fit.-2*. Medical Examiner. 26 LEA & BLANCHARD'S PUBLICATIONS.—(Midwifery.) THE NEW WORK—MEIGS' OBSTETRICS—Lately Issued. OBSTETRICS: THE SCIENCE AND THE ART. BY CHARLES D. MEIGS, M. D., Professor of Midwifery and the Diseases of Women and Children in the Jefferson Medical College, Philadelphia, &c. &c. With One Hundred and Twenty Illustrations. In one beautifully printed octavo volume, of six hundred and eighty large pages. As an elementary treatise—concise, but, withal, clear and comprehensive—we know of no one better adapted for the use of the student; while the young practitioner will find in it a body of sound doctrine, and a series of excellent practical directions, adapted to all the conditions ofthe various forms of labor and their results, which he will be induced, we are persuaded, again and again to consult, and always with profit. It has seldom been our lot to peruse a work upon the subject, from which we have received greater satis- faction, and which we believe to be better calculated to communicate to the student correct and definite views upon the several topics embraced within the scope of its teachings.—American Journal ofthe Medical Sciences. We are acquainted with no work on midwifery of greater practical value.—Boston Medical and Surgical Journal. Worthy the reputation of its distinguished author.—Medical Examiner. We most sincerely recommend it, both to the student and practitioner, as a more complete and valuable work on the Science and Art of Midwifery,, than any of the numerous reprints and American Editions of European works on the same subject.—JV. Y. Annalist. We have, therefore, great satisfaction in bringing under our reader's notice the matured views of the highest American authority in the department to which he has devoted his life and talents.— London Medical Gazette. An author of established merit, aprofessorof Midwifery, and a practitioner of high reputation and immense experience—we may assuredly regard his work now before us as representing the most advanced state of obstetric science in America up to the time at which he writes. We consider Dr. Meigs' book as a valuable acquisition to obstetric literature, and one that will very much assist the practitioner under manv circum- stances of doubt and perplexity.— The Dublin Quarterly Journal. These various heads are subdivided so well, so lucidly explained, that a good memory is all that is neces- sary in order to put the reader in possession of a thorough knowledge of this important subject Dr Mei°-s has conferred a great benefit on the profession in publishing this excellent work.—St. Louis Medical and Surgical Journal. No reader will lay the volume down without admiration for the learning and talents of the author An abler volume, on the whole, we do not hope soon to see.— Western Journal of Medicine and Surgery „£ safe and efficient guide to the delicate and ofuimes difficultduties which devolve upon the obstetrician - Ohio Medical and Surgical Journal. v un.uu»rciuUaii, One ofthe very best treatises on this subject, and worthy of being placed in the library of every American physician.—Northwestern Medical and Surgical Journal. y y -amerlcan He has an earnest way with him when speaking ofthe most elementary subjects which fixes the attention and^adds much value to the work as a text-book for students-British and Foreign Medico cfhturg^cal NEW EDITION, REVISED FOR THIS COUNTRY. THEORY AND PRAGTIGE OF MIDWIFERY. BY FLEETWOOD CHURCHILL, M. D., Hon. Fellow ofthe Royal College of Physicians of Ireland, &c. &c. WITH NOTES AND ADDITIONS BY ROBERT M. HUSTON, M. D., &c. THIRD AMERICAN EDITION, REVISED AND IMPROVED BY THE AUTHOR. With One Hundred and Twenty-eight Illustrations. In one very handsome octavo volume, of five hundred and twenty-six pages. This is certainly the most perfect system extant. It is the be=t adsntoH fnr n,» «,__ r . . u i that which he whose necessities confine him to one book, should selec in ZVtrlZT^sesnof a text-book, and Medical and Surgical Journal. ' snoulu 3elePl ln Preference to all others.—Southern SnaTiV v Tot7°rk °\MidWlftry "" J""6*1 fr°m thC AmeriCa" P'eSS -Charleston Medical Journal Certanv.), in our opinion, the very best work on the subject which exists.-iV. y. Annalist. m*.^"^^^^ ^permitted to choose, we wou.d -^■S*S:^ m°re USefU' a"d e'eSam Ma"ual *« D<- Churchill's Practice of Midwifery. LEA & BLANCHARD'S PUBLICATIONS.—(Midwifery.) 27 NEW EDITION, NOW READY. THE PRINCIPLES AND PRACTICE OP OBSTETRIC MEDICINE AND SURGERY, In reference to the Process of Parturition, BY FRANCIS H. RAMSBOTHAM, M. D., Physician to the Royal Maternity Charity, &c. &c. FIFTH AMERICAN FROM THE LAST LONDON EDITION. Illustrated with One Hundred and Forty-eight Figures on Fifty-five Lithographic Plates. In one large and handsomely printed volume, imperial octavo, with 520 pages. From Professor Hodge, of the University of Pennsylvania. To the American public, it is most valuable, from its intrinsic undoubted excellence, and as being the best authorized exponent of British Midwifery. Its circulation will, I trust, be extensive throughout our country. We recommend the student, who desires to master this difficult subject with the least possible trouble, to possess himself at once of a copy of this work.—American Journal ofthe Medical Sciences. It stands at the head ofthe long list of excellent obstetric works published in the last few years in Great Britain, Ireland, and the Continent of Europe. We consider this book indispensable to the library of every physician engaged in the practice of Midwifery.—Southern Medical and Surgical Journal. When the whole profession is thus unanimous in placing such a work in the very first rank as regards the extent and correctness of all the details of the theory and practice of so important a branch of learning, our commendation or condemnation would be of little consequence; but, regardingit as the most useful of all works ofthe kind, we think it but an act of justice to urge its claims upon the profession.—N. O. Med. Journal. We are disposed to place it first on the list of the numerous publications that have appeared on this subject; for there is none within our knowledge that displays in so clear and forcible a manner every step in the pro- cess, and that, too, under all imaginable circumstances,—N. Y. Journal of Medicine TYLER SMITH ON PARTURITION.—A New Work—Just Issued. ON PARTURITION, AND THE PRINCIPLES AND PRAGTIGE OF OBSTETRICS. BY W. TYLER SMITH, M. D., Lecturer on Obstetrics in the Hunterian School of Medicine, &c. &c. In one large duodecimo volume, of 400 pages. The work will recommend itself by its intrinsic merit to every member of the profession.—Lancet. We can imagine the pleasure with which William Hunter or Denman would have welcomed the present work; certainly the most valuable contribution to obstetrics that has been made since their own day. For ourselves, we consider its appearance as the dawn of a new era in this department of medicine. We have thus given a brief, but we believe accurate and succinct, outline of the original views contained in this volume. At every page of the work itself, practical deductions are drawn from the physiological doctrines as they are advanced; but we have for the present chiefly confined ourselves to the latter. In a future bibliographical sketch we shall, with equal care, go over these lectures, which aTe entirely devoted to practical points; and we are sure that the interest of our readers will not flag while they follow us in our task We would observe, that we do not pledge ourselves to all and every doctrine promulgated by Dr. Tvler Smith. This would be impossible, considering the magnitude of the subject itself, and the great vari- ety and importance ofthe topics discussed; but we do most cordially recommend the work as one absolutely necessary to be studied by every accoucheur. It will, we may add, prove equally interesting and instructive to the student, the general practitioner, and pure obstetrician. It was a bold undertaking to reclaim parturi- tion for Reflex Physiology, and it has been well performed.—London Journal of Medicine. LEE'S CLINICAL MIDWIFERY.—Now Ready. CLINICAL MIDWIFERY, ^[TlTp^^^^ BY ROBERT LEE? M. D., F. R. S., &c. From the 2d London Edition. In one royal 12mo. vol., extra cloth. 238 pages. More instructive to the juvenile practitioner than a score of systematic works.—Lancet. „_..-, W°» be consulted by every accoucheur who practices his art with the zeal which it ments.-Med. Gazette. An invaluable record for the practitioner.—JV. Y. Annalist. This admirable book of precedents.—Boston Medical and Surgical Journal A storehouse of valuable facts and precedenls.-^eman Journal ofthe Medical Sciences. DEWEES'S MIDWIFERY. A COMPREHENSIVE SYSTEM OF MIDWIFERY. ILLUSTRATED BY OCCASIONAL CASES AND MANY ENGRAVINGS. BY WILLIAM P. DEWEES, M. D. Tenth Edition, with the Author's last Improvements and Corrections. In one octavo volume, of 600 pages. 30 LEA AND BLANCHARD'S PUBLICATIONS. NEW AND ENLARGED EDITION-JUST READY. 1850. TAYLOR'S MEDICAL JURISPRUDENCE. MEDICAL JURISPRUDENCE. BY ALFRED S. TAYLOR, Lecturer on Medical Jurisprudence and Chemistry at Guy's Hospital, &c. SECOND AMERICAN, FROM THE THIRD AND ENLARGED LONDON EDITION. With numerous Notes and Additions, and References to American Practice and Law. BY R. E. GRIFFITH, M. D. In one large octavo volume. This work has been much enlarged by the author, and may now be considered as the standard authority on the subject, both in England and this country. It has been thoroughly revised, in this edition, and completely brought up to the day with reference to the most recent investigations and decisions. No further evidence of its popularity is needed than the fact of its having, in the short time that has elapsed since it originally appeared, passed to three editions in England, and two in the United States. We recommend Mr. Taylor's work as the ablest, most comprehensive, and, above all, the most practically useful book which exists on the subject of legal medicine. Any man of sound judgment, who has mastered the contents of Taylor's '• Medical Jurisprudence," may go into a court of law with the most perfect confi- dence of being able to acquit himself creditably.—Medico-Chirurgical Review. The most elaborate and complete work that has yet appeared. It contains an immense quantity of cases lately tried, whieh entitle it to be considered what Beck was in its day.—Dublin Medical Journal. TATLOS ON POISON'S. ON POISONS, E\T RELATION TO MEDICAL JURISPRUDENCE AND MEDICINE. BY ALFRED S. TAYLOR, F. R. S., &c. Edited, with Notes and Additions, BY R. E. GRIFFITH, M. D. In one large octavo volume, of 688 pages. The most elaborate work on the subject thatour literature possesses.— Brit, and For. Medico-Chirur. Review. Une ofthe most practical and trustworthy works on Poisons in our language — Western Journal of Med. It contains a vast body of facts, which embrace all that is important in toxicology, all that is necessary to the guidance ofthe medical jurist, and all that can be desired by the lawyer.—Medico-Chirurgical Review. It is, so far as our knowledge extends, incomparably the best upon the subject: in the highest degree credit- able to the author, entirely trustworthy, and indispensable to the student and practitionen-iV. Y Annalist. TRt^Sw^FhDnIC^L JU^PRUrDENCE.-Outlines of a Course of Lectures on Medical Jurisprudence- Kevised, with numerous Notes. In one small octavo volume of 234 pages. DUNGLISON ON HITMAN HEALTH. HUMAN HEALTH, OR THE INFLUENCE OF ATMOSPHERE AND LOCALITY, CHANGE OF AIR AND CLIMATE, SEASONS, FOOD, CLOTHING, BATHING, EXERCISE, SLEEr, &C. &C. &C, ON HEALTHY MAN. CONSTITUTING ELEMENTS OF HYGIENE. Second Edition, with many Modifications and Additions. BY ROBLEY DUNGLISON, M. D., &c. &c. In one octavo volume of 464 pages. BJIRTLETT OJV CERT*IJ\~T*-jrjY MElUCLJVE.-JVotc Ready. AN INQUIRY INTO THE DEGREE OP CERTAINTY IN MEDICINE, AND INTO THE NATURE AND EXTENT OF ITS POWER OVER DISEASE BY ELISHA BARTLETT, M D Author of "Fevers of th, Un.ted States," "Philosophy if Medical Science." In one small volume of 84 pages, crown 8vo., extra cloth. AN ESSAY ON THE PHILOSOPHYlr MEDICAL SCIENCE. BY ELISHA BARTLETT, M. D, Author of "Fevers ofthe United States." In one handsome octavo volume, of three hundred and twelve pages. DUNGLISON'S MEDICAL STUDENT.-The Medical Student or Airt..«.».- c. a ,». a- ■ and Modified Edition. 1 vol. royal 12mo„ extra cloth 312 pp ' he Study °f Medlcine- Revised MITCHELL ON FEVERS.-On the Cryptogamous Oriein of Mila™,,= „ a r- -a ■ *, royal 12mo. 138 pp. 5 g 01 Mala™>us and Epidemic Fevers. In 1 toL ££a' & BLANCHARD'S PUBLICATIONS. 31 CONTENTS OF THE AMERICAN JOURNAL OF THE MEDICAL SCIENCES, April, 1850. ORIGINAL COMMUNICATIONS. Memoirs and Cases, pp. 291-402. Art. 1. Warren on the Prevention of Constipation. II. Kirkbride on Heating and Ventilating Hospitals and other Buildings, by Steam and Hot Water. I With three wood cuts ) III. Atlee on Large Peritoneal Sec- tion. IV. Dallon's Case of Malformation of the Cranium, Encephalon, and Spinal Cord. (With eight wood- cuts.) V. Buckingham's Cases of Labor. (With five wood-cuts.) VI. Morland's Extracts from the Records of the Boston Society for Medical Improvement. VII. Bemis' Case of Hydrophobia. VIII Bennett on the Identity of Erysipelas and a certain form of Puerperal Fever, and its Contagiousness. IX. Peaslee's Case of Rupture ofthe Bladder, together with Seven Fractures ofthe Pelvis. X. McSherry's Medical Cases. Xf. Griffin's Case of Ascites, in which the patient was tapped, in ten years, one hundred and eighty-six times, and had seven hundred and fifty-one and three-fourths gallons of water drawn off. Reviews, pp. 402-434. XII. 1. Traite" Th^orique et Pratique des Luxations Cong^nitales du Fe'mur. Par le Doct. Ch. G. Pravaz. 4to. pp. 288, with ten plates. 2. A Treatise on the Etiology, Pathology, and Treatment of Congenital Disio* cations of the Head of the Femur. With plates. By M. Carnochan, M. D. 8vo. XIII. The Transactions of the American Medical Association. Vol. II. Bibliographical Notices, pp. 435-474. \XIV. A Universal Formulary: containing the Methods of Preparing and Administering Officinal and Mher Medicines. By R. E. Griffith, M. D 8vo. XV. An Introductory Lecture. By J. Knight, M. D. 8vo. XVI. Lectures on Electricity and Galvanism, By G. Bird, M. D. 12mo. Notes on the Medical Application of Electricity. By W. F. Chauning. 12mo. XVII. Curability de la Phthisie et des Scrofules. Par A. M. Riofrey. 8vo. XVIII Report of the Committee of the Legislature of Massachusetts on a Sanitary Survey of the Stale. 8vo. XIX. An Essay on Intestinal Auscultation. By C. Hooker, M. D. 8vo. XX. Observa- tions on Asiatic Cholera. By J. Evans. Svo XXI. The History of the Cholera in Exeter in 1832. By T. Shapter, M. D. 8vo. XXII. Remarks on Obstetrical Forceps. By J. P. White, M. D. 8vo. XXIII. A Practical Treatise on Inflammation of the Uterus and its Appendages. By J H. Bennet, M. D. 8vo. XXIV. A Treatise on Midwifery. By P. Cazeaux. Translated by R. P. Thomas, M. D 8vo. XXV. Medico Chi- rurgicale Cliniqueet Iconographie. Par M. Anvert. XXVI. An Introductory Lecture. By P. F. Eve, M. D. 8vo. QUARTERLY SUMMARY. FOREIGN INTELLIGENCE. " Anatomy and Physiology, pp. 475-478. 1. Blanchet on Hearing independent of the Auditory Nerve. 2, Brown Slquard on Regeneration of the Sciatic iMerve 3. Brown Siquard on the Pathological Changes that follow Section of the Sciatic Nerve. 4. De Froberville on the OstroNegroes of Eastern Africa. 5. Frerichs on the Uses of the Pancreas. Organic Chemistry, pp. 47S-481. 6. Plouviez on Experiments on the Use of Salt. 7. Jones'' Contributions to the Chemistry of the Urine. Materia Medica and Pharmacy, pp. 481-483. 8. Rivallier on Monohydrated Nitric Acid as a Caustic. 9. Hannon oh Pharmaceutical Preparations of Manganese. Medical Pathology and Therapeutics and Practical Medicine, pp. 483-503. 10. Jenner on the Identity or Non-identity of Typhoid Fe vet, Typhus Fever, and Relapsing Fever. "11. Ben- son on the Use of Cod-Liver Oil—its Effect in producing Congestion ofthe Lung. 12. Bennett on Tubercular Exudation into the Lungs—Diseased Aortic Valves—Musical Heart-Sounds—Ulceration in the Lungs check- ed by Cod-Liver Oil. 13. Walton on Conversion of Tubercle into Eariliy Matter. 14. Mauthner on Blood- letting in the Pneumonia of Children. 15. Sibson on Falling in of the Chest during Inspiration, in some Dis- eases of the Chest. 16. Fliess on Paralysis which attends Dentition. 17. Schneider on Sulphuric Acid in Singultus. 18. Brownon the Diagnosis of Ovarian Dropsy. 19. Popham on Obstructions ofthe Rectum from the Use of Diseased Potatoes. 20. Griffith on the so called Cholera.Bodies. 21. Robertson on Cholera (?) Cor- puscles. 22. Benson on the Gutta Perclia Stethoscope. Surgical Pathology and Therapeutics and Operative Surgery, pp. 503-521. 23. Banon on Aneurism ofthe Popliteal Space treated successfully by Compression. 24. Madden on Pop- liteal Aneurism treated by Compression. 25. Taylor on Chronic Hydrocephalus—Tapping—Death. 26. Ne- vins on Tapping in Spina Bifida. 27. A Case of Dislocation ofthe Neck successfully treated by Mechanical Means. 26. Sedillot on Gastrotomy for Impassable-Stricture ofthe CEsophagus—Death in twenty-four hours. 29. Field and Clarkson's two Cases of Complete Intestinal Obstruction arising from Disease of the Sigmoid Flexure of the Colon and the Rectum, in which the Descending Colon was successfully opened in the Loin. 30 Robert on Annular Stricture of the Rectum. 31. Frilze on the Statistics of the Mortality from Fractures ofthe Head. 32. Blick on Large Punctured Wound ofthe Rectum and Dislocation of the Coccyx. 33. Can- ton on Ununited Comminuted Fracture of the Surgical Neck of the Humerus; also, Unreduced Dislocation ot the Radius forwards at the Elbow, with Partial Luxation of the Ulna inwards. 34. Simpson on the Use of the Exploring Needle in Pelvic and other Tumors. 35. Raciborski's Method of Preventing the Ingress of Air in evacuating large Collections of Fluid. 36. Tufnell on Gutta Percha Bougies. Ophthalmology, pp. 521-525. 37. Jacob on Tumors of the Eye and Orbit. 38. Canton on Hystetical Ptosis. Midwifery, pp. 525-532. 39. Diday on a New Method of Plugging the Vagina, &c. 40. Lever on the Use and Advantages of Opium in the Practice of Obsleincy. 41. Webster on Insanity from the Use of Chloroform during Parturition. 42. Cormack on Puerperal Convulsions- their dependence on Toxaemia. 43 Kesteven's Examination iuto the Grounds of ihe Ovular Theory of Menstruation. 44. Gillette on Pregnancy in a Female who had never Men- struated. 45. Rogers on Vicarious Menstruation. 46. Tyler Smith on the Length of the Umbilical Cord. Medical Jurisprudence and Toxicology, pp. 532-537. 47. Miller on Infanticide—Retention of Life after Long Exposure. 48. Brachet on the Cessation ofthe Heart's Sounds as a Sign of Death. 49. Bernard on the Action of Amygdaline and Emulsine. 50. Landi- berg. Vagiius Uterinum. 51. Simpson on Faial Venous Hemorrhage from the Pudenda. 52. Christison on Poisoning with Hydrocyanic Acid. 53. Mitscherlich on the Poisonous Effects of Oil of Cinnamon. Miscellaneous, pp. 537-541. 54. Davy.on Action of Lime on Animal and Vegetable Substances. (Continued on Page 32.) 32 LEA St, BLANCHARD'S PUBLICATIONS. TWO MEDICAL PERIODICALS FOR FIVE DOLLARS. THE AMERICAN" JOURNAL OF THE MEDICAL SCIENCES, EDITED BY ISAAC HAYS, M. D., Is Published Quarterly ON THE FIRST OF JANUARY, APRIL, JULY, AND OCTOBER, By LE*1 A' ttljJUYCHJinn, Philadelphia. Each Number contains about Two Hundred and Eighty Large Octavo Pages, Appropriately Illustrated with Engravings on Copper, Wood, Stone, &c. Some estimate of the variety and extent of its contents may be formed from the very condensed summary of the Number for April, 1850. THE MEDICAL NEWS AND LIBRARY Is Published monthly, and consists of THIRTY-TWO VERY LARGE OCTAVO PAGES, Containing the Medical Information of the day, as well as a Treatise of high character on some prominent department of Medicine. In this manner its subscribers have been supplied with WATSON'S LECTURES ON THE PRACTICE OF MEDICINE, BRODIE'S CLINICAL LECTURES ON SURGERY. TODD & BOWMAN'S PHYSIOLOGY, AXD WEST ON THE DISEASES OP INFANCY AND CHILDHOOD. And the work at present appearing in its columns is MAEGAICJ^E'S OPERATIVE SURGERY, TRANSLATED AND EDITED BY BRITTAN, With Engravings on Wood. TERMS. THE SUBSCRIPTION TO THE AMERICAN JOURNAL OF THE MEDICAL SCIENCES I IS FIVE DOLLARS PER JUVJYUJfl. .-^«tl!,.8am0Unti8paidinadVanCe'the subs^ber thereby becomes entitled to the MEDICAL NEWS AND LIBRARY FOR ONE YEAR, GRATIS. When ordered separately,the price of the "News" is ONE DOLLAR per ann'um, ""ar'ably', For the small sum, therefore, of FIVE DOLLARS tt,» =..k u , . 'a Month* Journall-TtF.S^ FIFTEEN HUNDRED LARGE OCTAVO PAGES, With numerous Illustrations. Rendering these among The Cheapest of American Medical Periodicals Those who are desirous of subscribing are recommends tn e a 1 1UU1^aH>, time, as the increase ofthe subscriptio/listhas2 off £,7?"* ^ names withou* '°ss of the pubHshers cannot pledge themselves ^T^^X^*^** *' 1849>and CONTENTS OF JOU^lTlcTntinued from page 31) AMERICAN INTELLIGENCE Domestic Summary, pp.. 542-548 T™tment. ^ <* ."£ ^>r :^i i:3 *&: [*tf 2EP& & ■,*■"; H' - v.'-v.v :& a^ 'A*» • ^ '5ft ■Ay ?; -••*»£■-.< -, .'■. .y ■ir ^«N f?U,>;.'«' ^ <•■■<%