to form the umbilical cord, and belongs to the system of the child. The vessels, therefore, which supply the child, and those which connect these vessels with the mother, are perfectly distinct. Injection poured into the one set will not pass into the other, and the communication which they enjoy is maintained only through the interposition of cel lular cavities. Now, it is within these cellular cavities that the work of oxydation is supposed to go on; but, whether these cells possess the prerogative of absorbing carbon from the foetal vessels which lie on one side of them, and oxygen from the maternal vessels which ramify on the other, or whether of themselves they exert some change upon venous blood equivalent to oxydation, is at present unknown. Argument may be used in support of either view; but the investigation is so incomplete, that it would be vain to attempt to anticipate a result, which careful experiment and diligent research can alone obtain. One circumstance only is known with certainty, that the blood which the child sends to these cells is venous, that the blood which it receives from them is arterial, and that the alterations which the lungs are destined to work upon this fluid after birth are effected, during foetal life, by, or at least through the placenta. Having now concluded our description of the three great functions of lifedigestion, circulation, and respiration; having shown how the food is converted into blood, how the blood is circulated through the body, how the noxious qualities with which it becomes tainted during its journey are removed, and its purity maintained, it will be necessary to consider the ultimate design of such an elaborate system, to investigate the reason why blood must be so freely supplied by the stomach, so constantly circulated by the heart, and so regularly purified by the lungs. And this brings us to the subject of secretion. Secretion. tent of power, brings every resource of the system into full operation; while, in old age, functions no longer necessary are gradually withdrawn, and organs which have ceased to discharge duties no more required insensibly decay. The machinery by which this reparative system is conducted is extensive and complex. Nutritive matter taken into the stomach is there converted, as we have seen, into chyle; chyle is converted by the lungs into blood; blood is carried to every part of the system, and in every part of the system is changed into those substances which are necessary either for its growth or reparation. Wherever muscle is required, muscular fibre is deposited; where bone is needed, ossific particles are laid down in the requisite form and in the desired order; where it is necessary to construct cellular tissue, or to deposit fat, albuminous and oily matter are provided. The process by which these different substances are eliminated from the blood is termed secretion; that by which they are removed when no longer useful is called absorp-· tion: the result of both actions combined is denominated nutrition. The first process is preparatory to the second, and both are essential to the third. Were the old particles not regularly removed, new particles could not be regularly deposited; and were not fresh nutritive matter ready for deposition as absorption proceeds, emaciation would be the first, and death the ultimate and the speedy result. How the chyme is elaborated in the stomach by the gastric juice, how the chyme is converted into chyle in the duodenum, how the chyle is arterialized in the lungs by the action of the air, and how the arterialized blood is transmitted by the blood-vessels to every organ in the body, have been already shown; but it still remains to be explained how this common fluid, which exhibits the same properties in every part of the circulation, can be converted into bone and brain, into muscle and membrane, into cartilage and fat; how saliva and bile, how urine and mucus-fluids the most active and the most insipid-can be manufactured in determinate quantities, in certain textures, and at appropriate periods, out of a bland material totally unlike any and every substance which it generates! Ar no period of life is the human body perfectly complete. At birth the process of development is still unfinished; during infancy every organ is weak, and every function is imperfect; at the period of puberty, parts previously unexercised suddenly enlarge, and take on new actions: manhood, requiring in the discharge of its offices the utmost ex- THE apparatus by which the animal se Organs of Secretion. I cretions are formed assumes a thousand modifications of external aspect. Sometimes nothing can be discerned but a smooth vascular membrane, such as the pleura which lines the chest, or the pe A gland with an excretory duct. aa the substance of the gland irregular and lobulated; bb the small branches by which the excretory duct rises from the gland; e the trunk of the excretory duct fully formed. ritoneum which envelops the viscera of the abdomen. At another time this membrane is not smooth, but rough, and covered with small vascular elevations, called villi, giving to this entire surface a velvety appearance. In one organ the secreting membrane assumes the form of small bags, called follicles, which contain an aperture in their centre for the transmission of the substance secreted by their internal surface. In another it constitutes sacs, open at one end, denominated lacunæ, which are sometimes single, as in the nosesometimes ramified, as in the neck of the womb. Again, the wax which defends the passage to the internal ear seems to be secreted by small membranous cavities, which open, by means of small ducts, upon the surface of the auditory canal; while the oily matter which is formed beneath the cuticle, more especially in the armpits and groins, appears to be a secretion produced by bodies similarly constructed, and of the same figure. These small secreting bags are known by the name of glands, and the small tubes through which they convey the secretions they elaborate are styled excretory ducts. But different as the external characters of all these secreting organs may be, yet their general structure is essentially the same; and the more carefully the secreting apparatus is examined, the more convincing will appear the reason for believing that every external modification of structure has been adopted for the sake of convenience rather than be cause it was indispensable: nothing appears absolutely necessary to the performance of secretion, save capillary bloodvessel. The chain of connexion between the simplest secreting surface and the most complex gland, of which only the first links have been now given, might be easily completed, by tracing the gradual complication of an elementary secreting organ, as it appears in a serous membrane, up to the most elaborate specimen of glandular structure. When a bone is broken, blood is poured out into the fracture, and there coagulates. After a short period, vessels are seen to shoot into this coagulated blood, the blood gradually disappears, and gelatinous matter occupies its place. This gelatinous matter progressively hardens, osseous particles are slowly deposited, and thus the fracture is ultimately repaired. During all these to be principally employed, and that stages of renovation one agent appears agent is capillary blood-vessel. It is capillary vessel which shoots into the coagulated blood, which deposits the callus, which conveys the osseous substance, and which unites the fractured same process is established, excepting bone. When muscle is divided the that fibrin is deposited instead of ossific matter; and whether the secretion be nerve or cartilage, tendon or skin, no difference can be detected in the secreting vessel, and nothing essential but secreting vessel can be discovered. vessel be required to secrete such imIf, then, nothing but capillary bloodportant and different substances as pected that the fluids of the human body the animal solids, may it not be susare formed in the same way, and by the lubricates the nostrils is secreted by a same instruments? The mucus which fine and highly vascular membrane, which covers them internally. The perspiration which oozes from the skin is poured out through small apertures in the walls of capillary arteries, which lie beneath the cuticle. The water which exudes from such serous membranes as the pleura, which lines the chest, or the pericardium, which surrounds the heart, seems to be filtered in the same manner through these vascular strainers from all these, and in no analogous instances, the general mass of blood. In none of apparatus. Capillary blood-vessel is can there be found any specific secreting alone discoverable. Malpighi maintained that such glands as the liver are composed of very small bodies, called acini from their resemblance to the stones of grapes; that these acini are hollow internally, and externally covered with a net-work of blood-vessels; that these minute bloodvessels covering the acini pour into the cavities of these elementary cells the secreted fluid as it is eliminated by them, out of which it is afterwards removed by ducts. The minute dissections of Ruysch, however, induced this distinguished anatomist to dissent from the views of Malpighi; and, after a series of very accurate experiments, he was satisfied that the apparently hollow acini of Malpighi are merely convoluted vessels, perfectly continuous with the excretory ducts that carry away the secreted substances. This opinion was, for a time, strongly opposed; but it afterwards became general, and is now, perhaps, universally adopted. The chief, if not the only, difference between the secreting structure of glands and that of simple surfaces appears, then, to consist in the different number and the different arrangement of their capillary vessels. The actual secreting organ is in both cases the same-capillary blood-vessel; and it is uncertain whether either its peculiar arrangement, or greater extent in glandular texture, be productive of any other effect than that of furnishing the largest quantity of blood-vessel within the smallest space. Thus convoluted and packed up, secreting organ can be procured to any amount that may be required, without the inconveniences of weight and bulk. These acini are most distinctly seen in the liver and spleen; in the pancreas and kidneys they are less perceptible; and in the prostate and tonsils they cannot be detected by the finest glasses. Glandular structure has accordingly been arranged into three varieties. In the first are placed all those glands which are uniform in their external surface; which are composed internally of acini, bound together by cellular tissue, and which are enclosed in a general membrane. The liver affords a good illustration of this form of gland. In whatever part it may be examined, its texture appears uniformly the same. Nothing but acini can be seen-there are no larger and no smaller subdivisions; and when the structure of one acinus has been fully ascertained, the composition of the entire organ is understood. The anatomy of the second variety is somewhat dif This figure represents the human kidney, which is so finely injected, that what were considered acini or hollow globules by Malpighi, appear to be merely convoluted vessels. ferent. The glands pertaining to this order are obviously composed of subordinate divisions called lobes, which themselves consist of subdivisions called lobules; these lobules are formed of smaller bodies, which are again divisible into bodies still smaller, until we ultimately arrive at the elementary acini, which are, in common with the first class of glands, the rudimental constituents of the second class. As a fine example of this glandular tissue, the kidney may be taken. When viewed externally, it is manifestly lobulated, and its internal structure is strikingly beautiful. It consists of two distinct varieties of texture, the first, being external, is called cortical, and the second, from its office, is denominated tubular. The cortical portion is the secreting organ of the kidney; the tubular portion is mainly composed of tubes, which convey the urine formed within the cortical tissue into the reservoir, or pelvis, of the kidney. These tubes are at first, where they issue from the cortical substance, very small and numerous; but they increase by uniting as they proceed, and they form considerable trunks before they terminate in the small oval projections called papillæ, cccc. These papillæ, which are formed by the union of the uriniferous tubes, project into, and are surrounded by membranous cups called Fig. 80. The interior of the human kidney: a a, the cortical portion; bb, the tubular portion; c c cc, the papilla; d, the pelvis; e, the ureter. calyces, which receive the urine poured out by the papillæ from the extremities of the tubes, and convey it onwards to the pelvis or basin of the kidney, from which it is conducted by an excretory duct, styled ureter, into the urinary bladder. The two varieties of glands now described, differ only in the number of their component parts, not in the nature of their composition; and, because the first form of gland consists of parts of the same size and equally elementary, while the second divides and subdivides before its rudimental structure is attained, the former is generally termed a conglomerate, the latter a conglobate gland. The third and last form of gland is distinguished from the two preceding in having no discoverable specific structure. Without lobes, lobules, or acini, these organs exhibit neither parts nor divisions; they are dense, firm, lacerated with difficulty, and present everywhere a striking uniformity of aspect. Still they are highly vascular, and, when inflamed, produce excruciating pain; so that, in whatever other points these different forms of glandular structure may disagree, they are all equally remarkable for the profusion of blood-vessels with which they are supplied. It has been estimated that the glandular portion of the human body contains two-thirds of the entire mass of blood; and when successful injection has rendered all their vessels visible, six-eighths of their whole structure appear to be composed of capillary blood-vessels, for secreting the fluid which they are destined to elaborate, and of excretory ducts for removing that fluid as it forms. When all these facts are compared and considered, it would seem a reasonable inference that, as far as regards the vascular part of the apparatus, to constitute a secreting organ, blood-vessel alone is requisite; and that glands differ from mere secreting surfaces, as the pleura or peritoneum, chiefly in having a greater number of blood-vessels, only differently arranged. The intervening cellular tissue, by which these bloodvessels are tied up and connected, appears not to be essential, because in a mere serous membrane there is little or no such cellular tissue; nor are excretory ducts indispensable, since many glands have central apertures instead of ducts, through which their secretions are poured out. In the present imperfect state of our knowledge, it is impossible to estimate what effect difference of size, length, and direction in the secreting vessels, may exert upon the nature and quantity of the substance secreted. It is reasonable to believe that these circumstances may exert, and that they do exert, considerable influence upon the process; but, as far as regards the vascular part of the apparatus of secretion, all that is really known is told when it is stated, that wherever this function is performed, there is provided an abundant supply of capillary blood-vessels. To the blood-vessels of secreting organs, as to all other parts of the body, are added nerves; and the number of nerves which are distributed to secreting organs, bears a close relation to the number of blood-vessels. If the capillary arteries are countless, so also are the ultimate nervous filaments. Moreover, there are some anatomical peculiarities in the nerves of secreting organs, which deserve to be borne in mind, because it is probable that they are intimately connected with the function of secretion. While treating of the nervous tissue, it was stated that in all the more perfectly organized animals the nervous substance is disposed in four different modes, forming four distinct parts or organs-nerves, ganglia, or appendages to particular nerves, spinal cord, and brain. With one exception, a the splenic artery, separating into a rich variety of branches, which divide and subdivide into a number and minuteness so extraordinary, that the spleen, the gland which this artery supplies, seems to be almost entirely composed of blood-vessels. no nerve that comes off from the brain possesses a ganglion; but every nerve that issues from the spinal cord, is provided with one or more of these knotlike bodies. What object the ganglia serve is not understood; but we know that the nerves of sensation consist of one set of filaments, that the nerves of motion consist of another set of filaments perfectly distinct from the former, and that the nerves of secretion consist of these ganglionic nerves. These ganglionic nerves are destitute of feeling, the Occasional sensibility which they indicate being, probably, derived from the nerves of sensation with which they intermingle and communicate; and they are not obedient to the will. As soon as they issue from the ganglia, they proceed to the trunks of the secreting arteries, a a, blood-vessels; bb, a rich network of nerves investing the external coat of the blood-vessels. |