the principal branches of these veins ramify in the spaces between the lobules, they are termed inter-lobular veins.-On the other hand, the branches of the Hepatic Vein pass from the trunk to the centre of each lobule, from which they send out diverging capillary twigs (1, 1), towards the circumference; and these last, coming into connexion with the converging capillaries of the portal vein, establish a free capillary communication between the interior and exterior of each lobule. Thus the portal blood is first distributed to its exterior, then penetrates its substance, 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. 120). The precise relation of the capillaries of the hepatic artery with those of the portal

Connexion of the lobules of the Liver with the Hepatic vein:-1, trunk of the vein; 2, 2, lobules depending from its branches, like leaves on a tree; the centre of each being occupied by a venous twig,-the Intralobular Vein.

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 venæ portæ, to supply the materials of the secretory function, which are eliminated from it during its passage into the hepatic vein.

723. The Hepatic Ducts also seem to 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, has not yet been completely elucidated. There seems reason to believe, however, that the tubular plexus extends throughout the substance of the lobule, filling up the entire space not occupied by the blood-vessels (its membranous wall, however, being with difficulty traceable, owing to its extreme tenuity); and that the hepatic cells are contained within it, as within the follicles or tubes of ordinary glands. These cells (which are easily obtained in a separate condition by scraping a piece of fresh Liver) are of a flattened spheroidal or polygonal form; and their diameter is usually from 1-800th to 1-1600th of an inch. Each cell presents a dis

tinct nucleus; and it is usually around this, that the yellowish hue of the cell is the deepest. The cavity of the cell is chiefly occupied by biliary matter, much of which is in the condition of fine granular particles too minute to be measured. In the midst of these, there are usually

Horizontal section of two superficial lobules, showing the interlobular plexus of biliary ducts:-1, 1, intralobular veins; 2, 2, trunks of biliary ducts, proceeding from the plexus which traverses the lobules; 3, nterlobular tissue; 4, parenchyma of the lobules.

one or two large adipose globules, or five or six small ones (Fig. 122); but the amount of this fatty matter is liable to great variations (§ 754).

Fig. 122.

The biliary matter which these cells 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, which, 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.

Glandular cells of Liver:-a, nucleus;

b, nucleolus; c, adipose particles.

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 ducts 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 when 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 obtained from this receptacle, they probably over-estimate the proportion of solid matter contained in this secretion; which

Of this solid matter, about a

is usually stated at from 8 to 9 per cent. 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 substances employed to separate them; so that different Chemists, by employing 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; consisting of a very large proportion of carbon and hydrogen, and of a comparatively small amount of oxygen and azote. According to Prof. Liebig, the organic portion of ox-bile may be represented by the formula 76 Carbon, 66 Hydrogen, 22 Oxygen, and 2 Nitrogen, with a considerable proportion of Sulphur. 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 crystallizable fatty matter, somewhat resembling spermaceti, free from taste and odor, and composed almost entirely of carbon and hydrogen; its formula is 36 Carbon, 32 Hydrogen, and 1 Oxygen.-The coloring 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 from 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 complete summary; though it is impossible 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 recognised. A portion of the Bile unquestionably passes off, in Man and most other animals, with the fæces; this portion, which includes the coloring matter, is probably that which would be most injurious, if retained in the blood, and is most purely excrementitious. In bilious diarrhoea, and under the influence of purgatives, especially mercurials, a large quantity of bile is discharged per anum, apparently almost unchanged. But in the healthy state, a portion, at least, of the soapy compound seems destined for reabsorption. Just as ox-gall is commonly used to remove grease-spots, by its solvent power for fatty matter, 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 (§ 479). The fatty matter of the bile when reabsorbed. with that of the newly-ingested food, is probably, 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.The secreting action of the Liver, by which a certain product is entirely separated from the blood, constitutes, however, only a part of the action of that organ; since, as already shown (§ 493), the changes which it effects in the alimentary materials newly introduced into the current of the circulation, are at least equally important. A large part of the bulk of the Liver, in many of the lower animals, is made up of oleaginous matter; which appears to accumulate in the hepatic cells, giving them almost the character of fat-cells, in proportion as the respiratory function is inactive. Thus, the liver is very large and fatty in Mollusca and Crustacea; whilst, on the other hand, in Insects it is comparatively undeveloped. In Fishes, again, it is rich in oily matter, but in Mammalia it is much less fatty in the state of health; whilst in the liver of Birds, scarcely any traces of fat are to be found.

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. It may be pretty certainly affirmed, however, that biliary matter does not pre-exist as such in the blood; but that its elements must be originally present there, under some more pernicious form. For it is found that the total suspension of the secreting action of the Liver, whereby the excrementitious matter is left to accumulate in the blood, has a much more prejudicial effect upon the system, than the reabsorption of Bile after it has been secreted, in consequence of an obstruction to its discharge through the ductus choledochus; so that it may be inferred that the noxious products of the disintegration of the tissues are transformed into comparatively innocent components of Bile, in the very act of secretion.-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, beyond the amount that is requisite for the supply of the respiratory process, or that can be deposited as fat. For we continually witness the results of habitual excess in the amount of such substances, in producing that state of the system commonly termed bilious; of which all the symptoms are referable to the accumulation of the elements of the bile in the blood, and the consequent deterioration in the purity of 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.

3. Of the Kidneys and the Urine.

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 removal from the system. We have seen that, in the Lungs, the excretion of Carbonic acid is made subservient to the absorption of Oxygen; 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 invariably 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 development; and in ascending through the Vertebrated 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 ureter, and which are connected together by a loose web Fig. 123.

a

Fig. 124.

Kidney of foetal Boa; the urinary tubes as yet short and straight.

Embryo of Green Lizard;-a, heart; b, duplex aorta; c, vena cava; d, intestine; e, liver; f, rudiment of Wolffian body;g, g, rudiments of extremities.

of areolar tissue, 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 animals (Fig. 123, f). A similar condition is found in the true Kidney of higher animals at an early grade of development (as shown in Fig. 124); the tubuli uriniferi being short and straight. In their more advanced condition, however, they become long and convoluted; and the ramifications of the capillary vessels come into very close relation with them (Fig. 125). It is in the higher Reptiles, that we first meet with the distinction between the cortical and medullary substance; the former being the part in which the blood-vessels are most copiously distributed, and in which the tubuli have the most convoluted arrangement; and the latter consisting chiefly of straight tubuli, converging towards the points at which they discharge themselves into the ureter (Fig. 126). The bundles of tubuli and their vascular plexuses remain distinct, however, in Birds and in the lower Mammalia, so as to give to