Uses cf the liver.—The liver performs two most important functions in the animal eco nomy :-1, it separates from the venous blood of the chylopoietic viscera certain elements which are needful to digestion ; and, 2, it de purates the venous blood. The first of these functions constitutes the secretion of bile. The second is evinced in a comparative exami nation of two of the great depurating organs, the lungs and the liver, in the various classes of animals, where the latter will be constantly found in exact relation with the development of the respiratory organ, and with the neces sity for the removal of a larger quantit7 of hydrogen and carbon from the blood. Thus, in herbivorous animals, the liver is small; it is small also in monkeys and in man. It is large, and has reached its highest development amongst Mammiferous animals in Carnivora. In birds it is larger in proportion than in Car nivora, from the greater necessity of a highly oxygenated blood in that class of animals. In Reptiles, with cold blood and a low degree of respiration, it is large; it is large also and for the same reason in Fishes; and very large among the Invertebrata.
Secretion if* bile.—The bile, which would appear, from the existence of follicular recesses in the alimentary canal, to be produced in all animals from the lowest to the highest, is secreted in man and in vertebrate from the blood during its circulation through the lobu lar venous plexus in the lobules of the liver. Ilence it becomes a question of importance to physiology to decide from what kind of blood it is eliminated. If, according to Kiernan, all the arterial blood of the hepatic artery become venous previously to its passage into the lo bular venous plexus, the bile must be secreted from venous blood ; that venous blood being derived from the capillaries of the cliylopoietic organs, and from the capillaries of the hepatic artery. I have given Kiernan's reasons for the belief that this is the truth ; and in corrobo rating the results of his injections I must also add my own testimony to his view of the se cretion of the biliary fluid. Muller, enter taining, as I have already shewn, a different opinion with regard to the distribution of the vessels of the liver, believes that the bile is secreted from a mixed arterial and venous blood, resulting from the termination of both the hepatic artery and portal vein in the " vas cula ultima reticulata," or lobular venous plexus. From the undecided manner in which he expresses this opinion, I am tempted to give the quotation in which it is contained, that my readers may judge how far he be really in earnest in his assertion. " It is known that injection thrown either into the hepatic artery or into the portal vien, fills the same capillary net-work, from which, on the other hand, the hepatic veins likewise arise." Since reading the above paragraph I have injected twelve livers for the purpose of de ciding the question, in my own mind, of the ultimate termination of the hepatic artery; but I have in no instance succeeded in forcing in jection into the lobular venous plexus, although every other part of the organ has been beauti fully injected. I have therefore been forced to the conclusion that some mistake must exist with regard to this passage, and that, although perfectly true when confined to the portal vein, Muller cannot mean that the capillary net work (lobular venous plexus) from which the hepatic veins arise, is actually filled from the hepatic artery. But he continues, " It ap pears, therefore, that the arterial blood of the hepatic artery, and the venous blood of the porta, become mixed in the minute vessels of the liver, and that the secretion of bile probably takes place from both." Now, with deference to Muller's judgment, the question, with our present knowledge upon the exact anatomy of the liver, ought not to be one of probability or surmise ;—does it? or does it not? But he appears far from satisfied, in relying for the support of his argument upon his own peculiar theory of the arrangement of the he patic vessels, and, as if distrusting its effici ency, he exclaims in another page of his Phy siology, " But the possibility of bile being secreted from arterial blood is demonstrated by the cases in which the vena ports: enters the vena cava directly instead of being distributed through the liver. Mr. Abernethy observed this anomalous structure in a male child ten months old ; and Mr. Lawrence has detailed a case in which the same malformation existed in a child several years of age. In Mr. Abernethy's case however the umbilical vein was still pervious and branched out in the substance of the liver; it is possible therefore, as Mr. Kiernan remarks,
that the arterial blood, after having nourished the liver, was poured into the branches of the umbilical vein, just as it is in the normal con dition, according to his opinion, poured into branches of the portal vein, and the secretion of bile therefore might still have been derived from venous blood." " M. Simon and Mr. B. Phillipps have in ferred from experiments which they performed, that the bile is secreted from the blood of the portal vein. But Mr. Phillips found that after the vena portre had been tied the secretion of the bile still continued, though in di minished quantity ; and he concludes, there fore, that it is formed both from arterial and venous blood. He perceived no change in the biliary secretion when the hepatic artery was tied." The cases recorded by Wilson, Abernethy, and Lawrence are interesting, but they do not appear to me to affect in the slightest de gree the arguments on either side of the pre sent question. It is true that it might be asserted in behalf of Miiller's opinion, that the blood sent to and circulating in the liver was arterial, and that from this alone bile was secreted, for in both cases bile was found in the gall-bladder, while the vena porta emptied itself into the vena cava. On the other hand it was ascertained by Kiernan in the only one of the three cases in which the liver was pre served, that the umbilical vein (hepatic portal) was pervious, of considerable size, and rami fied as usual through the portal canals and terminated as usual in the lobular venous plexus. Now, although the hepatic portal vein (umbilical) did not obtain its accustomed supply of blood after the placental circulation was arrested, from the abdominal portal vein, yet there is no reason for supposing that it did not collect the venous blood from the capillaries of the arteries supplying the coats of the ex cretory ducts and other vessels. Again, the transmission of the remaining portion of the arterial circulation through the vaginal, the interlobular, and lobular arteries must have seriously affected its arterial character if it have not indeed altogether converted it into venous blood. Although Mayo, who took part in the examination of this liver, observed upon this point that " it cannot be supposed that the arterial blood, in its passage through the vase vasorum into the branches of the umbilical (hepatic portal) vein underwent the usual change into venous blood ; and it was still, he contended, arterial blood, though less pure in character, which was conveyed through venous canals into the secreting part of the liver." Now it may be fairly presumed that blood which is not arterial must be venous; but it must at the same time be admitted that the normal degrees of arterialisation are various in individuals, and different in different regions of the body at the same moment ; so that no satisfactory argument can be an assumption of the sub-arterial character of the blood. I would rather suggest another train of reasoning. The abdominal portal vein re turning blood possessed of peculiar properties from the chylopoietic viscera terminates in a rare anomaly in the inferior cava, so that the portal blood is mingled with the general venous current of the system. The lungs receiving this blood exert their appropriate influence in separating from it a portion of the noxious ele ments with which it is combined; but it cannot be supposed that this blood will return to the heart as pure in character as that which has circulated in the usual way through the other depurating organ, the liver. No; it still con tains the elements from which bile may be secreted, and a larger portion than usual is therefore sent to the liver, that this secretion may be eliminated. Hence we cannot treat the blood thus flowing into the liver from the aorta in a much larger currertrthan natural (" in ordinary cases one principal artery is found in each canal ; in this case two, and in some places three arteries of equal calibre were found in each canal") as mere arterial blood destined for nutrition alone; but we must re gard it as a fluid bearing in its course the ele ments of the bile; and therefore, whether it be poured through the capillary channels of the lobular venous plexus, or through those of its own developing in the substance of the lobules, it is nevertheless an abnormal influence which cannot be tested by man's decision, but is part of the compensating principle so admirably displayed by nature in all her operations.