A similar arrangement is seen upon the visceral surface of the liver in the Llama.
- The gall-bladder is absent in all invertebrata, the efferent ducts of the biliary organ termina ting for the most part by several openings in the digestive stomach. In Fishes the gall bladder is observed for the first time in the animal series, but it is not by any means con stant in its existence. It is absent in many genera, and in these cases is frequently re placed by a dilatation upon the hepatic duct and by several efferent tubes. In the class Reprints it is invariably present, and varies considerably in form, in the different genera. In serpents it is placed at the extremity or even beyond the liver, and occupies the space formed by the pyloric contraction of the sto mach. The cystic duct is consequently ex tremely long. Among the Chelonia the gall bladder is enclosed within the substance of the liver, and receives its secretion through the medium of cyst-hepatic ducts. Some of these ducts unite also with the cystic duct arid con stitute a ductus communis choledochus. In Birds the gall-bladder is occasionally absent, as in Pigeons, Toucans, &c. without supplying to the comparative anatomist a sufficient reason for the peculiarity; being present and absent in the same natural genera and under precisely the same circumstances of food and climate. The bile is brought from the liver by two ducts, a cyst-hepatic duct which opens into the gall bladder, and an hepatic duct which terminates in the duodenum near to the cystic duct. When the gall-bladder is absent, both hepatic ducts terminate in the duodenum. There is no instance in the whole class of a ductus com munis choledochus. In Mammalia, the gall bladder is by no means constant; it is deficient as a general rule, to which there are several exceptions, in herbivorous animals, as in the horse, stag, elephant, peccary, tapir, whilst it is present in the ox, sheep, goat, antelope, &c. In the first giraffe examined in this country by Owen it was absent; in the next he found two. Upon the hepatic duct in the elephant, near to the duodenum, there is a remarkable dilata tion. In the cat and seal the ductus communis choledochus is dilated in the same situation. It is not uncommon to find a double gall-bladder or two gall-bladders in the cat; in the kinkaju this is supposed to be the normal condition ; and in the Museum of the Royal College of Surgeons there is a preparation, preserved by Hunter, of the liver of a small animal in which are three gall-bladders.
Throughout Invertebrata the bile is secreted from arterial blood. In Fishes the portal vein is formed by veins returning from the tail and occasionally from the air-bladder and genital organs. In Reptiles a part of the blood from the lower extremities unites with that from the alimentary canal to constitute the portal circu lation. In Birds the portal vein also receives a part of its blood from the tail and lower extremities by means of its communication with the pelvic veins. ( Fig. 171, u, v, z, page 338, vol. i.) Injections of the portal vein carefully conducted, as well as injections from the internal iliac vein, have sliewn that a venous communication subsists between the smaller branches of the two systems in the large in testines, even in man. In support of this communication Muller, in his Physiology, quotes the observations of Retzius: " l'ro fessor Retzius, of Stockholm, however, has in formed me that he has discovered in man some minute communications between the veins of the intestines and the branches of the vena cava. When he injected the vena cava and lama portal with fine injection of different colours, he found that the whole meso-colon and colon sinistrum were injected with both colours, and veins belonging to the two systems at several places formed anastomoses. The veins of the
colon and meso-colon, which belonged to the system of the vena cava and entered the left renal vein, lay superficially, while those which belonged to the vena ports: lay for the most part nearer the mucous membrane. The ex ternal surface of the duodenum also had re ceived injection from the vena cava. M. Breschet too has filled the inferior mesenteric vein from branches of the inferior cava, and Schlemm has discovered distinct com munications of the inferior mesenteric vein with branches of the inferior cava about the anus." Besides these communications be tween the two systems occurring in the pelvis, Kiernan points to another most important com munication upon the surface of the liver. " The capsular veins," he says, " are branches of the portal vein; these vessels communicate freely with branches of the phrenic veins. In some cases of atrophy of the liver, and in cases in which the circulation through the liver has been for some time obstructed, a collateral circulation is established by means of the communications which take place between the capsular branches of the hepatic artery and portal vein and those of the phreoic artery and vein." In diving animals, as in the otter and seal, in which large venous reservoirs exist upon the inferior cava, for collecting the re turning blood during submersion, the hepatic veins are muscular. Kiernan observes with regard to the hepatic veins of the seal that they "differ in many respects from those of any other animal I have examined. The intra lobular veins at their exit from the lobules do not as in other animals terminate immediately in the hepatic veins: these vessels enter the hepatic venous canals, where they unite into branches, which, like the vaginal branches of the portal vein, are connected by a fine cellular tissue, with which they form around the he patic veins a cellulo-vascular sheath precisely similar to that surrounding the branches of the portal vein. The structure of the two is similar, but their uses are different. That of Glisson's capsule has been explained ; the capsule of the hepatic veins in the seal appears destined to admit of the muscular con tractions of these vessels." " The external coat of the hepatic veins is composed of circu lar fibres which in the larger vessels form a complete tunic. In the smaller vessels the fibres are arranged in the form of circular fas ciculi, which are connected with each other by oblique intermediate fibres. All the fas ciculi do not extend completely round the veins ; some, dividing into two portions, unite with fibres from those above and below, and form other fasciculi." " In the porpoise the hepatic veins are connected to their canals; no circular fibres are seen in their coats. Their external surface is reticulated, the ridges cor responding to the interlobular fissures, where the interlobular cellular tissue is continuous with the cellular coat of the veins. The mouth of an intra-lobular vein occupies the centre of each space circumscribed by the ridges." The distribution of the vessels in the liver in the three great classes, Reptilia, Ayes, and hlammalia, has been ascertained to be the same with that which has been so completely illus trated in the discoveries of Kiernan. In Fishes but few observations have been made, but analogy would lead us to infer that the ge neral arrangement must be the same.