Gall Bladder, the pear-shaped membranous reservoir, situated in a slight depression on the lower surface of the right lobe of the liver, which contains the bile during the intervals of digestion. The larger extremity is directed forward and to the right; the body of the organ is adherent above to the substance of the liver by dense areolar tissue, free below, covered by the peritoneum, and resting upon the pylorus, duodenum, and right arch of the colon; the neck is narrow and continuous with the cystic duct, about an inch and a half long, which unites with the hepatic duct from the liver, of about the same length, to form the common bile duct (ductus communis choledocua of anatomists). It is composed of an external serous coat, a middle areolar contractile tissue, and an internal mucous membrane; the arteries are derived from the hepatic branch of the coeliac axis, the nerves from the hepatic plexus, and the veins empty their contents into the vena portae. The hepatic duct is formed by the junction of the two principal branches (one from each lobe), the result of the union of the numerous ramifications from the interior of the liver.

During digestion the bile flows without obstruction into the duodenum, but in the intervals of this process, owing to the partial constriction of the common duct, a portion of the bile flows by the cystic duct backward into the gall bladder, whose office is essentially that of a reservoir, storing up a supply of the secretion in the intervals of digestion. The common duct is formed by the union of the hepatic and cystic ducts, and is about 3 1/2 in. long, opening obliquely into the duodenum near its last curve, by an orifice in the middle of a slight elevation. The stimulus of the food opens the intestinal orifice, and bile is discharged both from the liver and the gall bladder during digestion, its passage being effected by the contraction of the walls of the gall bladder and the ducts. Ordinarily containing a few ounces, the gall bladder may be so distended as to contain several pints, and it may be so atrophied as to be little larger than a pea; these cases, and the fact of the absence of the reservoir in many animals, show that its physiological importance is not great.

It is subject to ossification, cancer, and acute and chronic inflammation from the irritation of gall stones or extension of diseases from the intestine; its diseases may end in ulceration, and obliteration of the ducts. From its smallness and protected situation it is rarely directly wounded, though it is sometimes ruptured by great external violence. The gall bladder is absent in invertebrates, in which the bile ducts open directly into the digestive cavity; it is present in most fishes, all reptiles, and most birds. There seems to be no general law regulating its presence or absence in mammalia; it is wanting in many rodents (as the mouse), in the elephant, rhinoceros, tapir, camel, peccary, horse, stag, and dolphin; it is present in the monkeys, bats, carnivora, almost all edentates, and in many ruminants (as the ox, sheep, goat, and antelope). In the orycteropus of the Cape of Good Hope, an animal related to the ant-eaters, there are two gall bladders. With the exception of the dolphins, it seems that all mammals in which it is absent are vegetable feeders.