I. B. Comparative Anatomy of the Liver.—The liver, phylogenetically a very old organ, occurs in all vertebrates, for the cæcal diverticulum of the intestine of amphioxus ([Fig. 49]) has probably the significance of a hepatic outbud.

The primitive form of the liver is symmetrically bilobed, a type which is seen well in the chelonian organ (Fig. 238).

Fig. 238.—Pseudemys elegans, pond turtle. Alimentary canal. (Columbia University Museum, No. 1437.)   Fig. 239.—Stomach, mid-gut, pancreas, and liver of Boa constrictor, boa. (Columbia University Museum, No. 1832.)

In size the liver is subject to great variations. It is usually larger in animals whose food contains much fat. Hence carnivora in general have a larger liver than herbivorous animals.

Its shape also varies considerably, depending on the form of the body cavity and on the amount and disposition of the available space. Hence in the snakes the organ appears long drawn out, flattened, almost ribbon-like (Fig. 239), while the relatively very large coronal diameter of the body cavity in the turtles permits the liver to expand transversely (Fig. 238).

Fig. 240.—Liver of Macacus cynomolgus, Kra monkey. (Columbia University Museum, No. 28/1833.)   Fig. 241.—Liver of Pleuronectes maculatus, flounder. (Columbia University Museum, No. 1679.)

In general, when the liver is large and the available space for its reception limited, it is usually split into several (two to seven) lobes, which permit, by mutual displacement, the accommodation of the organ to varying space-conditions of the body cavity (Fig. 240). Under the opposite circumstances, on the other hand, even the primitive bilobed character may disappear and the liver is then unlobed (Fig. 241).

The presence or absence of a gall-bladder depends apparently largely on the character of the food and on the habitual type of digestion. In many vertebrates digestion is carried on nearly continuously, without marked interruption, especially in many ungulates, ruminants and rodents. In such animals the gall-bladder is absent. It is also absent in several birds (most Parrots, Doves, Ostrich, Rhea americana, the Cuculidæ, Rhamphastos, etc.). This variability emphasizes the morphological fact that the biliary bladder is only a modified portion of the hepatic duct system, as shown by the development above outlined.

A great variety is observed in the arrangement of the biliary ducts, through which, at the period of intestinal digestion, bile passes from the liver and gall-bladder into the intestine, while in the intervals of digestion the secretion is only carried from the liver to the bladder. The following main types of the biliary duct system may be recognized: