1. The hepatic duct joins the cystic to form the common bile-duct, entering the duodenum by passing obliquely through the intestinal wall (Fig. 242). This form is encountered in man and in most mammals. It is also found in some birds (Buceros), many amphibians, and in some fish (Lophius). Instead of one hepatic duct two may join the cystic duct separately to form the common bile duct (Phoca litorea), or the number of hepatic ducts may be further increased. The separate hepatic ducts then unite successively with the cystic duct. This occurs in many mammals (as Tarsius, Galeopithecus, monotremes) and in some fishes (Xiphias, Trigla, Accipenser) (Fig. 243).
2. Of two hepatic ducts only one helps to form with the cystic duct the common duct, while the other leads from the liver transversely into the bladder, especially into the neck, forming the hepatico-cystic duct (Fig. 244). This arrangement is found in several mammals (calf, sheep, dog).
3. No common bile-duct is formed. The hepatic and cystic ducts each empty separately into the intestine (hepato-enteric and cysto-enteric ducts), while a hepato-cystic duct carries the bile directly from the liver to the gall-bladder (Fig. 245).
Lutra vulgaris among mammalia, the majority of the birds and several reptilia present this type.
When the gall-bladder is absent a single large hepato-enteric duct is found, or instead a number of smaller ducts which enter the intestine successively.
I. C. Development of Vascular System of Liver.—In order to comprehend the peritoneal relations of the adult liver it is absolutely necessary to have a clear understanding of the development of the vascular system in connection with the gland.
For our purpose, in the first place, a serial consideration of the successive stages, illustrated by schematic diagrams, will prove most practicable. These diagrams represent the structures in the dorsal view, i. e., in the position which they would occupy in the adult liver with the gland resting on its upper or convex surface and with the ventral sharp margin turned toward the beholder (see [Fig. 259]).
The development of the venous system, especially in connection with the liver, presents a somewhat complicated series of successive conditions. After having become familiar with the principal typical embryonal stages, as shown in the following diagrams, the student is strongly recommended to cement this knowledge by the comparative examination of the venous system. The permanent veins of the lower vertebrates, while in many cases not strictly homologous to those of the higher forms, yet are excellent objects for study, since they serve to illustrate temporary stages in the development of the mammalian venous system, and to that extent are of aid in comprehending one of the most difficult and important chapters in human anatomy. At the conclusion of the diagrammatic consideration of the mammalian development a number of comparative facts will be put together for this purpose.