This symmetrical arrangement, and the direct connection of the umbilical veins with the sinus venosus, now becomes lost by the occurrence of the following changes:
1. At first ([Fig. 249]) all the blood carried to the liver by the omphalo-mesenteric veins passes through the hepatic capillary network before being conducted by the venæ revehentes to the sinus venosus. Very early, however, a new intrahepatic channel develops, the ductus venosus ([Figs. 250]-[253]), which passes obliquely between the entrance of the left omphalo-mesenteric vein into the capillary system (l. v. advehens) and the termination of the right omphalo-mesenteric vein (r. vena revehens) in the sinus venosus.
In human embryos of 4 mm. the ductus venosus can already be distinguished, and in embryos of 5 mm. the vessel has assumed considerable proportions.
2. A communication is next established on both sides between the capillary hepatic network in the portion of the liver nearest to the abdominal wall and the umbilical veins as they ascend imbedded in the abdominal wall ([Fig. 251]).
This connection is usually from the start larger on the left side and connects with the left omphalo-mesenteric vein just at the point where the same is about to be continued into the ductus venosus. This connection becomes rapidly larger, so that the ductus venosus, which at first appeared merely as an anastomotic channel between the left omphalo-mesenteric vein and the terminal portion of the right omphalo-mesenteric vein, now forms the main continuation of the left umbilical vein. This vessel grows very rapidly up to its connection with the ductus venosus and soon exceeds the right umbilical vein in size (Fig. 252). Beyond the ductus venosus on the other hand the proximal segment of the left umbilical vein diminishes in size, and loses its independent character by incorporation in the hepatic circulation. Only its terminal portion, emptying into the sinus venosus, is preserved. This is surrounded by the growing masses of hepatic cylinders and is converted into a vena revehens.
The connection of the right umbilical vein with the liver vessels is at first symmetrical to that on the left side, but less strongly developed. The effect of this connection is to reduce in the same way the proximal segment of the right umbilical vein and to convert its termination into a vena revehens. With the great development of the left vein, however, the vein on the right side gradually diminishes and finally loses its connection with the intrahepatic circulation altogether. The right umbilical vein is now reduced to a vessel of the ventral abdominal wall, which carries blood in the reverse of the original direction, i. e., from the abdominal wall caudad into the left umbilical vein (Figs. 253 and [255]).
The connection thus established between the umbilical vein and the portal circulation results in the formation of a single large (the original left) umbilical vein which, throughout the remainder of fœtal life, returns all of the placental blood (Fig. 253).
The newly developed hepatic portion of the left umbilical vein becomes, however, not only connected with the ductus venosus, but also with the right part of the upper venous ring, derived from the right omphalo-mesenteric vein (Fig. 253). This connection forms the left portal vein of the adult, and enlarges rapidly.
The terminations of the ductus venosus and of the venæ hepaticæ revehentes undergo a number of secondary changes in relative position. The left hepatic vein loses its direct connection with the sinus venosus, and now opens into the termination of the ductus venosus, into which the right hepatic vein also empties. This common vessel (v. hepatica communis) subsequently forms the proximal segment of the postcava when this vessel develops (Fig. 256).