2. The hepatic portal circulation corresponds to the mammalian stage in which the vitelline veins have become omphalo-mesenteric by joining the intestinal veins.
The spiral vein remains as a portion of the original vitelline vein corresponding to the subintestinal segment of the mammalian embryo (cf. [Figs. 248] and [249]).
The selachian portal vein represents the united vitelline veins, into which the veins of the digestive tract open.
In the liver we find a simple system of venæ advehentes, derived from the branching of the portal vein, a hepatic capillary network, and venæ revehentes, the proximal remnants of the original vitelline veins which carry the liver blood to the sinus venosus. The condition of the hepatic circulation corresponds therefore to the stage shown in [Fig. 250] of the mammalian development. There is as yet no association of the hepatic venous system with the representative of the umbilical vein (the lateral vein of the selachian).
Fig. 263.—Diagram of the veins of urodele amphibian (Salamandra maculosa). (Wiedersheim.)
The caudal vein bifurcates at the posterior extremity of the kidneys to form the afferent trunks of the renal-portal system along the lateral border of the kidneys, from which the advehent veins of the renal-portal system are derived. The iliac or femoral vein divides into an anterior and a posterior branch, the latter opening into the afferent renal-portal vein, while the former, uniting with the one of the opposite side, forms the abdominal vein, and receives vessels from the bladder, cloaca, and end-gut. The revehent veins of the renal-portal system, emerging upon the ventral surface of the kidneys, empty into a single median vessel, the distal or renal section of the postcava or vena cava inferior. Proceeding cephalad, the proximal or hepatic section of this vessel, after traversing the liver and receiving the revehent hepatic veins of the hepatic portal system, empties into the sinus venosus of the heart. Previous to entering the liver the postcava gives off the two posterior cardinal or azygos veins, which continue cephalad, receiving tributary segmental veins from the body-walls and reach the sinus venosus by joining the subclavian veins. These latter uniting with the anterior cardinal (jugular) veins form the ducts of Cuvier (precaval veins).
The abdominal vein continues cephalad in the ventral mesogastrium to the liver, giving off a number of smaller branches, which enter the hepatic portal circulation by penetrating the ventral surface of the liver between the layers of the ventral mesogastrium, while the main continuation of the vessel joins the hepatic portal vein at its point of entrance into the liver.
The hepatic portal vein is formed by tributaries returning the blood from the digestive tract (intestinal canal, spleen, pancreas). The blood, after traversing the hepatic portal circulation, is conducted by the hepatic revehent veins to the proximal section of the postcava. A number of secondary or accessory portal veins pass from the anterior portion of the intestinal canal (œsophagus, stomach) directly to the liver.