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 (oesophagus, stomach) directly to the liver.] In Fig. 264 the dissected venous system of _Necturus maculatus_, the mud puppy, is shown in an injected preparation. [Illustration: FIG. 264.--Dissection of veins of _Necturus maculatus_, mud-puppy. (Columbia University Museum, No. 1835.) The postcava has been divided at the cephalic end of the liver just before entering the sinus venosus, and the postcardinals have been cut prior to their junction with the subclavian veins. The stomach has been turned caudad. The abdominal vein has been divided after the common trunk has been formed by branches from the iliac veins. The latter are seen entering the afferent renal-portal vein, derived from the bifurcation of the caudal vein, along the lateral border of the kidneys. The junction of the main trunk of the abdominal vein with the hepatic portal vein takes place close to the liver under cover of the pancreas. A series of accessory portal veins continuous with the abdominal vein enter the ventral surface of the liver between the layers of the ventral mesogastrium. The inter-renal segment of the postcava receives the revehent renal-portal veins. The iliac vein enters the advehent renal-portal veins derived from the caudal vein.] [Illustration: FIG. 265.--Venous system of _Rana esculenta_, frog. (Ecker.)] (_b_) =Anure.=--The venous system of _Rana esculenta_ is shown in Fig. 265. Comparison with venous system of _urodele_: 1. The abdominal vein, corresponding to the mammalian umbilical vein, has assumed a greater importance in reference to the hepatic circulation. It is a large trunk, continuous below with the pelvic vein, terminating above in two branches, which enter the liver as afferent veins, being joined just prior to the division by the hepatic portal vein. 2. A small cardiac vein, coming from the heart, empties into the angle of bifurcation of the abdominal vein. 3. The postcava is well developed, formed by large efferent renal veins. It entirely replaces the posterior cardinal veins which are absent in the adult animal. 4. A right and left praecaval vein is formed by the union of two jugular trunks with the vein of the anterior extremity and a large musculo-cutaneous vein. Comparison with the mammalian development: the venous system of this amphibian can be used to illustrate the mammalian embryonal stage shown in schema Fig. 252, in which the abdominal or umbilical vein has become the most important vessel in the afferent hepatic venous system.