(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 præcaval 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.

The communication existing by means of the cardiac vein between the heart and the hepatic afferent system may suggest, but purely for illustrative purposes, the direct connection of the umbilical vein with the heart by the ductus venosus in the mammalian embryo (cf. schema [Figs. 250]-[256]).

3. Reptile.—In Iguana the renal portal system is well developed. The caudal vein, returning the blood from the tail and the cavernous tissue of the genital organs, continues for a short distance upon the fused caudal end of the two kidneys ([Fig. 269]) and then divides into two afferent renal veins which ascend on the ventral surface of the glands, giving branches to the renal capillary system. About the middle of the kidney each afferent vein is joined by a large transverse branch from the abdominal vein ([Fig. 266]).

Fig. 266.—Systemic veins of Iguana tuberculata. The alimentary canal and appendages,together with the hepatic portal vein and the intrahepatic segment of the postcava, have beenremoved. The liver occupies the space between the divided ends of the postcava. The vertebralvein represents the rudimentary proximal segment of the postcardinal vein corresponding to themammalian azygos vein. (Columbia University Museum, No. 1320.)

 

Fig. 267.—Veins of Iguana tuberculata. Connection of systemic veins with sinus venosus ofheart. The rudimentary system of the vertebral (azygos) veins and their proximal connectionwith the subclavian vein are shown. (Columbia University Museum, No. 1859.)

 

Fig. 268.—Corrosion preparation of venous system of liver in Iguana tuberculata. The hepaticportal system and its connection with the abdominal vein, as well as the relation to the postcava,are shown. The preparation supplements [Fig. 266], showing the parts which have been removedin the latter. (Columbia University Museum, No. 1860.)