328 COMPARATIVE ANATOMY 



Thus the postcaval is made up of a posterior or renal portion, and 

 of an independently developed anterior or hepatic portion. 



In Ceratodus, the posterior cardinal and postcaval are directly 

 continuous with the caudal vein, and the renal portal, receiving 

 branches from the posterior end of the body, arises from the iliac vein, 

 which also gives off a pelvic branch. The latter unites with its 

 fellow in the middle line to form a median abdominal vein, com- 

 parable to that of the Amphibia, and opening into the sinus 

 venosus. 



The two pulmonary veins unite into a single trunk before 

 opening into the left atrium (p. 309). 



Amphibia. — A large postcaval vein arises in essentially the 

 same manner as in the Dipnoi, its renal section being formed by 

 the fusion of the two posterior cardinals in this region. The 

 hepatic portion apparently arises in part from the right omphalo- 

 mesenteric vein, and in part independently, while the hepatic 

 portal vein is developed from the left omphalo-mesenteric. The 

 postcaval receives blood from the kidneys and generative organs, 

 as well as indirectly from the posterior extremities, body-walls, and 

 tail (when present). The anterior part of hoth posterior cardinals 

 persists in Urodeles and in Bombinator as the paired azygos vein, 

 and this may exceptionally be present on one or both sides in other 

 Anurans. It communicates with the corresponding precaval 

 (Fig. 268). 



A renal portal system is present, and is formed, as in Fishes, 

 by the bifurcation of the caudal vein, which is wanting in adult 

 Anura ; into the renal portal open the veins from the hind-limb, 

 and vessels from the body-wall often also communicate with it. 

 The blood from the kidneys passes into the postcaval. Connecting 

 the right and left renal portals (or femorals) is a transverse pelvic 

 vein, from which, in the medio-ventral line of the body, an abdominal 

 or epigastric vein arises,as in Ceratodus: this is primitively paired, and 

 corresponds genetically with the lateral veins of Elasmobranchs ; 

 it extends forwards in the ventral body-wall into the liver, in 

 which it breaks up into capillaries, becoming secondarily connected 

 by anastomoses with the hepatic portal vein (Fig. 268). The ab- 

 dominal vein receives blood from the cloaca, bladder, and body- 

 walls. In Urodeles remains of the subintestinal vein also open 

 into the hepatic portal system. 



The arrangement of the anterior cardinals (external and inter- 

 nal jugulars) is essentially similar to that seen in Fishes and 

 Dipnoans. 



Amniota. — The section of the right posterior cardinal vein in 

 the region of the embryonic kidney (mesonephros, p. 341) gives 

 rise, as in the Dipnoi, to the hinder part of the postcaval : the 

 hepatic section of the latter arises as in Amphibia. In the Saur- 

 opsida, the anterior portions of both posterior cardinals disappear. 



