426 COMPARATIVE ANATOMY 



A renal portal system is absent in Cyclostomes and certain 

 Elasmobranchs, and is inconstant and very variable amongst 

 Teleostomes : in many instances the caudal vein communicates 

 directly with one or with both posterior cardinals, and in the former 

 case the other cardinal shows a tendency to become reduced in size : 

 a similar reduction occurs in many of the forms described below. 



The chief point of interest as regards the veins of Dipnoans 

 (Fig. 328) is the presence of a large unpaired pos,tc.aral vein, 

 derived in part from the posterior cardinal, and comparable to 

 that of the Amphibia and Amniota. A renal portal system is 

 present, and the blood from the kidneys is collected into two veins 

 having the relations of posterior cardinals. Only the left of these, 

 however, opens anteriorly into the corresponding precaval, the 

 right, which is much the larger of the two, passing along the 

 dorsal border of the liver to open independently into the sinus 

 venosus in the middle line. The renal portion of this vein is 

 evidently homologous with the corresponding part of the posterior 

 cardinal, the anterior portion of which can no longer be recognised. 

 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 (cf. Fig. 329) and opening into 

 the sinus venosus. Afferent renal veins also enter the postcaval 

 and cardinal, the corresponding efferent vessels opening into the 

 left cardinal and caudal veins ; the renal portal system is thus 

 more complicated than in other Dipnoans. The two pulmonary 

 veins unite into a single trunk before opening into the left 

 atrium. 



Amphibians. 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 both posterior cardinals 

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

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

 other Anurans. It communicates with the corresponding precaval 

 (Fig. 329). 



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

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



