240 STRUCTURE OF THE VERTEBRATES 



auricle and joins the cardinals. Therefore the posterior part of 

 the postcava is morphologically the older cardinal vessels. 



In the reptiles the cardinals lose their posterior connection 

 with the postcava, and are small paired vessels draining the 

 thoracic region. The postcava drains the posterior part of the 

 body (the abdominal veins having disappeared) and the cardinal 

 vessels have further degenerated. The left vein loses its connec- 

 tion with the jugulars, and develops one or more transverse con- 

 nections with the right cardinal. The left postcardinal is called 

 the hemiazygos; and the right half, which empties into the 

 jugular, is the azygos. As in the reptiles, it drains the costal 

 muscles. 



The embryonic development of the veins of the mammal will 

 review these changes. The embryo has two postcardinals carry- 

 ing the blood from the primitive kidneys (mesonephroi) to the 

 ducts of Cuvier. Subcardinal vessels develop on the median sides 

 of the kidneys, taking over the smaller vessels of the post- 

 cardinals. The postcava grows posteriorly and joins the sub- 

 cardinals, taking its definitive form. In the meantime the post- 

 cardinals have begun to degenerate in the kidney region, and 

 the cross connections have developed. The left cardinal then de- 

 generates along its anterior course, the blood from both cardinals 

 emptying through the right azygos into the jugular. 



7. The Renal Portal System. A portal vein is one which 

 breaks into capillaries before reaching the heart. In the dogfish 

 the caudal vein divides and passes to either kidney, the lateral 

 veins making a connection with the two renal portal vessels. 

 Blood from the tail, therefore, may pass to the heart either 

 through the kidneys, or around the kidneys into the lateral 

 veins. 



The vessels entering the kidneys break into capillaries around 

 the tubules, with the result that both venous and arterial blood 

 is received. The renal portal system is present, almost fully 

 developed, in the amphibia; but with the degeneration of the 

 cardinals in the reptiles the portal veins become less important. 

 In the mammals the kidneys are supplied by large arteries, and 

 the renal portal veins disappear. 



8. The Hepatic Portal SYSTE^L It has been mentioned that 

 the paired omphalo-mesenteric vessels pass anteriorly around the 



