428 COMPARATIVE ANATOMY 



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 its paired condition may even be retained in the adult 

 posteriorly (cf. Fig. 322) ; it corresponds genetically to the lateral 

 veins of Elasmobranchs. This vein extends forwards in the ventral 

 body- wall into the liver, in which it breaks up into capillaries, 

 becoming secondarily connected by anastomosis with the hepatic 

 portal vein. The abdominal, 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 in- 

 ternal jugulars) is essentially similar to that seen in Fishes. 



The venous blood from the integument of the body and 

 frequently that of the tail also, passes into a great cutaneous or 

 musculo-cutaneoiis vein, which extends forwards along the longi- 

 tudinal muscles of the trunk, and then makes a wide curve 

 towards the fore-limb, where it unites with the subclavian vein 

 (Figs. 322 and 329). Shortly before opening into the sinus venosus 

 the combined trunk receives a small vein from the laryngeal 

 mucous membrane. 



Amniota. The section of the right posterior cardinal vein in 

 the region of the embryonic kidney (mesonephros) gives rise to the 

 hinder part of the postcaval : the hepatic section of the latter arises 

 as in Amphibia. In the Sauropsida the anterior portions of both 

 posterior cardinals disappear to a greater or less extent, and are 

 replaced by vertebral veins, while in Mammals they may persist 

 and may form part of the azygos veins. An anastomosis is 

 formed between these latter, and eventually the anterior part of 

 the left disappears more or less completely, the blood from both 

 sides passing into the right azygos (hemiazygos), which opens into 

 the right precaval (Figs. 330 and 331). As the azygos and post- 

 caval both receive part of their blood from the lumbar and pelvic 

 regions, the former forms an import ant communicating channel 

 between the latter and the precaval. 



The anterior cardinal gives rise, as in lower Vertebrates, to the 

 jugular, which, as well as the subclavian and vertebral or azygos, 

 opens into the precavals. In Reptiles, Birds, Monotremes, and 

 Marsupials, as well as in many Rodents, Insectivores, Bats, and 

 Ungulates, both precavals persist throughout life ; in other 

 Mammals the main part of the left disappears, all the blood from 

 the head and anterior extremities passing into the right precaval. 

 The coronary veins open into the base of the left precaval (coronary 

 sinus, Fig. 317). 



