658 VEINS OF THE CHICK. 



It soon receives a mesenteric vein bringing the blood from the viscera, 

 which is small at first but rapidly increases in importance. 



The common trunk of the vitelline and mesenteric veins, which may be 

 called the portal vein, becomes early enveloped by the liver, and gives off 

 branches to this organ, the blood from which passes by the hepatic veins 

 to the vena cava inferior. As the branches in the liver become more 

 important, less and less blood is directly transported to the heart, and finally 

 the part of the original vitelline vein in front of the liver is absorbed, and the 

 whole of the blood from the portal system passes from the liver into the 

 vena cava inferior. 



The last section of the venous system to be dealt with is that of the 

 anterior abdominal vein. There are originally, as in the Anura, two veins 

 belonging to this system, which owing to the precocious development of the 

 bladder to form the allantois, constitute the allantoic veins (fig. 370, vu}. 



These veins, running along the anterior abdominal wall, are formed 

 somewhat later than the vitelline vein, and fall into the two ductus Cuvieri. 

 They unite with two epigastric veins (homologous with those in the Anura), 

 which connect them with the system of the posterior cardinal veins. The 

 left of the two eventually atrophies, so that there is formed an unpaired 

 allantoic vein. This vein at first receives the vena cava inferior close to the 

 heart, but eventually the junction of the two takes place in the region of the 

 liver, and finally the anterior abdominal vein (as it comes to be after the 

 atrophy of the allantois) joins the portal system and breaks up into capillaries 

 in the liver 1 . 



In Lizards the iliac veins join the posterior cardinals, and so pour part of 

 their blood into the kidneys ; they also become connected by the epigastric 

 veins with the system of the anterior abdominal or allantoic vein. The 

 subclavian veins join the system of the superior venae cavas. 



The venous system of Birds and Mammals differs in two important 

 points from that of Reptilia and Amphibia. Firstly the anterior abdominal 

 vein is only a foetal vessel, forming during foetal life the allantoic vein ; 

 and secondly a direct connection is established between the vena cava 

 inferior and the veins of the hind limbs and posterior parts of the cardinal 

 veins, so that there is no renal portal system. 



Aves. The Chick may be taken to illustrate the development of the 

 venous system in Birds. 



On the third day, nearly the whole of the venous blood from the body 

 of the embryo is carried back to the heart by two main venous trunks, 

 the anterior (fig. 125, S.Ca.V) and posterior (V.Ca) cardinal veins, joining on 

 each side to form the short transverse ductus Cuvieri (DC), both of which 

 unite with the sinus venosus close to the heart. As the head and neck 

 continue to enlarge, and the wings become developed, the single anterior 



1 The junction between the portal system and the anterior abdominal vein is 

 apparently denied by Rathke (No. 300, p. 173), hut this must he an error on 

 his part. 



