THE VENOUS SYSTEM. 541 



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 ox'gan, the blood from which passes by the hepatic veins 

 to the vena cava inferior. As the bi-anches in the liver become more im- 

 portant, 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 [)recocious development of the 

 bladder to form the allantois, constitute the allantoic veins (tig. 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 car- 

 dinal 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'. 



In Lizards the iliac veins j(jin the posterior cardinals, and so pour part 

 of their blood into the kidneys ; they also become connected by the epi- 

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

 The subclavian veins join the system of the superior venae cavae. 



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 fuetal 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.Ga. V) and posterior (C. Fa) 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 

 cardinal or jugular vein (fig. 371, J), of each side, is joined by two new 

 veins : the veitebral vein {Su. V.), bringing back blood from the head and 

 neck, and the subclavian vein from the wing (IF). 



On the third day the posterior cardinal veins are the only veins whicli 

 return the blood from the hinder part of the body of the embryo. 



About the fourth or fifth day, however, the vena cava inferior (fig. 

 371, V.C.I) makes its appearance. This, starting from the sinus venosus 

 not far from the heart, is on the fifth day a short trunk running backward 

 in the middle line below the aorta, and speedily losing itself in the tissues 



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

 apparently denied by Ratlike (No. 300, p. 173), but this must be an error on his part. 



