VENOUS SYSTEM 419 



caudal aorta is spoken of as the median sacral artery, and the aorta 

 here appears to be directly continued, not by it, but by the 

 common iliac arteries, which pass outwards into the pelvic 

 region (Fig. 322). 



Each common iliac artery becomes divided into an internal 

 iliac, or hypogastric, supplying the viscera of the pelvis and 

 derived from the proximal portion of the embryonic allantoic 

 artery, and an external iliac, which is continued into the femoral or 

 crural and supplies the hinder extremity. In some cases (e.g. 

 Sauropsida) the internal and external iliacs come off separately 

 from the aorta (Fig. 323), thus indicating the primary segmental 

 nature of the arteries supplying the embryonic extremity. 



The arteries of the hind-limb, like those of the fore-limb, have 

 undergone considerable modifications in the course of phylogeny. 

 Thus it is highly probable that the femoral artery was not 

 originally the chief vessel of the posterior extremity, but that the 

 main flow of blood passed along the sciatic artery arising more 

 posteriorly from the aorta, as is still the case in the Amphibia 

 and Sauropsida and in certain embryonic stages in Mammalia. 1 



VENOUS SYSTEM. 



Ampbioxus. The blood from the intestine passes into a 

 subintestinal vein which extends forwards as the hepatic portal vein 

 to the ventral side of the rudimentary liver, where it divides 

 up into a capillary network (Fig. 324). From this it passes into 

 a Jicpatic vein on the dorsal side of the liver, and this vein becomes 

 continuous with the contractile ventral aorta. A series of seg- 

 mental veins in the body walls opening into anterior and 

 posterior cardinal veins, comparable to those of Fishes and com- 

 municating with the ventral aorta by precaval veins, have also 

 been described (Fig. 324). 



Fishes. Taking the Elasmobranchii more particularly into 

 consideration, a few of the more important facts as regards the 

 development of the veins must first be considered (cf. p. 397). 



The first veins to appear in the embryo are the paired omphalo- 

 mesenteric veins, which bring back the blood from the surface of 

 the yolk and from the walls of the gut (Figs. 300,326, I, II). The 

 vessels from the former region are known as mtelline veins, while 

 those from the latter give rise to subintestinal veins (Fig. 326, III 

 VII) running beneath the embryonic intestine, which primarily 



1 The tibialis antica and tibialis postica, like the radial and ulnar arteries of 

 the fore-arm, do not represent the chief vessels of the shank. At an earlier stage 

 all these were relatively small branches, while the main stream passed along an 

 internal interosseous and then a median artery in the fore-arm, and a peroneal 

 or branches of a primitive sajihenou* artery in the shank. 



E E* 



