322 COMPARATIVE ANATOMY 



by the common iliac arteries, which pass outwards into the pelvic 

 regiou. 



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 crural or 

 femoral and supplies the hinder extremity (Fig. 262). In some 

 cases the internal and external iliacs come off separately from the 

 aorta (Fig. 263). The function of the femoral may be largely taken 

 by a sciatic artery arising separately from the aorta (Birds). The 

 main vessels again branch out in the limb. 



Venous System. 



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 (comp. p. 301). 



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 (Fig. 265, 1, II). The vessels 

 from the former region are known as vitelline veins, while those 

 from the latter give rise to suhintestinal veins (III- — VII), 

 running beneath the embryonic intestine, which primarily extends 

 into the caudal region as the post-anal gut. On the disappearance 

 of the latter, the posterior part of the suhintestinal vessels gives 

 rise to the caudal vein, which now lies directly beneath the caudal 

 aorta and loses its direct connection with the anterior part (VIII — 

 XII). As the liver is gradually developed, the main trunk of the 

 left omphalo-mesenteric vein breaks up into capillaries within this 

 organ, and these again unite anteriorly, opening into the proximal 

 ends of both omphalo-mesenteric veins. The latter thus give rise 

 to the hepatic veins, which open into the sinus venosus (or precaval, 

 e.g., in Cyclostomes). New vessels from the various parts of 

 the alimentary canal (gastric, splenic, and mesenteric veins) are 

 gradually developed, the pre-caudal portion of the suhintestinal 

 vein becoming of minor importance ; all these vessels unite to 

 form what is now known as the liepatic portal vein, and thus 

 pour their blood through the capillaries of the liver (Figs. 270, 

 264—268). 



Anteriorly to the heart, a paired precaval vein (ductus Cuvieri) 

 is developed (Figs. 264 — 268), and opens into the sinus venosus. This 

 is formed, on either side, by the confluence of an anterior omA a, posterior 

 cardinal vein, the former bringing back the blood from the head 

 {external and internal jugular veins),^ and the latter from the body, 



^ A single or paired inferior jiujular from the ventral part of the head may 

 also be present (Fig. 266). 



