CZ-.rru 



VEINS 605 



the dorsal body-wall and main muscles (cardinals or vena? cava?), 

 the ventral body-wall (abdominals) and the lungs (pulmonarics). 

 All of these are originally paired, but there is a general tendency, 

 both in ontogeny and in phylogeny, 

 for them to join to form or be replaced 

 by single vessels. 



As we have seen, the two vitelline 

 veins fuse to form a subintestinal 

 vessel. The anterior part of this 

 becomes the heart and ventral aorta, 

 while the posterior part drains the 

 gut. As the liver grows, the vein in 

 it breaks up into capillaries so that an 

 anterior hepatic portal vein, emptying 

 into the heart, is separated by them 

 from a posterior hepatic vein. A 

 portal system is one in which the 

 blood, having passed through the 

 capillaries of the general circulation, 

 passes through another set on its 

 way back to the heart. Apart from 

 variations in the disposition of the 

 branches coming from the gut, and 

 one point which we shall mention 

 later, the hepatic portal system is 

 much the same in all vertebrates. 



The cardinal veins run almost the 

 whole length of the body, and are 

 divided into anterior and posterior 

 cardinals by a transverse common 

 cardinal vein, or Cuvierian sinus, 

 which carries blood from both sides 

 into the sinus venosus. This condition 

 is found in embryos, and also in the adults of selachians, except 

 that, before going into the main posterior cardinals, the blood 

 from the posterior part of the body passes through capillaries 

 in the kidney, making the renal portal system. In actinoptcry- 

 gians only part of the blood goes through this, the rest going 

 through the posterior cardinals direct to the heart. In lung fishes 

 and tetrapods a branch of the hepatic portal vein runs dorsally 

 and connects with the right posterior cardinal ; blood then flows 



Fig. 467. — The principal 

 arteries of a lizard. 



a.m., Anterior mesenteric ; c, coeliac; 

 c.a., carotid arch ; c.c, common 

 carotid; cm., coeliaco-mesenteric; 

 cd., caudal ; d.ao., dorsal aorta ; 

 d.B., carotid duct; /.. femoral or 

 external iliac ; l.ov., left ovarian ; p, 

 pulmonary ; p.m., posterior mesen- 

 teric ; r., renal ; r.h., right hepatic ; 

 the left hepatic is a branch of the 

 coeliac ; r.ov., right ovarian ; scl., 

 subclavian ; sci., sciatic or internal 

 iliac; sy.a., s\'Stemic arch; tr.l., 

 tracheo-lingual. 



