THE MODIFICATIONS OF THE VASCULAR SYSTEM. 87 



run along the anterior wall of tlie abdominal cavity, either as 

 two trunks, or united into one. These vence ahdominales an- 

 teriores are eventually distributed to the liver, along with the 

 branches of the proper veriw portm. 



In £irds, the sinus venosus is not distinct from tlie rio'ht 

 auricle, and there are tv/o anterior venm cavm. The vena 

 cava inferior arises, as in Mammals, by the union of the two 

 common iliac veins. It receives both the right and the left 

 hepatic veins, and, in addition, the anterior abdominal vein no 

 longer enters the portal system, but passes up the anterior 

 wall of the abdomen and through the hepatic fissure to join 

 the inferior cava. 



The caudal and pelvic veins unite into three principal 

 trunks, of which one is median and tv/o are lateral. The 

 median enters into the portal system. The lateral branches 

 pass along and through the kidney, receiving veins from it, 

 but giving none to it ; and eventually, after receiving the 

 ischiatic veins, unite with the crural veins to form the common 

 iliacs. Thus there is no renal portal system in birds. 



In Mammalia^ the sinus venosus is not distinct from the 

 right auricle. The anterior cavm are frequently reduced to 

 one, the right. The ve7\a cava inferior commences in the 

 caudal region, and receives all the blood of the posterior 

 moiety of the body, except so much as is carried away by the 

 azygous veins. The anterior abdominal veins are represented 

 only during foetal life, by the umbilical vein or veins. The 

 efferent veins of the kidneys open directly into the trunk of 

 the inferior vena cava, and the portal vein is comjDosed 

 exclusively of radicles proceeding from the chylopoietic 

 viscera. 



Many of the veins of AmpTiioxus^ the portal vein of MyX' 

 ine^ dilatations of the caudal vein in the Eel, the vena3 cavas 

 and the iliac and axillary veins of many Anipliihia^ the veins 

 of the wing of Bats, possess a rhythmical contractility, which, 

 in combination with the disposition of their valves, assists the 

 circulation of the blood. 



In Vertehrata of all classes, and in very diverse parts of 

 the body, both veins and arteries occasionally break up into 

 numerous branches of nearly equal size, which may or may 

 not unite again into larger trunks. These are called retia 

 mirabiUa, 



Modifications of the Heart. — Great changes go on m the 

 structure of the heart, j(9ar^Je>a5Sw with the modifications of the 



