206 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



4. The aorta proper gives off large branches, such 

 as the cceliaco-meseiiteriic, hepatic, renals to 



the viscera of the body cavity, and two large hinder 

 (iBiac) arteries which pass to the hinder limbs. The 

 final termination of the aorta is the caudal artery, 

 and the size and extent of this and its branches are, 

 of course, in direct relation to the size of the tail. 



As the arterial vessels get smaller and smaller the 

 blood from them flows into the capillaries, and thence 

 begins to make its way back to the heart by the 

 veins. 



1. The blood brought back by the hind-limb enters 

 the pelvic vein, and then passes either by the iliac 

 or the anterior abdominal into the vena cava 

 inferior ; in all Vertebrates, except Mammals and 

 Birds, the blood that passes along the iliac veins breaks 

 up again into a system of smaller veins within the 

 substance of the kidneys, forming in them a renal 

 portal 1 * system. Again collecting, the renals, just as 

 in mammals, pass into the inferior vena cava. 



3. The blood from the intestinal viscera is collected 

 into a portal"* vein, which breaks up in the substance of 

 the liver into a portal system ; this hepatic portal 

 system obtains in all Vertebrates. The blood brought 

 from the liver by the hepatic veins likewise passes 

 into the inferior vena cava. 



It is impossible to understand the arrangements of 

 the collecting portion of the heart and of the superior 

 veins without a knowledge of what obtains in all 

 Vertebrates at an early period of development, and 

 in Fishes throughout their lives. The unpaired vena 

 cava is preceded by a single subintestiiial vein, 

 which collects the blood from the yolk sac, and this by 



* The term PORTAL SYSTEM owes its name to the fact that in 

 man the so-called portal vein enters the substance of the liver 

 by its fissure (or PORTA, as the older Latin-speaking anatomists 

 called it). 



