CIRCULATION AND RESPIRATION IN ANIMALS 167 



glands. The fine capillaries lead into veinlets and these into 

 veins of constantly increasing calibre which sooner or later 

 complete the circuit by returning the blood to the heart. 



The return current, however, is not quite so simple as 

 would appear from the above statement because, just as all 

 the outgoing stream is interrupted for the respiratory inter- 

 change in the gills, so a part of the return current is tempora- 

 rily side-tracked through the liver. The veins returning blood 

 from the digestive organs merge to form the PORTAL VEIN 

 which proceeds to the liver, where it resolves into capillaries 

 to allow that organ to regulate certain of the blood constit- 

 uents in particular, to store up sugar after a meal and 

 later dole it out to the blood as needed. These capillaries 

 then pass the blood into the HEPATIC VEIN, which conveys 

 it toward the heart. Thus the liver receives blood from two 

 sources: an artery providing blood primarily for the use of 

 the organ itself and a vein (portal vein) delivering blood con- 

 taining a large amount of food material solely to receive 

 special treatment before being sent back to the heart and then 

 all over the body. This special arrangement for a venous 

 blood supply to the liver is known as the HEPATIC PORTAL 

 SYSTEM. Moreover, in Vertebrates lower than the Birds, the 

 venous blood from the posterior part of the body makes a 

 detour through the capillaries in the kidneys on its way back 

 to the heart. This constitutes what is termed the RENAL 

 PORTAL SYSTEM. Therefore in these forms the kidneys as 

 well as the liver receive blood from two sources, an artery 

 and a vein. It will be noted that both the hepatic portal vein 

 and the renal portal vein arise in capillaries and terminate in 

 capillaries. (Figs. 91-93.) 



Such is the general plan of the blood vascular system of the 

 lower Vertebrates. The modifications of this which occur in 

 higher forms are related chiefly to changes in the respiratory 



