CIRCULATION 173 



of cells. These capillaries, in turn, merge into larger vessels (ef- 

 ferent branchial arteries) which finally lead into the chief 

 artery of the body, the dorsal aorta. This extends along the 

 median dorsal line of the body, just below the vertebral column, 

 and sends branches to the various organs. 



The branches of the dorsal aorta, on reaching the location which 

 they supply with arterial blood, break up into capillaries similar 

 to those in the gills, so the blood can deliver food, oxygen, etc., to 

 the tissues. The blood receives in return various waste products 

 of metabolism, including carbon dioxide and, in certain cases, 

 absorbed food materials from the intestine, and special secretions 

 chiefly from endocrine glands. The fine capillaries lead into vein- 

 lets and these into veins of constantly increasing caliber 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 interchange in the gills, 

 so a part of the return current is temporarily 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 constituents. From the capillaries the blood then 

 passes 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, containing 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 con- 

 stitutes 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. 115, 120, 121.) 



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



