STRUCTURE OF THE VERTEBRATES 225 



be traced from points of origin to the heart. Blood in the muscles 

 of the body is collected by the paired lateral veins and a single 

 cutaneous vein. These empty directly into the heart. The lateral 

 veins are joined by the subclavian and iliac veins from the 

 appendages. The head region is drained by the anterior cardinals 

 and the jugular veins. These may unite before entering the heart, 

 or enter separately. 



The caudal vein drains the tail region, and at the level of the 

 cloaca splits into two vessels which enter the kidneys and there 

 break into capillaries. These two vessels are the renal portal 

 veins. Observe that the blood passing to the excretory organs is 

 both arterial and venous in origin. The nitrogenous wastes are 

 taken from the arterial blood. The venous blood from the renal 

 portal resorbs sugars from the kidney tubules, and collects the 

 blood carried in by the metameric and renal arteries. It is here 

 also that the tonus of the blood is maintained. The capillaries 

 from the kidneys collect into a number of small veins along the 

 median sides of the kidneys, and these form the large paired 

 posterior cardinal vessels which pass anteriorly, close to the 

 dorsal median line, to empty into the heart. 



The digestive tract and mesenteries are drained by the hepatic 

 portal system. Food materials are taken from the absorptive 

 cells of the intestine and taken to the liver. There the hepatic 

 portal vein breaks into capillaries and the sugars are to a large 

 extent deposited as glycogen. The capillaries in the liver re-form 

 as several vessels which unite to form the hepatic veins which 

 enter the heart. 



The large vessels carrying blood to the heart are classified 

 as two groups: (1) the systemic veins which enter the heart 

 directly, and (2) the portal veins which break into capillaries 

 before entering the heart. In the lower groups of vertebrates the 

 veins do not enter the auricle, but into a sinus venosus, the thin- 

 walled posterior chamber. The sinus is extended laterally in 

 the Elasmobranchs as the ducts of Cuvier. The sinus venosus is 

 present in the embryos of all vertebrate classes, but grows pro- 

 gressively smaller as a definitive structure until it disappears 

 entirely in the birds and mammals. 



