410 



VERTEBRATE LIFE AND ORGANIZATION 



Tr-uncuS 



a-rfceriostxs 



PolTnocutaneouS 



-Aortic 



Spiral 

 va-lve 



A^rio- 



ventr-icu-liT 



value 



ABC 



Figure 21.13. The frog's heart. A, Dorsal view of the surface of the heart; B, 

 ventral view of the surface; C, ventral view of a dissection of the heart. L.A., left atrium; 

 P.V., pulmonary vein; R.A., right atrium. In C, blood entering the ventricle from the 

 right atrium is more darkly shaded than the blood entering from the left atrium, and 

 the classic hypothesis of the separation of the blood within the single ventricle is shown. 



the hepatic portal system. Various tributaries from the viscera unite to 

 form a large hepatic portal vein, which enters the hver and breaks up 

 into many capillary-like spaces among the liver cells. Absorbed materials, 

 therefore, pass directly from the gut to the liver, which, as explained in 

 Chapter 5, has an important role in the metabolism of food. The liver 

 receives blood from the aorta by the hepatic artery and is drained by 

 hepatic veins, which empty into the large posterior vena cava. 



Much of the blood from the hind legs and the back enters a pair 

 of renal portal veins leading to capillaries within the kidneys. The kid- 

 neys are drained by renal veins which enter the posterior vena cava. As 

 the vena cava continues forward it also receives veins from the repro- 

 ductive organs and the liver. Some blood from the legs passes through 

 pelvic veins to the ventral abdominal vein. This vessel continues for- 

 ward, draining the urinary bladder and ventral body wall, and finally 

 joins the hepatic portal as the latter enters the liver. 



Blood from the head, shoulders and arms returns to the heart 

 through a pair of anterior venae cavae (Fig. 21.12). Certain of the tribu- 

 taries of the anterior venae cavae, e.g., the musculocutaneous vein from 

 the skin, and tributaries of the jugulars from the mouth lining, come 

 from respiratory membranes and carry blood with a relatively high 

 oxygen content. 



The lungs are drained by a separate pair of vessels, the pulmonary 

 veins, which unite and enter the heart independently of the anterior 

 venae cavae. 



Heart. The frog's heart consists of a series of chambers having 

 muscular walls to force the blood along and valves to prevent its back- 



