Chap. 34 



AMPHIBIANS 



699 



Fig. 34.17. Networks of blood vessels in the web of a frog's foot, a, the 

 arterioles; v, venules and the capillaries between them; x, direct connections be- 

 tween arterioles and venules; pigment spots are scattered along the capillaries. 

 (Courtesy, Maximow and Bloom: Histology, ed. 6. Philadelphia, W. B. Saunders 

 Co., 1952.) 



The frog is midway between fishes and higher vertebrates and its heart is 

 midway between the two-chambered heart of fishes and the four-chambered 

 hearts of reptiles, birds, and mammals. It contains two auricles and a single 

 ventricle (Figs. 34.18, 34.19). On its dorsal side is an important entrance 

 chamber, the sinus venosus, to which three great veins bring blood from all 

 parts of the body except the lungs. The auricles have thin, elastic walls 

 strengthened by narrow bands of muscle. The right one, larger than the left, 

 is separated from it by a partition. The ventricle has a relatively very thick 

 wall containing interlacing muscles. It is separated from the auricles by a par- 

 tition whose location is indicated on the outside by a prominent constriction. 

 On the ventral side a great artery, the truncus arteriosus, is the only exit for 

 the blood. It runs forward a short distance and divides into two trunks, the 

 right and left aortic arches, each of which splits into three branches that supply 

 the entire body. 



