FOOD AND AJR TURNED INTO FLESH AND ENERGY. 95 



distribute it to the capillaries of tlie general body-tissues. 

 From these it is gathered by the veins and carried back 

 to the auricle to begin again. In the course of circu- 

 lation the blood reaches every part of the body, picking 

 up certain substances here, leaving others there, thus 

 accomplishing the results already pointed out as the 

 objects of the circulation. 



In the circulation of the higher vertebrates the most 

 striking difference from that of the fish is in the structure 



Fig. 59. — Diagram of circulatory system of a fish; v, ventricle; a, auricle. 

 (After Parker and Haswell.) 



of the heart, which adapts the circulation to lungs instead 

 of gills, and in the more perfect control and regulation 

 of the action of heart and blood-vessels by the nervous 

 system. 



The circulation of a gilled frog tadpole is on the same 

 plan as that of a fish. In the adult frog, however, there 

 is no longer a circulation through gills but one through 

 the lungs. Moreover, the adult has two auricles instead 

 of one (fig. 60). Of these the right receives the blood 

 from veins draining the tissues, the other blood from the 

 lungs. All this blood, however, is thrown together into 

 the one ventricle, from which, mixed as it is, it is sent out 

 both to the lungs and to the tissues through arteries. 



In reptiles there are two auricles, as in the frog, and a 

 partition partially separates the ventricle into halves (fig. 

 61), so that the blood coming from the tissues is kept 



