314 



COMPARATIVE ZOOLOGY 



purified blood from the lungs into the left. Both throw 

 their contents into the ventricle, which pumps the mixed 

 blood in two directions partly to the lungs, and partly 

 around the system (Fig. 273). Circulation is, therefore, 

 incomplete, since the whole current does not pass 

 through the lungs, and three kinds of blood are found 

 in the body arterial, venous, and mixed. In many 

 animals, however, arrangements exist which nearly 

 separate the venous from the arterial blood. 



The ventricle of reptiles is partially divided by a par- 

 tition. In the crocodile, the division is complete, so that 

 there are really four cavities two auricles, and two 

 ventricles. But both ventricles send off aortas which 

 cross one another, and at that point a small aperture 

 brings the two into communication. The venous and 

 arterial currents are, therefore, mixed, but not within 

 the heart, as in other reptiles, nor so extensively. In 



the structure of the heart, 

 as well as in that of the 

 gizzard, crocodiles approach 

 the birds. 



The Highest Form of the 

 Circulating System is pos- 

 sessed by the warm-blooded 

 vertebrates, birds and 

 mammals. Not a drop of 

 blood can make the circuit 

 of the body without passing 



throu s h the lun s s ' the dr - 



culation tO and from thoSC 



Organs being aS perfect aS 



, ,. ., f . , 



the distribution of arterial 

 blood. The heart consists of four cavities a right 

 auricle and ventricle, and a left auricle and ventricle. 

 In other words, it is a hollow muscle divided internally 



separated than in higher animals: E, 

 right ventricle; L, left ventricle; D, 

 right auricle; F, pulmonary artery; 

 K, left auricle; A, aorta. 



