CIRCULATION IN WARM BLOODED ANIMALS. 137 



ing the several parts as they actually exist. The upper 

 a, shows the pulmonary vein, the lower , the vena cava ; 



Fig. 87. 



b, b, the right and left auricles ; c, c, the right and left 

 ventricles ; e, e, the right and left branches of the pul- 

 monary artery ; d, d, the aorta. 



But notwithstanding the two hearts are thus united 

 within a single envelope, the right and left cavities are 

 perfectly distinct from each other as represented by Fig. 

 86 ; the two ventricles having between them a strong 

 muscular partition, which allows of no communication 

 from one side to the other. 



In the lower orders of animals, as already shown, the 

 circulating fluid is composed of one half venous blood, 

 or blood which has not passed through the purifying 

 and renovating influence of the lungs. In the heart just 

 described, the two systems of circulation are so separat- 

 ed as to entirely prevent the two kinds of blood from 

 mixing with each other. 



The color of the arterial blood is light red, while 

 that of the veins is dark purple. This change is pro- 

 duced by the exposure of the venous blood to the air in 

 its passage through the lungs, by which it either loses 



Are the pulmonic and systematic circulation perfectly distinct in the 

 warm blooded animals 1 What is the color of arterial blood ? What 

 is the color of venous blood? How is this change of color produced? 



