CIRCULATION IN WARM-BLOODED ANIMALS. 



133 



mg the several parts as they actually exist. The upper 

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



Fis. S7. 



6, 5, the right and left auricles ; c, c, the right ana left 

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

 nary artery ; d, d, the aorta. 



396. 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. 



397. 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 de- 

 scribed, the two systems of circulations are so separated as 

 to entirely prevent the two kinds of blood from mixing 

 with each other. 



398. The color of the arterial blood is light red, while 

 that of the veins is dark purple. This change is produced 

 1 y the exposure of the venous blood to the air in its pas- 

 sage through the lungs, by which it either loses a portion 



Are the pulmonic and systematic circulation perfectly distinct in the 

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

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



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