CIRCULATION OF THE NUTRIENTS 



111 



left ventricle with those of the right, one is struck with the 

 great thickness of the former. The left ventricle does much 

 more work than the right ; it forces blood to the top of the 

 head, to the tips of the fingers and toes, and to every other 

 organ of the body. The right ventricle, on the other hand, 

 pumps blood only to the lungs (Fig. 33). 



A = right heart. B = left heart. 



Fig. 33. — Cavities of heart. 



157. Action of the hoart. — • The blood flows into the right 

 and left auricles and thence into the corresponding ventricles. 

 When the ventricles are nearly full of blood, the two auricles 

 contract and force downward enough blood to fill the two 

 ventricles completely. These muscular chambers then con- 

 tract and force the blood out into the arteries that lead to 

 the lungs, or to other parts of the body. When the con- 

 traction of the ventricles takes place, it is evident that blood 

 would be driven back into the auricles were there not some 

 means of preventing this back flow. Hence, between each 



