THE CIRCULATW.X 



53 



from the air, and there must be another pump to send the 

 pure blood front the limgs back to the body. Hence there 

 are two pumps bound together into one heart, beating at 

 the same time like two men keeping step, or like two car- 

 penters keeping time with their hammers. There are 

 valves in the heart, as in other pumps. These valves are 

 so arranged that when any part of the heart contracts and 

 forces the blood onward, the blood cannot return after that 

 part of the heart relaxes. Are the pumps placed one 

 behind the other? Or is one above the other? Neither; 

 they are side by side, with a 

 fleshy partition between them 

 (Fig. 54). The pump on the £ 

 right moves the impure blood $ 

 from the body to the 



. , . , purmonary 



lungs, and the one on the veins 

 left moves the pure blood 

 from the lungs to the . body. 

 There is no direct connection 

 between the right and left sides 

 of the heart. 



To trace one complete circuit 

 of the blood (Fig. 54), let us 

 begin with the blood in the 

 capillaries of the outer tissues, 

 such as the skin or muscles. 

 The blood goes through small 

 veins which unite into tzvo 

 large veins, through which it 

 enters the receiving chamber, or right auricle, goes through 

 the tricuspid valve into the expelling chamber, or right 

 ventricle, then through a semilunar valve into the pulmo- 

 nary artery leading to the lungs. Becoming purified while 



Fig. 54. — Diagram of Heart. 



Notice the two dark spots in the right 

 auricle, and four dark spots in left 

 auricle, where the veins enter. Does 

 the aorta pass in front of, or behind, 

 the pulmonary artery? 



