THE CIRCULATION 89 



the pressure from the ventricle soon sends it through the 

 swollen artery into the lungs (Plates IV and VIII). 



146. When the ventricle 

 has emptied itself, it re- 

 laxes. The semilunar valve 

 is composed of three pock- 

 ets, which the swollen pul- 

 monary artery fills with 

 blood as soon as the ventri- 

 cle begins to relax. The 

 pockets of the valve are 

 thus pressed together, and FIG. 87. The Blood 



FIG. 86. The Right -, , i n i i going through the 



Side of the Heart. HO blood flows back into Pulmonary Artery 



The blood flowing the ventricle. The auricle to the Lungs. 



through the tricus- .. . . .. . Trictisnid valve closed 



pid valve into the was relaxing while the ven- Tn s c e ^fj^ e ^s 



tricle was contracting, and P en - 



it is already filled with blood that has flowed in from the 

 veins. After a short pause, it again contracts ; and the 

 same action is repeated. More and more blood is thus 

 driven by the right ventricle through the semilunar valve 

 into the pulmonary artery, so that the blood which is 

 already in the artery is sent on through the numerous 

 small branches and through the multitude of fine tubes 

 called capillaries ', which go through every part of the lungs. 



147. In the lungs the carbon dioxid passes into the air 

 passages, and the oxygen brought bythe breath goes into 

 the blood of the capillaries, which changes in color to 

 bright red. The capillaries unite again to form the pul- 

 monary veins, which lead back to the heart. We thus 

 see how the blood is sent from the heart through the 

 lungs and back to the heart. How is the blood sent through 

 the body and back to the heart ? We shall find that this 

 is done by the left side of the heart; that the two pumps 

 work in unison ; that, in fact, a wave of muscular contrac- 

 tion starting at the top of the heart passes downward over 



