THE HEART 



I6 5 



a 



ventricle and out into an artery, while preventing any flow 

 in the opposite direction. 



As blood enters the heart it passes through the auricles 

 into the ventricles. Just before the ventricles are full the 

 auricles suddenly contract and drive the blood into the 

 ventricles, which are 

 thus filled full and im- 

 mediately begin to con- 

 tract, while the auricles 

 relax. The pressure 

 closes the mitral and 

 tricuspid valves and 

 opens the semilunar 

 valves. The blood is 

 thus prevented from 

 flowing back to the auri- 

 cles, but flows through 

 the open entrance to the 

 arteries. During the 

 contraction of the ven- 



Diastole of the heart. 



a blood entering auricle. 



tncles, the auricles re- b auricle. 



open valve to permit blood to flow into the 

 ventricle. 



the blood returning to d ventricle. 



e artery. 



/ closed semilunar valve. 



main relaxed and receive 



the heart. 



When all the blood is 

 expelled from the ventricle it relaxes, and the blood falls 

 back upon the semilunar valves, closing them so that none 

 returns. At the same time the blood in the auricles presses 

 open the mitral and tricuspid valves, and again fills the 

 ventricles. 



287. Rate and time of the heart's action. The contrac- 

 tion of the heart is called its systole, and its relaxation its 

 diastole. At each systole from two to four ounces of 



