x CIRCULATION OF THE BLOOD 135 



amount of blood that is to go through the heart all the 

 time. 



101. The Heart Beat. The heart beats on an average 

 about seventy-two times per minute. The rate, how- 

 ever, varies widely with rest and exercise, health and 

 disease. Besides, some persons have naturally a 

 higher average and others a lower. 



TABLE SHOWING VARIATIONS WITH AGE 



At birth, from 140 to 130 



First year, from 130 to 115 



Second year, from 115 to 100 



Third year, from . . \ . . . . . 100 to 90 



Seventh year, from ........ 90 to 85 



Fourteenth year, from . . . . . . . 85 to 80 



Adult, from 80 to 70 



Old age, from 70 to 60 



Decrepit, from 75 to 65 



Count your own heart beat per minute at different times of the 

 day, for different positions of the body, and under different condi- 

 tions of exercise and rest. Each person should know his own pulse. 



When does the heart rest ? The two auricles con- 

 tract at the same time and force the blood into the 

 ventricles. Then they relax and rest while the two 

 ventricles work together and force the blood into the 

 arteries. In this way the auricles and the ventricles 

 rest a little more than half the time. It is an interest- 

 ing fact that the muscles of the heart contract in about 

 the same time, whether it beats slow or fast. Now the 

 faster the beat, the shorter must be the period of rest 

 between the beats. 



