THE EXERCISE WE NEED 



I6 7 



In every case the heart beats because the muscles 

 which are working must have more blood ; and the heart 

 meets the demand by pumping the blood into them as 

 promptly as possible. 



Moreover, with circulation as with respiration, it isn't 

 so much the use of the small muscles as the use of the 



large muscles that makes 

 the heart work. You can 

 increase the heart rate a 

 trifle by using the hand 

 vigorously, but move- 

 ments of the body and 

 movements of the legs, as 

 in running, increase the 

 circulation very much more 

 than exercise of the small 

 muscles. 



Exercise also tends to 

 make the muscles harder. 

 Take hold of your right 

 upper arm with your left 



hand; clinch the fist of your right hand as hard as possible; 

 slowly bend the right arm, and you will feel the muscle 

 of that arm increasing in size. This is because it is get- 

 ting shorter. If you do this a good many times each 

 day, the muscle itself becomes harder ; that is, it has less 

 fat in it and is better adapted for doing hard work. 



HARDENING A MUSCLE 



