1 68 APPLIED PHYSIOLOGY 



they are large and hard, and he can run very fast and 

 long without getting tired. But using the muscles of the 

 leg does not make the arm stronger. We must use every 

 muscle if we would get strong all over. 



If we stop using our muscles, they soon become weak 

 again. If you break your leg and have to keep it still, it 

 will soon become small and weak. Then the other one will 

 grow larger, for it has to work harder to carry you around. 



If you use your muscles until you strain them, they will 

 grow weaker instead of stronger, for you will wear them 

 out faster than the blood can feed them. You ought not 

 to strain yourself trying to lift as much as a grown man 

 lifts, or to run as far as a large boy runs. 



335. Round shoulders. The muscles of our backs hold 

 us erect and keep our shoulders thrown back. A lazy boy 

 lets his shoulders fall, and supports himself by leaning 

 against the wall. Sometimes braces are used to keep the 

 shoulders back. This rests the muscles, and lets them 

 grow still weaker. The best way to make the body 

 straight is to make the muscles hold the shoulders back. 

 This is what soldiers do. Military drill tends to make 

 boys straight. Working women of Europe often carry 

 heavy burdens upon their heads. In order to do this, they 

 must walk erect and steady. These women are noted for 

 their straight backs and graceful walk. A drill in carrying 

 loads upon the head would be of great value in making 

 young girls walk gracefully. 



336. How exercise makes the body healthy. The mus- 

 cles are the engine of the body. The food is the fuel. 

 Oxygen burns the food and makes heat, which the muscles 

 turn to power. When a muscle acts, it needs a great deal 

 of heat. So it must have more food and air, which is 

 brought by the blood. In order to support it, the heart 



