MUSCULAR MOVEMENT AND EXERCISE 373 



age about seventy times every minute during our whole 

 lifetime, and so it does far more work than all the other 

 muscles together. If it stops working, death occurs 

 almost at once. The heart is about as large as a closed 

 fist, lying in the middle of the chest (not on the left 

 side, as often stated), with the lungs on each side of it. 



There are really two hearts, a right and a left, side 

 by side, looking like one organ. Each of these acts like 

 a syringe : the blood enters from above and is squeezed 

 out below with considerable force into the arteries. 



When a small artery or vein is cut, a blood clot (page 

 364) soon stops the bleeding. When a larger one is cut, 

 we must stop it by tying a cord or handkerchief around 

 the arm or limb and twisting it tightly with a pencil or 

 piece of wood. If the blood comes in spurts (artery), 

 we place the cord on the side nearest the heart; if it 

 comes evenly (vein), on the side away from the heart. 

 The wound should be disinfected (see page 254). There 

 is much more danger of bleeding to death when an 

 artery is cut; arteries are protected by being deep in 

 the body, while veins are near the surface. 



Effect of exercise. When a muscle begins to work 

 (contract), it burns more food, and the temperature 

 goes up; we soon become warm from exercise. At 

 the same time waste substances are formed by the burn- 

 ing (carbon dioxide and poisonous substances), and if 

 they cannot be taken away fast enough, the muscles 

 get stiff and sore. They are in fact poisoned. 



