4 2 HUMAN BIOLOGY 



the voluntary muscles are arranged in pairs and cause 

 motion in opposite directions ; they are said to be antago- 

 nists. The biceps (Fig. 43) bends the arm. Its antagonist 

 is the triceps on the back of the arm. By feeling them 

 swell and harden as they shorten, locate on your own 

 body the muscles mentioned in Fig. 44. 



How a Muscle grows Stronger ; its Blood Supply. 

 Nature has provided that any part of the body shall receive 

 more blood when it is working than when it is resting. 

 When it works the hardest, the blood tubes expand the most 

 and its blood supply is greatest. So whenever a muscle is 

 used a great deal, an unusual amount of material is carried 

 to it by the blood, the cells enlarge and multiply, and the 

 muscle grows. The walls of the capillaries are so thin that 

 the food which is in the blood readily passes from them to 

 the muscle. Because of the oxidation taking place, a work- 

 ing muscle is warmer than one at rest. By use a muscle 

 grows large, firm, and of a darker red ; by disuse, it be- 

 comes small, flabby, and pale. But if muscles are worked 

 too constantly, especially in youth, their cells do not have 

 time to assimilate food and oxygen, and their growth is 

 stunted. 



Unless the meal has been a very light one, vigorous 

 exercise should not be taken after eating, as the blood will 

 be drawn from the food tube to the muscles and the secre- 

 tion of the digestive fluids will be hindered. Persons 

 whose entire circulation is weak may find that light exercise 

 after a meal, such as walking slowly, may help circulation 

 and digestion. 



Why the Muscles work in Harmony. When a boy throws 

 a stone, almost every part of the body is concerned in the 

 action. His arms, his legs, his eyes, the breathing, the 

 beating of the heart, are all modified to assist in the effort. 



