194 HUMAN PHYSIOLOGY CHAP. 



it. Over this nerve a message may be sent from the 

 brain, causing the muscle to contract. We can make a 

 muscle do just what we please if we have learned how 

 to stimulate it. 



When a muscle contracts it pulls its ends nearer 

 together ; it gets shorter and causes motion in the part 

 to which it is fastened. We say the muscle does work. 

 In some way the heat obtained from the oxidation of 

 foods in the muscle cells is transformed into mechanical 

 energy and used in doing work. The thicker a muscle 

 the greater the load it can move. 



In every contraction a muscle consumes a certain 

 amount of food material, causes the cells to break down 

 somewhat, and produces waste products. The muscle 

 cells must be repaired, and new energy must be stored 

 up for future use. To accomplish this more food must 

 get into the blood and be carried to the cells. Working 

 gives us an appetite, and we now see the reason for it. 



A muscle cannot remain strongly contracted for a 

 long time without becoming tired. It is difficult to 

 stand perfectly still in one position, to hold the arm 

 straight out, or to look steadily at a quiet object. A 

 contraction must be followed by a relaxation. The 

 muscles are generally arranged in pairs so that when 

 one contracts the other relaxes ; when one works the 

 other rests. The muscle used in bending the arm can- 

 not straighten it, but another must be called into action. 



1. By means of a tapeline, measure the muscles of the right arm 

 (i) when extended, and (2) when flexed. 



2. Measure the left biceps in the same way. 



3. Measure other large muscles of the body. 



