46 



MUSCLES AND TENDONS 



Clavicle . 

 Scapula 



Short head of biceps- 

 Long head. 



soon begins to waste away, or atrophy. In other- words, 

 the muscle is paralyzed and cannot act. 



Tonicity. In a healthy muscle with its nerve unin- 

 jured, we find a constant slight elastic resistance, or 

 tension, which persists even when the muscle is not con- 

 tracted. This tension is 

 called the tonicity of 

 the muscle, since it 

 gives it the elastic tone 

 characteristic of health. 

 It is due to a continu- 

 ous but very slight con- 

 traction of the muscle. 

 The contraction is in 

 turn due to a constant 

 slight stimulation of 

 the muscle by its nerve 

 and a consequent con- 

 tinuous setting free of 

 energy by combustion. 

 Each muscle, even when 

 idle, is thus given suf- 

 ficient exercise to keep 

 it in a healthy condition. 

 Moreover, by being kept 

 tense, it is instantly ready for action. The constant 

 combustion is further useful in helping to keep the 

 body warm during inactivity and sleep. 1 



Voluntary muscles. Most of our muscles are under 

 our control and can be made to do what we will. Be- 

 cause of this, they are called voluntary muscles. They 



1 When the body is chilled, the shivering which results consists 

 of the rapid contraction of many muscles. Its purpose is to develop 

 heat enough to warm the body. 



FIG. 38. Lengthening of biceps when arm 

 is extended. 



