MUSCLES 



27? 



the shortening of the muscle will lift the arm. After the arm 

 I has been thus lifted, the muscle may remain contracted for 



a time and the arm held up, but it requires a constant effort 



to keep the muscle contracted, and just as soon as the effort 



ceases, the arm falls of its own 



weight. The muscle has no 



power of forcibly lengthening 



and pushing the arm down, but 



as the arm falls, it pulls out the 



muscle to its elongated form 



again. On the back of the arm 



is another muscle which acts in 



opposition to the biceps, these 



two muscles thus forming a pair, 



each of which produces an 

 action opposed to that of the 

 other. They cannot both act 

 at once. 



Nerve Control of Voluntary 

 Muscle Action. While there are 

 some muscles in the body (the 

 heart, for example) which perform 

 very regular, apparently spon- 

 taneous contractions, all the body muscles are more or less 

 under the influence of nerves, and the striped muscles act 

 only when stimulated by the brain or spinal cord. It is 

 because they are under the control of the will that they are 

 called voluntary muscles. If a single muscle contracts, it 

 produces motion of a single bone in a single direction. The 

 motions of the body are, however, rarely simple, but 

 generally very complicated. In the process of walking 

 nearly a hundred muscles are first contracted and then 

 relaxed in regular order. In throwing a baseball, nearly 

 all of the three hundred muscles of the body are brought 

 into use to some extent, and the remarkable thing is 



FIG. 140. DIAGRAM 



Showing that the muscle shortens 

 though it does not change its bulk 

 when it contracts. 



