SKELETAL MUSCLE AND THE NERVOUS SYSTEM 



93 



nerve fibers, as found in the white matter of the cen- 

 tral axis and in the nerves, have the greatest resistance 

 to injury by use. 



Reciprocal Innervation. It will be recalled that the 

 skeletal muscles are generally organized into opposing 

 groups. It has also been said that these muscles are 

 rarely so completely relaxed as they can be. In other 

 words, they have tone. Now it is an interesting fact 

 that when a movement is made perhaps the bending 

 of the elbow just as the muscles directly responsible 

 for the movement go into contraction there is an aboli- 

 tion of tone in the antagonistic muscles. Thus they are 

 kept from hindering the act. This is an instance of 

 inhibition; it is supposed that the nerve cells presiding 

 over the muscles which show the extra relaxation are 

 restrained at this moment from a habitual, mild ac- 

 tivity which expresses itself in the ordinary muscular 

 tone. "The Law of Reciprocal Innervation" is to the 

 effect that when any muscles are thrown into contrac- 

 tion through the agency of the central nervous system 

 their opponents are inhibited. 



This law has been verified for the slender muscles 

 which rotate the eyeball. They are six in number. 

 One is so placed as to turn the eye toward the temple 

 while its antagonist can turn it toward the nose. The 

 contraction of either one of these muscles can be shown 

 to be simultaneous with a slackening of its mate. Of 

 course this is true only when the action is controlled 

 through the gray matter; there would be no trace of it 

 if local electric stimulation were employed. 



