92 



HUMAN PHYSIOLOGY 



Summary. A skeletal muscle consists of a vast 

 number of working units, muscle fibers, which are 

 modified cells. The activity of 

 these fibers is not spontaneous or 

 automatic but dictated through the 

 nervous system. Each perfect fiber 

 in the muscle has a motor end-plate 

 which is the place where the stimu- 

 lus is applied. Each end-plate is 

 the terminus of a branch of a motor 

 nerve fiber. A motor nerve fiber, 

 or more precisely its axon, is an 

 outgrowth from a cell in the gray 

 matter of the nervous system. Such 

 a nerve cell, upon receiving stimu- 

 lation by way of its dendrites, sends 

 out over its axon the rapidly tra- 

 velling form of energy which we 

 call the nerve-impulse. 



A system composed of one motor 

 nerve cell, its far-reaching fiber 

 which branches within the muscle, 

 and the cluster of dependent mus- 

 cle fibers with their end-plates may 

 be termed a neuromuscular unit. 

 When this little system is at work, 

 the chief expenditure of fuel sub- 

 stance is in the muscle. It follows 

 that muscular fatigue, in the strict 

 sense, is a possibility. But ac- 

 tivity is accompanied by some de- 

 structive processes at the end-plates 

 and in the nerve cells. Hence, fa- 

 tigue at these points has also to be 

 considered. It is probable that fa- 

 tigue becomes effective in the following order: first, at 

 the end-plates, second, in the muscle substance itself, 

 third, in the gray matter. There is no doubt that the 



FIG. 23. The princi- 

 ple of reciprocal innerva- 

 tion. The right eye is 

 shown; (ri) is the nose. 

 (C.C.) is the symbol of a 

 coordinating center 

 which presides over two 

 subordinate centers. 

 When the eye is to be 

 turned out one of these 

 (C.e.) which commands 

 the external rectus mus- 

 cle is excited ( +) ; the 

 center for the opponent 

 (C.i.) is inhibited ( -) 

 and the tone of the inter- 

 nal rectus is abated. 

 (Page 93) 



