SKELETAL MUSCLE AND THE NERVOUS SYSTEM 91 



to believe that the end-plates are vulnerable parts of 

 the association and may set a limit to our voluntary 

 performances. A common experiment makes this ap- 

 pear probable. Suppose that a frog's muscle is pre- 

 pared with its nerve. We can then choose whether we 

 will apply our electric shocks directly to the muscle 

 substance or to the nerve which will carry to the muscle 

 the effects of our stimulation. 



If we stimulate the nerve a great many times we shall 

 witness at length a total failure of response. If we then 

 shift our application to the muscle itself we may find 

 it capable of still further work. Something has evidently 



FIG. 22. A " neuromuscular unit" as defined in the text. The 

 motor nerve cell is united through its branched fiber and end-plates 

 with five muscle fibers. The typic number would be much larger. 



fatigued more rapidly than the muscle proper. Other 

 experiments, which we cannot outline here, forbid us 

 to think that the nerve fibers can have been injured 

 and only one conclusion remains possible: that the 

 trouble is with that which is neither muscle or nerve, 

 the end-plate that intervenes between the two. It is 

 quite likely that the comparatively early failure of the 

 end-plates protects the muscles against excessive strain 

 and damage. A muscle which one regards as tired is 

 probably a muscle containing end-plates that are more 

 or less fatigued. There are still other elements in the 

 fatigue of daily life which we shall have to indicate at 

 another time. 



