THE NEUROMUSCULAR SYSTP:M : FATIGUE 107 



another afferent nerve. Such a shift is followed by 

 renewed activity on the part of the muscle to which 

 attention is directed (Fig. 19). 



In this case the motor elements used in the earlier and 

 in the later part of the trial are presumably the same. 

 It is the afferent neurons which are different. Fatigue of 

 nerve-fibers is negligible, and the conclusion must be that 

 failure to conduct has occurred at the place where the 

 impulses of peripheral origin must exert their effect upon 

 the motor neurons. This is the same as saying that we 

 have here an instance of synaptic fatigue. Much poten- 

 tial capacity for action may still reside in the efferent 

 equipment, but a new line of approach must be adopted 

 to demonstrate it. Forbes has used animals in which only 

 the cord and not the brain has been intact, thus simplify- 

 ing his conditions as far as he could. It seems legitimate 

 to extend the same principle to muscular performances in 

 which the brain is concerned. This we shall now under- 

 take to do. 



The neurons, which have their perikarya in the cortex 

 of the cerebrum and send their dependent projection 

 fibers down the spinal axis, are not self-stimulated. They 

 are thrown into action, like the neurons of lower rank, 

 by impulses affecting them through synapses. Now we 

 may fairly suppose that the efficiency of these cerebral 

 motor neurons is greatest when they are stimulated from 

 all sides, so to speak, rather than in a one-sided fashion. 

 Here we have a key to the invigorating influence of 

 rhythmic music upon the walker and perhaps to many 

 other phenomena of the same class. 



The tired man to whom we have so often referred has 

 been kept in motion through the day by a variety of 

 external stimuli. Some of these have had a very direct 

 effect upon the lower centers, as in the case of the im- 

 pulses returning from the soles of the feet at every con- 

 tact with the ground. These, together with others from 

 the joints, the tightening tendons, and probably from 

 the muscles themselves, have been ever prompting the 



