THE NEUROMUSCULAR SYSTEM: FATIGUE 103 



commented upon (see page 39). But fatigue of the 

 motor perikaryon is to be reckoned with, and so is fatigue 

 at the synapses through which its discharges are deter- 

 mined. It is not wise to make dogmatic assertions as to 

 the precise order in which failure of these unlike struc- 

 tures is to be anticipated when they are kept steadily at 

 work. 



If a proper balance between destructive and recupera- 

 tive processes can be struck, a neuromuscular mechanism 

 need not exhibit fatigue in spite of what seems like con- 

 tinuous use. This is the case with the diaphragm, which 

 bears its part in the taking of every breath, but seems not 

 to show any weakening of response or loss of irritability 

 until it shares the general decline of old age. As the 

 diaphragm is not automatic, but contracts only in response 

 to stimulation traceable to the brain, we must suppose 

 that in the nervous as well as in the muscular portions of 

 this combination restoration normally keeps pace with 

 decomposition, and that the waste-products are not suf- 

 fered to gather at the seat of action. (The diaphragm 

 is chosen rather than the heart to exemplify this balance 

 because the beating of the heart is not dictated from 

 without.) 



If we leave out of further account the muscles used in 

 breathing and their presiding neurons, we shall be ready 

 to agree that in general our neuromuscular equipment is 

 subject to daily if not more frequent fatigue, and calls for 

 rest that it may be renewed. The renewal must include 

 the washing away or perfect neutralization of the fatigue 

 substances and the introduction of fresh fuel supplies. 

 To make our discussion concrete, let us fix our attention 

 upon an imaginary pedestrian who is found toward the end 

 of the day covering the last mile of a long tramp. In what 

 respects is his system altered from its state at setting out 

 in the morning? 



His leg muscles have done the heaviest work. They have 

 produced in the course of the walk a great quantity of 

 carbon dioxid. This has, presumably, been removed quite 



