Ill' T1IK NKKVors SYSTI.M \M) ITS O INSKUVATK >.\ 



securing responds because of conditions existing at the 

 junction of nerve and muscle may lie one of the charac- 

 teristics of the untrained, a-~ it is of the fatigued, state. 

 The increa>ed endurance demonstrated by those who have 

 put themselves upon sparing diets i Fletcher, Chittenden, 

 et al.) is most easily explained on the theory that the 

 end-plates were previously subject to a chronic impair- 

 ment owing to the presence of avoidable metabolic prod- 

 ucts. Such a situation would be equivalent to continuous 

 fatigue and would limit at all times the working capacity 

 of the muscles. Exercise may be assumed to bring about 

 a more favorable condition at this important locality in 

 the neuromuscular complex. 



Still another result of training may be more extensive 

 and effective innervation. We shall make much of this 

 if we accept the Lucas doctrine of the fractional response 

 of muscles. According to this view, the muscle which has 

 not been much used contains elements which are not 

 accessible 1 to stimulation under any ordinary state of 

 affairs. With regular use the number of these idle fiber- 

 may very likely be steadily reduced and a corresponding 

 increase of force and endurance realized. The defect 

 overcome may be at the end-plates or it may be in the 

 central nervous system. If it was originally at the end- 

 plates, we may picture the perikarya which preside over 

 the untrained muscle as being all active, but unable to 

 call portions of it into contraction. If it was central, we 

 are to conceive of idle perikarya among those which are 

 at work, each unemployed neuron having a cluster of 

 dependent muscle-fibers which remain at rot. 



So far we have spoken of peripheral improvements, 

 -ave that in the last instance we have introduced the idea 

 of a possible central factor. We have recogni/ed the ad- 

 vantages accruing (h from the actual growth of the 

 muscles, (12) from improved contractile substance, (3) 

 from more reliable end-plates, and (I) from better "team 

 work" among the neurons governing single muscles. 

 Now. by a simple extension of the last-named factor, we 



