THE REFRACTORY PERIOD AND FATIGUE 187 



As a result of all these investigations, linked together in a sys- 

 tematic series, the proof has now been obtained that the nerve 

 like all other living substances is fatigable. Its fatigue is solely 

 the manifestation of a prolonged refractory period and the exten- 

 sion of the latter by continuous stimulation is, as in all aerobic 

 substances, a result of relative deficiency of oxygen. 



WO 



CL 



200 



300 



If 20 4 3 



Fig. 38. 



5 5 



10 



Curve of irritability as demonstrated by action current of two nerves 

 in nitrogen, which are alternatively stimulated (plain line) and at 

 rest (dotted line). Recovery in nitrogen is always merely partial 

 and relative. It only increases on introduction of oxygen. (After 

 Thorner.) 



To briefly summarize in conclusion, I will repeat that just as 

 all living systems show a refractory period after an excitation, in 

 which irritability is reduced, all living systems are likewise capable 

 of fatigue. Both are most intimately connected and are based 

 fundamentally on the facts of metabolism. 



An excitating stimulus disturbs the metabolic equilibrium of 

 rest by suddenly bringing about increased decomposition of cer- 

 tain substances. During and directly after the breaking down, 

 irritability is reduced in the same degree as the amount of sub- 

 stances required for disintegration in response to a succeeding 

 stimulus is decreased and the quantity of the decomposition 



