230 IRRITABILITY 



tigations showed that in a normal nerve in air the first typical 

 beginning of fatigue resulting from faradic stimulation can be 

 demonstrated in the characteristic summation of excitations. 

 This is shown by the nerve after fifteen minutes of stimulation 

 with faradic shocks applied for short intervals. The irritability, 

 when tested with single induction shocks, is at the same time 

 reduced. Thereby the amount of fatigue of the nerve, that is, 

 the amount of the reduction of irritability, is dependent upon the 

 strength and frequency of stimulation producing fatigue. When 

 the nerve is stimulated with weak faradic shocks of a slow rate 

 of frequency, there is a slight or a complete absence of the reduc- 

 tion of irritability. On the other hand, if the nerve is fatigued 

 with strong faradic shocks of great frequency, the irritability 

 falls very considerably. This shows that when the nerve is stimu- 

 lated for a longer tim.e, even under conditions favorable to the 

 supply of oxygen, a diminution of irritability occurs and with it 

 naturally an actual diminution of the wave of excitation, a diminu- 

 tion the intensity of which becomes greater as the strength of 

 the stimulus increases. In other words, long-continued faradic 

 stimulation converts the nerve from a system isobolic in character 

 to that which is heterobolic in that the intensity of the excitation 

 which is conducted differs depending upon the intensity of the 

 stimulus. We have found other cases in the investigation of the 

 nervous system in which, as in fatigue, an isobolic is converted 

 into a heterobolic system. Veszi^ has shown that the centers of 

 the strychninized frog, which are isobolic in character, when 

 fatigued by weak faradic stimuli can be brought to react again 

 when the faradic stimulation is increased. According to this and 

 other experiments of a like nature, it is beyond doubt that an iso- 

 bolic system during the refractory period may assume a hetero- 

 bolic character, and only after completion of the refractory period 

 and entire recovery of the equilibrium of metabolism does the iso- 

 bolic character return. This permits us to understand the charac- 

 teristic properties of an isobolic system more accurately and pre- 

 cisely than has thus far been possible. The "all or none law" 



1 Veszi: "Zur Frage des Alles oder Nichtsgetzes beim Strychninfrosche." Zeitschr. 

 f. allgem. Physiologic Bd. XII, 1911. 



