232 IRRITABILITY 



mental evidence, therefore, must be brought forward to show 

 that faradic stimulation of short duration produces the above- 

 mentioned alteration in the character of the system. Thorner in 

 his experiments on the nerve stimulated it faradically at least four 

 minutes and always found after this that excitation was reduced. 

 After shorter intervals of stimulation Thorner made no test of 

 the state of excitation. It is, however, highly probable that a 

 reduction of excitation is much more quickly reached. Indeed, 

 we are unavoidably compelled to accept the assumption that even 

 after the first single stimulus of the faradic current, alterations 

 of a slight degree are present which, after repeated stimulation, 

 become constantly greater and give to the system a heterobolic 

 character. As a result of fatigue, as we have already seen, the 

 refractory period becomes more and more prolonged. As the 

 individual shocks in faradic stimulation follow each other at 

 regular intervals, a necessary consequence is that the shocks are 

 operative before the refractory period has completely disap- 

 peared, otherwise Thorner could not have obtained fatigue pro- 

 duced by continued stimulation. The intervals of the individual 

 shocks must be somewhat shorter than the duration of the refrac- 

 tory period, even in fatigue of a very slight degree. It is very 

 interesting in this connection that Thorner invariably obtained 

 positive evidences of fatigue by the application of stimuli at the 

 rate of 10-12 per second. When the number of stimuli per second 

 was less than this the above-mentioned result was not always 

 obtained. From this we can easily estimate the refractory period 

 of the nerve, which is present after reaching a state of equilib- 

 rium under certain conditions. If we assume ten stimuli per 

 second to be the number required to produce slight fatigue when 

 stimulation is prolonged, we can conclude that the refractory 

 period in this state is somewhat longer than one tenth of a second. 

 Even though Gotch in his investigations already cited placed the 

 refractory period of the normal nerve at about .005 second, this 

 statement is in no way contradictory to the figure which we have 

 just given. Gotch measured simply the duration of the absolute 

 refractory period of the normal nerve, in other words, the dura- 

 tion of the period in which no excitation at all could be brought 



