508 COMPARATIVE ELECTRO-PHYSIOLOGY 



idea ever gained currency, unless, indeed, it was due to the 

 tyranny imposed on our thought by these arbitrary classifi- 

 cations themselves. 



Before entering, however, on the question whether the 

 excitatory reaction in nerve finds motile expression or not, 

 we shall first examine the only method at present available 

 for the detection of the condition of excitation. Since ex- 

 cited nerve has hitherto been supposed to exhibit no visible 

 change, it followed that the only method possible for the 

 detection of the excitatory change was the electrical. In- 

 vestigations on nerve, therefore, had perforce to be carried 

 out by this means, through the medium either of the capillary 

 electrometer or of the sensitive galvanometer. But the elec- 

 trical method labours under certain inherent disadvantages, 

 and first of these is the objection which it raises to the free 

 employment of the most convenient form of stimulus, that, 

 namely, by induction shocks. For we have seen that unless 

 extraordinary precautions are taken, we have here, owing to 

 the possible escape of current, an element of error and un- 

 certainty in the results. If, on the other hand, it should 

 become possible to obtain mechanical response from the 

 nerve, this particular form of stimulation might be employed 

 without misgiving. 



The second limitation which the electrical mode of 

 detection imposes upon us is that arising from the differ- 

 ential character of the response which it indicates. For 

 stimulus induces electrical changes at both the contacts- 

 proximal and distal the record made being finally due to 

 the algebraical summation of the two. It is true that the 

 excitability of one contact is artificially depressed by injury. 

 But it is often difficult to say how far this injury has been 

 effective in completely abolishing the excitability of this 

 point. The depression of excitability, due to partial injury, 

 will sometimes disappear to a certain extent, with lapse of 

 time, and much uncertainty sometimes occurs as to whether 

 a certain curious variation in the response of the nerve- 

 negative followed by positive is due to this or some other 



