SEC. 3. THE NATURE OF THE CHANGES THROUGH 

 WHICH AN ELECTRIC CURRENT IS ABLE TO GENE- 

 RATE A NERVOUS IMPULSE. 



Action of the Constant Current. 



In the preceding account, the stimulus applied in order to give 

 rise to a nervous impulse has always been supposed to be an 

 induction shock, single or repeated. This choice of stimulus has 

 been made on account of the almost momentary duration of the 

 induced current. Had we used a current lasting for some consider- 

 able time, the problems before us would have become more com- 

 plex in consequence of our having to distinguish between the 

 events taking place while the current was passing through the 

 nerve from those which occurred at the moment when the current 

 was thrown into the nerve or at the moment when it was shut 

 off from the nerve. These complications do arise when instead of 

 employing the induced current as a stimulus, we use a constant 

 current, i.e. when we pass through the nerve (or muscle) a current 

 direct from the battery without the intervention of any induction- 

 coil. 



Before making the actual experiment, we might perhaps 

 naturally suppose that the constant current would act as a 

 stimulus throughout the whole time during which it was applied, 

 that, so long as the current passed along the nerve, nervous 

 impulses would be generated and thus the muscle thrown into 

 something at all events like tetanus. And under certain conditions 

 this does take place ; occasionally it happens that at the moment 

 the current is thrown into the nerve, the muscle of the muscle- 

 nerve preparation falls into a tetanus which is continued until the 

 current is shut off. But such a result is exceptional. In the vast 



