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 



74. 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. Thisj^hoice 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 _jiis_tinguish 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 stimu- 

 lus 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 that these would throw the muscle into some- 

 thing at all events like tetanus. And under certain conditions this 

 does take place ; occasionally it does happen 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 

 majority of cases what happens is as follows. At the moment that 

 the circuit is made, the moment that the current is thrown 

 into the nerve, a single twitch, a simple contraction, the so-called 

 making contraction, is witnessed ; but after this has passed away 



