EVENTS ACCOMPANYING A NERVOUS IMPULSE 257 



up, and the demarcation current is restored to its original strength. If, 

 while the demarcation current is at its height, we stimulate the other end of 

 the nerve with an interrupted current, the needle of the galvanometer swings 

 back towards zero, i.e. there is a negative variation of the resting current. 

 In order to demonstrate the wave-like progression of the electrical change 

 from the excited spot along the nerve, it is necessary, as in the case of muscle, 

 to make use of a very sensitive capillary electrometer or a string galvano- 

 meter. It is then found that the change progresses along the nerve at the 

 same rate as the nervous impulse, i.e. 28 to 33 metres per second in the frog. 

 But it lasts only an extremely short interval of time at each spot, viz. six to 

 eight ten- thousandths of a second. Thus the length of the excitatory wave 

 in nerve is about 18 mm. 



17 



