504 MANUAL OF PHYSIOLOGY. 



THE ELECTRIC CHANGE IN NERVE. 



Negative Variation. The natural current of a nerve, like that 

 of muscle, undergoes a diminution at the moment the nerve is 

 stimulated ; this is termed the negative variation. It occurs with 

 any other form of stimulation as well as when an electric shock 

 is used, so it is not dependent on an escape of the stimulating 

 current. In the case of a single stimulation, the negative varia- 

 tion is so rapidly over lasting only .0005 sec., that the inertia 

 of the needle of the galvanometer prevents the change in the cur- 

 rent being indicated. In tetanus, however, it makes a decided 

 impression on the galvanometric needle. The strength of the 

 negative variation depends on the condition of the nerve and the 

 strength of the stimulus ; being stronger when the nerve is fresh 

 and irritable and has a good natural current, and when a strong 

 stimulus is applied. 



The negative variation of the natural currents passes along the 

 nerve from the point of stimulation in both directions, just as does 

 the nerve impulse ; and with a galvanometer the electric change 

 may be traced from the nerve to the muscle. Moreover, it has 

 been made out that the negative variation travels along the nerve 

 at just the same velocity as the impulse does from the point of 

 stimulation ; namely, about 27 metres per second; and further, 

 this rate is said to be influenced in the same way by the passage 

 of a constant current through the nerve (to be presently described) 

 as is the impulse derived from stimulus. These points seem to 

 lead to the belief that the nerve impulse and the negative varia- 

 tion are identical. This peculiar electric change and its accom- 

 panying impulse pass then along the nerves as a kind of wave of 

 activity, the speed and the duration of which we know to be 27 

 metres per sec., and .0005 of a sec. respectively; the length of the 

 wave we therefore calculate to be'about 18 millimetres. 



ELECTROTONUS. 



If the two poles leading to a galvanometer be applied to the 

 centre and to the end of a nerve respectively, so as to indicate 

 the natural current, and at the same time another part of the 



