80 ELECT ROTONIG CURRENTS. [BOOK i. 



go far to shew that a nervous impulse is generated only when 

 a nerve passes suddenly from a normal condition into the phase of 

 katelectrotonus (making contraction) or returns from the phase of 

 anelectrotonus to a normal condition (breaking contraction), in 

 other words, when it passes suddenly from a phase of lower to 

 a phase of higher irritability. 



The phenomena of electrotonus are also interesting practically 

 in as much as they shew that in the constant current appropriately 

 applied we have the means of changing at will the irritability of 

 this or that nerve, decreasing it when we wish to lessen pain or 

 spasm, increasing it when we wish to heighten sensibility or 

 muscular action. For the increase or decrease is observed in the 

 case of nervous impulses passing towards the central nervous system 

 as well as in those passing to muscles. 



Electrotonic Currents. During the passage of a constant current 

 through a nerve, variations in the electric currents of the nerve analogous 

 in some respects to the variations of the irritability of the nerve may be 

 witnessed. Thus if a constant current supplied by the battery P 

 (Fig. 15) be applied to a piece of nerve by means of two non-polarizable 

 electrodes p, p, the " currents of rest " obtainable from various points of 

 the nerve will be different during the passage of the polarizing current 

 from those which were manifest before or after the current was applied; 

 and, moreover, the changes in the nerve-currents produced by the polariz- 

 ing current will not be the same in the neighbourhood of the anode (p) 

 as those in the neighbourhood of the kathode ( p'). Thus let G and H be 

 two galvanometers so connected with the two ends of the nerve as to 

 obtain good and clear evidence of the "currents of rest." Before 

 the polarizing current is thrown into the nerve, the needle of H will 

 occupy a position indicating the passage of a current of a certain 

 intensity from h to h' through the galvanometer (from the positive 

 longitudinal surface to the negative cut end of the nerve), the circuit 

 being completed by a current in the nerve from h' to h, i.e. the current 

 will flow in the direction of the arrow. Similarly the needle of G will 

 by its deflection indicate the existence of a current flowing from g to g' 

 through the galvanometer, and from g to g through the nerve, in the 

 direction of the arrow. 



At the instant that the polarizing current is thrown into the nerve 

 at pp'j the currents at gg', hh f will suffer a "current of action" correspond- 

 ing to the nervous impulse, which, at the making of the polarizing 

 current, passes in both directions along the nerve, and may cause a 

 contraction in the attached muscle. The current of action is, as we 

 have seen, of extremely short duration, it is over and gone in a small 

 fraction of a second. It therefore must not be confounded with a 

 permanent effect which, in the case we are dealing with, is observed in 

 both galvanometers. This effect, which is dependent on the direction 

 of the polarizing current, is as follows: Supposing that the polarizing 

 current is flowing in the direction of the arrow in the figure, that is, 

 passes in the nerve from the positive electrode or anode p to the negative 

 electrode or kathode p f t it is found that the current through the 

 galvanometer G is increased, while that through H is diminished. We 



