ix ELECTRICAL EXCITATION OF NERVE 145 



and yhi. Here again the true form of the curve is uncertain, and 

 this merely represents the general relations. The curve abc 

 shows that nearly the whole of the intrapolar tract is thrown 

 with minimal currents into a state of augmented excitability 

 (katelectrotonus), while the indifferent point in this case is near the 

 anode. Excitability increases gradually from this point on the 

 one side, while on the other it is correspondingly diminished. 

 The alteration reaches its maximum in the immediate proximity 

 of the two electrodes, whence it declines again to 0. The curve 

 (clef} from currents of medium strength is essentially the same, 

 but is distinguished by the larger tract of nerve which it embraces, 

 and by higher ordinates, while the indifferent point lies about 

 midway in the intrapolar region. These differences correspond 



FIG. 185. 



with the fact that the electrotonic alterations of excitability 

 increase in intensity and extent of diffusion with the strength of 

 the polarising current. The same applies to the curve (glii} of 

 strong currents, which contrasts with abc, inasmuch as the 

 indifferent point lies near the kathode, so that almost the whole 

 intrapolar tract is in a state of anelectrotonus. It would also 

 be easy to record the after-effects of an- and katelectrotonus, since 

 the excitability of every point, at all events immediately after 

 breaking the current, is exactly opposite to the effect at closure. 



We have up to this point considered only the effect of 

 strength of polarising current upon magnitude and diffusion of 

 electrotonic alterations of excitability, yet, as Pniiger has shown, 

 the length of tract traversed, as well as the time-relations of the 

 passage of the current, are not unimportant factors. The first of 

 these two (besides the older experiments of Humboldt, Eitter, 

 and others) was investigated by du Bois-Eeyrnond. According 

 to Ohm's law, the intensity of an electrical current is directly 

 proportional with its E.M.F. and inversely proportional with the 



VOL. II L 



