268 ELECTRO-PHYSIOLOGY CHAP. 



nerve, with both ends cut, is isoelectrically disposed, i.e. is in contact 

 with the galvanometer electrodes at points symmetrical with the 

 equator, laterally to which it is traversed by the polarising cur- 

 rent. The situation may then be expressed as follows : If a con- 

 stant electrical current is led through a portion of a medullated nerve, 

 the entire nerve (while preserving its original electromotive activitij) 

 becomes electrically active in the direction of the polarising current 

 at all points, each point of the nerve being negative to every other 

 that is anterior to it in the direction of the current. 



The magnitude of the electro tonic deflections diminishes (as 

 is prima facie obvious from the distribution of potential) with the 

 distance of the leading-off tract from the poles, as appears plainly 

 in the vicinity of the latter ; it is further in ratio with the 

 strength of the polarising current. Moreover, the electrotouic 

 effect increases constantly with increased intensity of current, 

 and never seems to find a limit. Certain experiments of du 

 Bois-Eeymond (23), which were intended to determine the 

 eventual maximal value of electrotonus, were unsuccessful, although 

 they showed that the E.M.F. of the incremental current on the 

 side of the anode and kathode (Griinhagen's anodic and kathodic 

 current) may exceed that of the normal longitudinal-transverse 

 current by more than twenty-two times, without finding a limit. 

 As expressed in units of a Daniell, the E.M.F. of the anodic 

 current = 0'5 Dan., that of the kathodic current 0'05 Dan. This 

 difference of E.M.F. in the an- and katelectrotonic incremental 

 currents, which finds similar expression in regard to intensity, is 

 distinct in every case, and is the reason that in a graphic represent- 

 ation the curves of potential are shorter upon the side of the 

 kathode, and the corresponding ordinates lower, than on the 

 anodic side (Fig. 203). Under all conditions the maximum of 

 anelectrotonus exceeds that of katelectrotonus. 



A further factor in the magnitude of electrotonus is the 

 length of tract traversed by the polarising current. If the 

 electrodes are gradually shifted so as to lengthen the tract of 

 nerve excited, the incremental currents diminish pari passu with 

 the extension of the tract through which current is passing ; 

 this diminution is, however, obviously due to the weakening of 

 the polarising current, from the increased resistance of the 

 conductor. If (as was first effected by du Bois-Eeymond) the 

 intensity of the polarising current is kept constant by introducing 



