210 ELECTRO-PHYSIOLOGY CHAP. 



analogous data for direct excitation of the muscle. These ex- 

 periments also show that mere variations of current density in 

 the nerve (as also in the muscle) may excite equally with the rise 

 or fall of the current from or to zero (make or break of the circuit). 

 Later on, 0. Nasse (53), Hermann (53), du Bois-Eeymond (53), 

 and others attacked the question of how far the absolute height 

 of previously existing polar alterations affects the discharge of 

 a twitch from the muscle, in sudden variations of intensity in the 

 (" electrotonic ") changes of the nerve that occur during closure at 

 the kathode, after opening at the anode. Nasse showed (by a fall 

 apparatus which closed or opened an incremental current, derived 

 through a rheochord) a positive or negative variation of intensity 

 superposed upon the existing battery current. The positive 

 variation of descending currents was found to be increased with 

 weak constant currents, to be diminished with stronger currents, 

 while the negative variation of ascending currents was depressed 

 at all strengths of the constant current. Hermann sums up the 

 result of his investigations, on the Eckhardt-Pfluger method, in 

 the dictum that the effect of a given induction current is raised 

 (as in muscle) by homodromous constant currents (provided these 

 do not exceed a certain range of intensity), and depressed (to 

 abolition) by opposite currents. Since, as Hermann concludes, 

 increase of a homodromous current is equivalent to closure of a 

 homodromous or opening of a heterodromous current, while its 

 sudden diminution corresponds with closure of an opposite or 

 opening of a homodromous current (so that in the former case 

 the seat of excitation coincides with a pre-existing katelectrotonus, 

 but otherwise with previous anelectrotonus), the experimental 

 results of indirect, as of direct, excitation of the muscle seem to 

 be intelligible from the same standpoint. 



If the exclusively kathodic excitation is thus to be regarded 

 as ascertained for weak induced currents, we must, on the other 

 hand, concede the probability that with strong currents even brief 

 duration may develop an electrotonus adequate to produce ex- 

 citation in the descending portion also. This is indicated in 

 certain observations of Fick, Lamansky, and others. These 

 refer, in the first place, to a characteristic feature of the height of 

 twitch, in indirect excitation of the muscle with very brief 

 constant currents (current impacts), on changing the intensity, 

 duration, and direction of the latter. Tick determined (supra) 



