136 



ELECTRO-PHYSIOLOGY 



CHAP. 



simultaneously by Heidenhain and by Pfliiger, who pointed out 

 that the " law of contraction " was a function not merely of direc- 

 tion of current and of excitability, but also of current intensity. 

 Beginning with the weakest currents, Heidenhain (18) obtained 

 the following table of effects from freshly-prepared nerve : 



Heidenhain does not seem to have exceeded a certain average 

 strength of current, for Ritter's fifth stage (and Nobili's third), 

 (i.e. closure twitch only with descending, opening twitch with 

 ascending direction of current, in the fresh nerve), do not appear. 

 All later observers state that at a given medium strength of 

 current both closure and opening are effective, with both descend- 

 ing and ascending directions. It is only with stronger currents 

 that the contrary effect of opposite direction of current becomes 

 apparent. In regard to minimal currents, on the other hand, 

 there are considerable differences of opinion. Heidenhain noted 

 as the first contraction the closure twitch of the ascending, as 

 the second the opening twitch of the descending current, while 

 most later observers give the closure twitch with both directions 

 of current as the first effect of minimal currents (Bibliography to 

 Herm. Hamlb. ii. 1, p. 61), the only difference being whether the 

 descending or ascending current was the first to act. The 

 formula given .by Pfliiger (2) must undoubtedly be accepted as 

 the most correct expression of the law of contraction 



