162 ELECTRO-PHYSIOLOCrY CHAP. 



from the cross-section, the described reaction undergoes no change, 

 or such only that the closure twitches discharged at different 

 points of the nerve, with uniform distance of electrodes and 

 strength of current, are of different magnitudes corresponding 

 with the fact that the excitability of a divided nerve is, as a rule, 

 not merely greater in the vicinity of the cross-section than at 

 the periphery, but also that certain special points in its continuity 

 are characterised by a higher excitability. 



If with descending direction of current the central electrode 

 is placed quite close to the cross-section, there is invariably an 

 opening twitch along with the closure twitch, at any given posi- 

 tion of the other electrode, and that at very low intensity of the 

 exciting current, hardly exceeding the threshold of activity. 

 With ascending direction of current, on the other hand, the 

 peripheral electrode must be brought to within a few millimetres 

 of the kathode at the cross-section, in order to demonstrate the 

 opening twitch, as well as that at closure. The closure twitch is 

 then, with low intensity of current, very small, and often entirely 

 absent ; the same applies, after reversing the current, to the 

 opening twitch. 



The dependence of the opening excitation upon the proximity 

 of the cross-section of a nerve to the anode stands out with 

 especial clearness when (as was first shown by Heidenhain, 29) 

 the electrodes are applied anywhere along the continuity of a 

 nerve, and the " centropolar " tract so shortened by amputation 

 that the cross-section is brought into the immediate vicinity of 

 the upper electrode. The opening twitch is at first seen only 

 with descending direction of current, and it is not till 

 the intrapolar region is shortened, whether by bringing the 

 lower electrode close to the upper, or, as was said before, by 

 making the cross-section within the intrapolar tract itself, and 

 applying the cut ends -together again, that the opening twitch 

 appears with ascending direction also, while the closure twitch 

 becomes less, or fails altogether. 



The most obvious interpretation of the efficacy of even very 

 weak opening stimuli in the immediate proximity of the cross- 

 section of a nerve is that derived from the well-known observa- 

 tions of Heidenhain (29), to the effect that on applying a cross- 

 section either to a fresh nerve or to one in which excitability 

 is already declining, the response of each point not unduly remote 



