IX ELECTRICAL EXCITATION OF NERVE 161 



Hermann (34), finally, points out the possibility "that the 

 opening excitation, which is dependent on the disappearance of 

 some change in the nerve, may be influenced by a certain resist- 

 ance of the nerve to deeper effects of the current (at an earlier 

 stage of excitability)." 



It is remarkable that the different accounts of the initial 

 appearance of the opening excitation, on stimulating with weak 

 currents, i.e. at the first stage of Pfliiger's law of contraction, should 

 again vary in the case of nerve separated from its centre. Pfliiger 

 himself lays down that the make twitch is the primary consequence 

 of stimulation with either direction of current, agreeing with the 

 observations of Bernard, Schiff, v. Bezold, and Eosenthal. Heiden- 

 hain (18), on the contrary, finds in most cases that the closure 

 twitch with ascending, the opening twitch with descending direc- 

 tion, is the first effect of stimulation with minimal currents. 

 Sometimes, however, he obtained only a closure twitch with both 

 directions of current, Wundt (35) made similar observations. 



The perfectly regular effects of excitation, with a given arrange- 

 ment and distance of electrodes in divided or partially killed nerve, 

 gives rise to the conjecture that the differences cited may perhaps\ 

 be explained by differences in the position of the electrodes upon| 

 a nerve divided from its centre. 



We have already seen that in exciting the cut end of a freshly 

 exposed nerve (one electrode being applied to the cross -section 

 itself, or to a point of the nerve close to it) the immediate effect 

 of closure of a minimal descending current is a twitch of the 

 muscle. A slight increase of current elicits a closure twitch with 

 ascending, as well as an opening twitch with descending direction 

 of current. These are approximately equal, and weaker than the 

 descending closure twitch at the same strength of current. The 

 ascending opening twitch only makes its appearance with much 

 greater strength of current. The effects of stimulation are very 

 different, under otherwise uniform conditions, with weak and even 

 with medium currents, if both electrodes are applied to a section] 

 of the same nerve lying a little deeper. For then, without excep- , 

 tiou, only the closure twitch occurs with both directions of current, 

 as was pointed out by Eosenthal and v. Bezold. 



The electrodes may be shifted to the immediate vicinity of 

 the muscle, or middle portions of the nerve may be excited ; so 

 long as the most central contact is sufficiently distant (about 1 cm.) 



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