AFTER ELECTRICAL STIMULATION. 269 



and with the same strength and duration of current, negative varia- 

 tion is seen to be less at each successive stimulation. The subse- 

 quent weakening of the nerve current is at the same time greater, 

 and the impression is given of increasing exhaustion of the nerve. 

 If the strength of current be increased by degrees, the negative 

 variation still increases, as does at the same time the subsequent 

 weakening of the nerve current. During stimulation with very 

 strong currents, the nerve current disappears altogether, and it 

 returns only partially and very slowly after stimulation has ceased. 

 Finally, this may proceed to such an extent that the nerve current 

 may reverse its direction during stimulation. After ceasing to 

 stimulate, this reversed current gradually dies away ; if stimulation 

 is again repeated, the reversed current may become still stronger, and 

 after ceasing to stimulate may continue for a period in the negative 

 direction. At other times a lasting disappearance of nerve current 

 ensues, the nerve appearing to lose permanently its electromotive force. 



But these last-named deep-seated alterations in the electrical 

 condition of the nerve are solely a consequence of change in the 

 neighbourhood of the galvanometer electrodes, especially of the upper 

 one. If the altered portion of the nerve be cut away and the elec- 

 trodes be connected, one with the new section, the other with a point 

 proportionately higher up the length of the nerve, a strong nerve 

 current is again obtained. If now the other end of the nerve be 

 stimulated afresh, the whole series of changes will be repeated. 



It cannot therefore be supposed that this weakening or total 

 disappearance of the nerve current depends on a corresponding 

 decrease or total annihilation of electromotive power of the whole 

 nerve ; we are not dealing here with ' complete exhaustion of 

 animal excitability V A nerve is a relatively durable structure, 

 and it is astonishing how long it holds out even against strong 

 stimuli. It is only locally that it is permanently altered by strong 

 currents. The portion which is not directly stimulated, although 

 it may be in a condition of strong propagated excitation, suffers 

 much less even when stimulated for a long period. 



The ease with which a nerve is exhausted has been greatly over- 

 rated. If unipolar action be excluded, the nerve can be repeatedly 

 stimulated, or stimulated for a long period, without any appreciable 

 weakening of the nerve current or its negative variation, provided 

 that the stimulating currents be not too strong. 



1 Compare on this point N. Wedenskii's recent communication, ' Centralblatt fur 

 die medicinische Wissenschaft ' of Feb. 2, 1884. 



