246 ELECTRO-PHYSIOLOGY . (HAP. 



At closure of both descending and ascending currents there is 

 often a considerable diminution of E.M.F. between the longitudinal 

 and transverse sections in the nerve, which is, however, much less, 

 and continues for a shorter time, with the ascending direction. 

 If the current remains closed for a sufficiently long period, the 

 negative variation will either be entirely abolished in the course 

 of a few seconds, or (with descending direction) there may be a 

 remainder of negative variation, which only disappears if at all 

 on opening the exciting circuit. 



The magnitude of the negative variation is therefore almost 

 independent of the distance between leading-off and exciting 

 electrodes. It does not increase perceptibly when the intermediate 

 tract is shortened, nor will it exceed a certain (easily attained) 

 maximum on strengthening the descending exciting current. 

 If the current, on the other hand, is ascending, the negative 

 variation declines with its increasing intensity, and is finally 

 abolished, exhibiting therefore, in this respect, a reaction precisely 

 similar to that of the closing excitation with ascending direction 

 of current. With respect, lastly, to the sequelae of the break- 

 effect, there is again complete uniformity between the reaction of 

 the opening excitation (in so far as it is expressed on exciting 

 the motor nerve by changes of form in the muscle) and the 

 alterations of the demarcation current, as above. This is more 

 especially true of the dependence of the negative opening variation 

 upon the strength and duration of the ascending exciting 

 current. It invariably appears first with a much higher 

 intensity of current than that demanded by the closing effect, 

 and increases in proportion with the duration of closure in the 

 exciting circuit. Hence it is possible, with sufficiently pro- 

 longed closure, to produce a distinct negative opening variation, 

 even with comparatively weak ascending currents. With 

 descending battery currents there is seldom a pronounced negative 

 variation at break ; in most cases it is merely indicated by a 

 transitory delay in the return of the scale. The most striking 

 coincidence is that exhibited between the galvanic effects of 

 excitation in non-medullated molluscan nerve, and the mechanical 

 effects in an ordinary nerve-muscle preparation, on employing an 

 electrical stimulus corresponding with the first or third stage of 

 Pfiiiger's law of contraction. In the first of these two cases a 

 negative variation (closing excitation) is apparent at closure of 



