x ELECTROMOTIVE ACTION IN NERVE 245 



deflection i.e. increase of the nerve current, to which we shall 

 return below is either (as in the majority of cases) smaller than, or 

 equal to, or even larger than the previous negative variation. Its 

 appearance seems to accompany the most favourable conditions of 

 excitability in the nerve, from which we can understand how 

 there comes often to be a distinct positive after-effect with the 

 first excitations, that subsequently fails altogether. As regards 

 dependence of these excitation effects upon strength of current, it 

 must be stated that visible effects upon the galvanometer appear 

 first at a comparatively marked intensity of the exciting current, 

 and always in the first place with the descending direction. These 

 rapidly increase in magnitude, and attain a maximum that 

 is not exceeded with any subsequent increase of the descending 

 current, while the negative variation at closure of the ascending 

 direction, on the contrary, declines, and even fails altogether when 

 the current is increased beyond a certain point. And as ascend- 

 ing closure inevitably produces a weaker negative variation of the 

 demarcation current, its subsidence is invariably more rapid than 

 after the closure of a descending current. If the exciting tract 

 is remote from the led-off galvanometer tract, the magnet usually 

 returns to its position of rest even while the current is passing. 

 With less distance between the two, there are, on the contrary, 

 very striking effects (infra), which have nothing to do with the 

 excitatory manifestations we are here discussing. 



The dissimilar action of descending or ascending currents is 

 again characteristic at break of the exciting circuit. While a 

 negative " opening variation " rarely appears distinctly with 

 descending direction of current, it may with the ascending 

 direction be equal to, or larger than, the initial deflection 

 at closure of the exciting circuit, which then, as a rule, recedes 

 considerably, or even fails altogether. With tolerably strong 

 ascending currents, and a longer distance between leading-off 

 and exciting tracts, the negative opening variation is, as a rule, 

 the sole effect of stimulation. 



It is evident from these data that there are simple and 

 regular relations between the magnitude of negative variation 

 induced by closing or opening an adequate battery current, and 

 the intensity, direction, and duration of the latter, proving 

 immediately that we are in presence of the consequences of 

 make and break excitation. 



