248 ELECTRO-PHYSIOLOGY CHAP. 



than at closure of au ascending current. On the other hand, 

 there is usually in this last case (as the galvanic expression of 

 opening tetanus) a negative deflection, conditioned essentially by 

 the duration of the previous passage of current, which declines 

 very slowly. 



We have already seen that the negative variation in nou- 

 medullated molluscan nerve is followed, under given circumstances, 

 by a distinct 'positive after-variation i.e. increase of the demarca- 

 tion current. Hering (18) had previously observed the same 

 effect in inedullated frog's nerve during tetanus, where also the 

 negative variation of the nerve current is generally followed by a 

 positive variation that makes its appearance at the close of 

 excitation, immediately subsequent to the negative variation. Thus 

 <lu Bois-Reymond's statement that " the needle of the multiplier 

 in variably returns to rest more or less imperfectly after tetanisation" 

 (which he attributes to loss of E.M.F. in the nerve in consequence 

 of the previous excitation) is, as Hering points out, inadequate. 



The effect is most distinct when the nerve current has been 

 previously compensated. " The negative variation is then 

 expressed in the deflection of the magnet from its point of rest, 

 in an opposite direction, by the now predominant compensation 

 current. At close of excitation, however, the magnet does not 

 merely return to its zero-point, but passes beyond it, and is then 

 immediately, or shortly after, reversed, and slowly settles down at 

 zero. This is the positive after-variation." Owing to its rapid 

 subsidence, and to the sluggishness of the magnet, the effects are 

 generally still stronger when the galvanometer circuit remains 

 open during excitation, and is only closed immediately afterwards. 

 ' The positive after-effect increases up to a certain point with the 

 duration of excitation. It is already visible when the stimulus 

 lasts only a fraction of a second, and reaches considerable pro- 

 portions after excitation for one second. The increment, as pro- 

 duced by duration of excitation, is only conspicuous in very 

 brief periods of stimulation. It subsequently grows lint 

 little with an increased duration of stimulation, declines again 

 when the excitation exceeds a certain limit, and disappears 

 altogether with further prolongation" (Hering). Head (10), 

 who studied the positive after-variation more closely under 

 Hering's guidance, found that it increased within certain limits 

 with intensity and duration of stimulation. Under favourable 



