156 ON NERVE-EXCITATION BY THE NERVE-CURRENT. 



by moving- the slider the derived current is increased until there is 

 a secondary contraction at closure. This requires, according- to the 

 excitability of the preparation, either an extremely small distance 

 of the rheochord slider, or with comparatively poor preparations, 

 a distance of at most a few centimeters. At opening- of this weak 

 current there is no secondary contraction. The above-mentioned 

 rheochord binding--screw is now withdrawn, so that the entire 

 current of the battery is sent throug-h the nerve : there is no longer 

 any secondary contraction at closure, because the electrotonic altera- 

 tion of the nerve at the anode blocks the excitation proceeding 

 from the kathode. Opening- of this current gives strong- secondary 

 contraction with sufficiently excitable preparations ; with such as are 

 less so, the contraction fails in this case also. If now the weak 

 derived current is restored, its closure gives a contraction, and its 

 opening- has no effect, whereas renewed establishment of the strong- 

 current has no effect at closure, but gives an opening contraction. 

 In this manner the alternate excitation with weak and strong 

 currents can be carried on so long as the preparation remains excit- 

 able ; the weak current giving only a make-contraction, the strong 

 current only a break-contraction, or, if the excitability is already 

 depressed, no contraction at all, at a time when the weak current 

 can still give rise to secondary contraction at make. 



But if in the case of the strong current the electrodes are brought 

 so near to the femoral branches of the nerve that electrotonic 

 currents act upon the fibres of these branches which are contiguous 

 with excited fibres, the paradoxical contractions of du Bois-Reymond 

 are produced, and now the strong descending (atterminal) battery-current 

 gives a make-contraction ; but the weak current applied to the same 

 point generally gives no contraction, for its strength was such that it 

 was only just adequate to evoke secondary contraction when it was 

 applied to the lower transverse section, i. e. to the most excitable 

 part of the nerve. 



It appears thus, that, as regards the true secondary contraction, 

 everything depends upon whether a sufficiently strong excitation 

 wave is propagated to the central end of the primary nerve-fibres, 

 there to give rise to excitation of adjacent fibres by means of the 

 negative variation of the nerve-current. Very weak currents are 

 sufficient for this, provided they are applied to the most excitable 

 portion of the primary nerve. The distance between the excited 

 point and the origin of the fibres to be excited secondarily is in the 

 main indifferent. But in the case of du Bois-Reymond's secondary 



