68 CONTRIBUTION TO THE 



polarisation-current flowing in the opposite direction ; the sum of 

 all these residual polarisation-currents make in the end a polarisa- 

 tion-current sufficiently strong- to block the descending break- 

 excitation. 



My theory explains therefore, in a very simple way, all the facts 

 formulated in the law of contraction, and makes it directly intel- 

 ligible why ' the disappearance of anelectrotonus is a much weaker 

 stimulus than the appearance ofkatelectrotonus? since the polarisation- 

 current is always weaker than the polarising, and both stimulate 

 the nerve in a like manner, namely, at their negative pole. 



If we bear in mind that the break-contraction arises through the 

 passage of a current (the polarisation-current), we can quite simply 

 formulate the law of contraction. We require in fact only two 

 strengths of current : 



1. Weak currents, in which the excitation at the negative pole 

 overpowers the blocking at the positive, and 



2. Strong currents, in which the excitation at the negative pole' 

 is overpowered by the blocking at the positive. 



These two cases comprise the whole law of contraction, if we only 

 remember at the same time that every polarising current produces 

 polarisation, and also, that every current, to liberate an excitation, 

 must be of a certain strength. 



During the closure of a current the excitability of the nerve is 

 lowered in the region of the positive pole, both on the extrapolar 

 and on the intrapolar side, and it is raised in the region of the 

 negative pole. 



Immediately after breaking, that is while the polarisation-current 

 is still of sufficient strength, the excitability is seen to be diminished 

 at the negative pole (the positive pole of the polarisation-current) 

 and strengthened at the positive pole (the negative pole of the 

 polarisation-current). Like the polarisation-current itself, the 

 changes in excitability on breaking the polarising current are very 

 evanescent. The agreement of the facts with the theory could not 

 be more striking than it is. 



After the stage of diminished excitability at the positive pole of 

 the polarisation-current has passed off, a rise of excitability shows 

 itself for a certain time throughout the whole extent of the nerve ; 

 and this phenomenon must be regarded as the real and only after- 

 effect of the electric current. 



Moreover, the circumstance that, in experiments on living animals 

 and on nerves unseparated from their connections, only make-con- 



