GENERAL LAW OF NERVE EXCITEMENT. 151 



differ entirely in duration. The closing inductive cur- 

 rent increases slowly, and decreases just as slowly, 

 while, on the other hand, the opening inductive current 

 very rapidly attains its full strength and ends just as 

 quickly. It is to this difference that the latter evi- 

 dently owes its greater physiological effect.^ 



Let us return to the experiment as first arranged 

 with the rheochord. Instead, of pushing along the 

 slide between A and S^ it may be moved backward or 

 forward between any two points. The current in the 

 nerve, in this case, never ceases, but is either strength- 

 ened or weakened according to the direction in which 

 the slide is moved. If the latter is moved suddenly 

 and with great speed, it may produce excitement ; but 

 the nerve always remains unexcited when the move- 

 ment is gradual. It therefore appears that it is not 

 the actual closing and opening of a current which is 

 required to excite the nerve, but that any change, 

 whether it strengthens or weakens the current, is suffi- 

 cient to effect this, provided that the alteration is 

 sufficiently great and sufficiently rapid. Closing and 

 opening are but special cases of alteration of the cur- 

 rent in which one of the limits to the strength of the 

 current = 0. The following law regarding the electric 

 excitement of nerve may therefore be stated: any 

 change in a current traversing a nerve may excite the 

 latter if it is sufficiently strong^ and if it occurs luith 

 sufficient speed. We have however seen that this law 

 has very many exceptions. For under certain circum- 

 stances a greater alteration (the closing of a strong 

 ascending current) may appear to be without effect, al- 

 though one less strong takes effect. If, however, it is 

 * See Notes and Additions, Xo 6. 



