MODIFICATION^' OF THE EXCITABILITY. 131 



2. Apart from these changes in the excitability 

 which are thus observable while a continuous current 

 passes through the nerve, others can also be shown to 

 occur immediately after the opening of the current. 

 Indeed, the excitability altered in electrotonus does not 

 immediatelv revert to its normal value when the cur- 

 rent is interrupted, but only regains this after the lapse 

 of a short time. The duration of the chano-es in the 

 excitability observable after the opening of the current 

 is greater ir proportion as the current is stronger and 

 its duration is longer. These changes, which, to dis- 

 tinguish them from the electrotonic changes, are called 

 modifications of the excitability, are not merely the 

 continuance of an electrotonic condition, but are some- 

 times completely different from the latter. If, for in- 

 stance, the experiment is tried at a point near the 

 anode, at which the excitability is decreased during the 

 continuance of the current, the excitability is found 

 to be increased immediately after the opening of the 

 current, and it is not till after this that the original 

 normal excitability is regained. Similarly, in the neigh- 

 bourhood of the kathode, the excitability decreases for 

 a short time after the opening of the current, after 

 which it again increases, and only gradually regains its 

 normal condition. As a rule, these modifications do not 

 last more than a few parts of a second. If, however, 

 the constant current has been long present in the nerve, 

 these modifications may endure for a somewhat longer 

 period. On account of their transient nature it is diffi- 

 cult to observe and test them. The change of condi- 

 tion which follows the opening of the current within the 

 nerve may, moreover, lead to excitement in the latter ; 

 so that, on the opening of a current which has been 



