CURVE OF EXCITABILITY. 121 



bility is very soon destroyed, and in a very irregular 

 manner. 



We have seen that the nerve dies gradually from 

 the top downward. This death does not, however, 

 consist in a simple falling off in the excitability from 

 its original degree till it completely dies out. If the 

 excitability is tested from time to time at a point some 

 distance from the cut end, it is found to increase at 

 first until it reaches a maximum, at which it remains 

 for some time stationary, and it is not till after this 

 that it gradually decreases and finally expires. The 

 further the point experimented on is from the point 

 which has been cut, the more slowly do all these 

 changes occur ; but their sequence is in all cases essen- 

 tially alike. The explanation of this may be that the 

 upper parts of the nerve, which directly after the pre- 

 paration is made usually exhibit the highest degree 

 of excitability, are really already changed. It must be 

 assumed that these changes intervene very quickly at a 

 point close to the section, so that it is impossible to 

 submit these points to observation until they are al- 

 ready in the condition which does not intervene till 

 later at the lower points in the condition, that is, of 

 increased excitability. This view is confirmed by the 

 following experiment : if the excitability is determined 

 at a lower point of the nerve, and the latter is then cut 

 through above this point, the excitability increases at 

 the point tested, and this takes place more quickly in 

 proportion as the cut was made nearer to the tested 

 spot. Each of the lower points may, therefore, be 

 artificially brought under the same conditions under 

 which only the upper parts of the nerve usually lie, 

 that is, it may be arranged that they are near the 



