ELECTROTONUS 129 



here also. If the irritant is near the positive electrode, 

 the excitability is lowered ; near the negative electrode 

 it is increased ; and between the two occurs a point at 

 which no noticeable change in the excitability takes 

 place under the influence of the constant current. 



From all these experiments we may infer that a 

 nerve, one part of the length of which is traversed by a 

 constant current, passes throughout its whole length 

 into an altered condition, and that this is expressed in 

 the excitability. One part of the nerve, that on the 

 side of the positive electrode, exhibits decreased excita- 

 bility; the part of the nerve corresponding with the 

 negative electrode exhibits increased excitability. This 

 altered condition is spoken of as the electrotonus of the 

 nerve, the condition which exists on the side of the 

 anode being distinguished as anelectrotonus ; that on 

 the side of the kathode as katelectrotonus. Where the 

 anelectrotonus approaches the katelectrotonus, a point 

 occurs between the electrodes at which the excitabilitv 

 remains unchanged ; this is called the neutral point. 

 The neutral point does not, however, always lie exactly 

 between the electrodes ; but its position depends on 

 the strength of the applied currents. When the cur- 

 rents are weak, it lies nearer the anode ; when they 

 are stronger, it is situated nearer the kathode ; and 

 when the currents are of a certain medium strength, 

 the neutral point is exactly midway between the two 

 electrodes. 



This electrotonic condition of the nerve may be ex- 

 hibited as in fig. 31. In this n n f indicates the nerve, 

 a and k the electrodes, a signifying the anode, k the 

 kathode. The direction of the current within the nerve 

 is, therefore, that indicated by the arrow. In order to 

 7 



