180 ELECTROTONUS. [CH. xvi. 



smaller contractions, or none at all. On removing the polarising 

 current, the after-effect is increase of excitability. 



The following figure is a reproduction of a tracing from an 

 actual experiment. The after-effects are not shown. N repre- 

 sents a series of contractions obtained when the nerve is normal, 

 K when it is katelectrotonic, A when it is anelectrotonic. 

 . Exactly similar results are obtained if one uses mechanical 

 stimuli, such as hammering the nerve, instead of induction shocks. 

 The same is true for chemical stimuli. If the exciting electrodes 

 are removed, and salt sprinkled on the nerve near the muscle, 

 the latter soon begins to quiver ; its contractions are increased 



Fig. 190. Electrotonus. M, make. B, break. 



by throwing in a descending and diminished by an ascending 

 polarising current. 



The increase in irritability is called katelectrotonus, and the 

 decrease is called anelectrotonus. The accompanying diagram 

 (fig. 191) shows how the effect is most intense at the points (a, k) 

 where the electrodes are applied, and extends in gradually dimin- 

 ishing intensity on each side of them. Between the electrodes 

 the increase shades off into the decrease, and it is evident that 

 there must be a neutral point where there is neither increase nor 

 decreaseof irritability. The position of this neutral point is found to 

 vary with the intensity of the polarising current when the current is 

 weak the point is nearer the anode, when strong nearer the kathode. 



Electro tonic alterations of conductivity. When a con- 

 stant current is passed through a nerve, not only is its excita- 



