132 ELECTROTONUS. [BOOK i. 



increase and decrease of irritability are most marked in the 

 immediate neighbourhood of the electrodes, but spread for a 

 considerable distance in each direction in the extrapolar regions. 

 The same modification is not confined to the extrapolar region, 

 but exists also in the intrapolar region. In the intrapolar region 

 there must be of course a neutral or indifferent point, where the 

 katelectrotonic increase merges into the anelectrotonic decrease, 

 and where therefore the irritability is unchanged. When the 

 polarizing current is a weak one, this indifferent point is nearer the 

 anode than the kathode, but as the polarizing current increases in 

 intensity, draws nearer and nearer the kathode (see Fig. 23). 



The amount of increase and decrease is dependent: (1) On the 

 strength of the current, the stronger current up to a certain limit 

 producing the greater effect. (2) On the irritability of the nerve, 

 the more irritable, better conditioned nerve being the more affected 

 by a current of the same intensity. 



In the experiments just described the increase or decrease of 

 irritability is taken to mean that the same stimulus starts in the one 

 case a larger or more powerful and in the other case a smaller or 

 less energetic impulse ; but we have reason to think that the mere 

 propagation or conduction of impulses started elsewhere is also 

 affected by the electrotonic condition. At all events anelectrotonus 

 appears to offer an obstacle to the passage of a nervous impulse. 



FIG. 23. DIAGRAM ILLUSTRATING THE VARIATIONS OF IRRITABILITY DURING ELECTRO- 

 TONUS, WITH POLARIZING CURRENTS OF INCREASING INTENSITY (from Pfliiger). 



The anode is supposed to be placed at A, the kathode at B ; AB is consequently 

 the intrapolar district. In each of the three curves, the portion of the curve below 

 the base line represents diminished irritability, that above, increased irritability. 

 y l represents the effect of a weak current ; the indifferent point x 1 is near the 

 anode A. In y. 2 , a stronger current, the indifferent point x. 2 is nearer the kathode 

 B, the diminution of irritability in anelectrotonus and the increase in katelectro- 

 tonus being greater than in y l ; the effect also spreads for a greater distance along 

 the extrapolar regions in both directions. In y 3 the same events are seen to be still 

 more marked. 



76. Electrotonic Currents. During the passage of a constant 

 current through a nerve, variations in the electric currents belonging 

 to the nerve itself may be observed ; and these variations have certain 

 relations to the variations of the irritability of the nerve. Thus if 

 a constant current supplied by the battery P (Fig. 24) be applied 



