CHAP, ii.] THE CONTRACTILE TISSUES. 79 



tonic 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. 14). 



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 affected 

 by the electrotonic condition. At all events anelectrotonus appears 

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



These variations of irritability at the kathode and anode respec- 

 tively must be the result of molecular changes, brought about by 

 the action of the constant current. They are interesting theoreti- 

 cally because they shew that the generation of a nervous impulse 

 as the result of the making or breaking of a constant current is 

 dependent on the change of a nerve from its normal condition into 

 either katelectrotonus or anelectrotonus, or back again from one 

 of these phases into its normal condition. And certain results as 

 to the occurrence or absence of a contraction at the make or at 

 the break, according as the current is strong or weak, ascending, 

 or descending (results which need not detain us here but which 

 have been formulated as the co-called "law of contraction") 



FIG. 14. 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. 

 2/j represents the effect of a weak current; the indifferent point x l is near the 

 anode A. In y v a stronger current, the indifferent point x^ 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 */ 3 the same events are seen to be still 

 more marked. 



