590 COMPARATIVE ELECTRO-PHYSIOLOGY 



incipient molecular distortion, in the same direction as that 

 which precipitates the excitatory response, hence kat-electro- 

 tonus should prove to be excitatory. But a moderate 

 anode, with its incipient molecular distortion in the opposite 

 direction, will retard the normal response, and thus appear 

 to be depressory. I must here point out that these terms 

 excitatory and depressing have ordinarily speaking no ab- 

 solute meaning, and can only acquire a definite significance 

 when we have first fixed on that form of response which is 

 to be regarded as normal. If, instead of contraction, we had 

 regarded expansion as the normal response, then the effect 

 of anode would have been regarded as excitatory, and that 

 of kathode as depressing. We must therefore recognise that 

 the very fact of contractile response being taken as excita- 

 tory, entails as a consequence the designation of all agencies, 

 such as K-tonus, which predispose the tissue to contraction, 

 as excitatory, or better pro-excitatory, while those which, like 

 an-electrotonus, oppose this, must be regarded as depressory, 

 or better anti-excitatory. 



From what has been said, it will be understood that it is 

 the direction of the molecular derangement which determines 

 the character of the response. That molecular upset, which 

 expresses itself as excitatory contraction, we may call the 

 K-effect, and the reversed molecular movement, expressed 

 as expansion, the A-efTect. Thus, under anode, in fig. 360, 

 the molecular distortion in one direction induces the expan- 

 sive A-effect. On the cessation of this, the rebound of 

 recovery causes a movement in the opposite direction, which 

 may carry the molecules back, not merely as far as the 

 equilibrium position, but beyond this. This movement, 

 however, is in the same direction as that induced by K-make. 

 Hence we may understand how excitation is caused, not only 

 by K-make, but also by strong A-break. We may also 

 understand how it is that the excitatory effect is much 

 enhanced when A-break is immediately followed by K-make. 

 We also see, in a general way, that a particular-directioned 

 molecular movement would have the most intense excitatory 



