THE MOLECULAR THEORY OF EXCITATION 



en 



- 



found to be transmitted through the kathodic area, inducing 

 enhanced response of the indicating muscle, if the polarising 

 current be weak. But when the intensity of the kathode is 

 made stronger, even the strongest stimulus will fail to induce 

 response. This is evidently due to the fact that a strong 

 kathode induces a depression or abolition of conductivity. 



Moderate K-tonus, then, we have seen to induce enhanced 

 conductivity, because of the favourable molecular disposition 

 which it brings about. Even on the cessation of K-tonus 

 this disposition remains, owing to molecular ' retentiveness,' 

 with its concomitant enhanced conductivity as an after- 

 effect. This induction of a favourable molecular disposition 

 habit is an interesting phenomenon, which we shall meet 

 with again. 



We shall next study the enhancement or depression of local 

 xcitability by K- or A-tonus. We saw, in Chapter XXXIII., 

 that by means of the Con- 

 ductivity Balance we might 

 determine the variations, not 

 only of conductivity, but also 

 of local excitability. In mag- 

 netic experiments the responsive 

 area at the right-hand end of 

 the balance may be made either 

 K-tonic or A-tonic, by bringing 

 near it one or other pole of 

 a permanent magnet. Under 

 induced A-tonus, the molecular 

 excitability is depressed, and 

 the balance upset in a down- 

 ward direction ; while under 

 K-tonus excitability is enhanced, 

 the resulting response being up- 

 wards (fig. 376). 



A still more interesting case is that in which the stimulus 

 itself fashions, as it were, the path for its own conduction. 

 The receiving coil is placed at such a distance from S 



R R 2 



FIG. 376. Effects of K- and 

 A-Tonus on Magnetic Ex- 

 citability 



From the resulting upset of the 

 balance, A is seen to induce de- 

 pression, and K enhancement, 

 of excitability. 



