488 COMPARATIVE ELECTRO-PHYSIOLOGY 



NaBr, applied on the right-hand side, is shown by the 

 resultant up-responses. But after a time, E being now 

 depressed by the continued action of NaBr, the effect of 

 NaCl, applied on the left, becomes relatively predominant, 

 a fact demonstrated by the upset of the balance in the oppo- 

 site direction, with concomitant down-responses. 



We shall next take up the subject of variations induced 

 in conductivity. We have seen that dilute solutions of 

 Na 2 CO 3 have the effect of exalting responsive excitability. 



FIG. 297. Photographic Record of Effect of Dilute ('5 per cent. ) Solution 

 of Na 2 CO s on Variation of Conductivity 



Reagent applied on right arm c. Record shows immediate enhancement 

 of conductivity giving rise to up-curves, followed by depression, seen 

 in down-curves. Note the appearance of a down-twitch at the be- 

 ginning of the sixth response due to the later arrival of excitation at E. 

 Note further the replacement of up- by increasing down-responses. 



Long-continued applications, or strong solutions, however, 

 have the effect of inducing a depression. Similarly, I find 

 that this reagent has the effect of enhancing conductivity, 

 provided the solution is sufficiently dilute. In the case of 

 the petioles of ferns, a 2 per cent, solution was found to 

 induce a preliminary exaltation of excitability, followed 

 by a depression (p. 136). In dealing with the conductivity- 

 variation in certain isolated vegetable nerves, however, a 

 2 per cent, solution was found to induce a depression of con- 

 ductivity, but a *5 per cent, solution caused an enhancement 



