a 



5 





ELECTROTONUS 573 



contiguity of the anode, while E is depressed by that of the 

 kathode. Hence, by the concordant action, at each end of 

 the balance, of conductivity and excitability changes, and 

 owing to the opposite nature of these changes at opposite 

 ends, the original balance is disturbed, and we obtain 

 resultant down responses, showing the greater excitation and 

 galvanometric negativity caused at E' than at E, when anode 

 is to the left and kathode to the right. This is exhibited in 

 the first pair of down responses in figure 353 b. When, 

 however, the polarisation current is reversed (fig. 352), the 

 excitation at the right-hand side, E being now near the anode, 

 is relatively the greater, and we find the resultant responses 

 to be upwards, as seen in the third record in fig. 353 c. To 

 go back to the question of the relative directions of electro- 

 tonic and responsive currents, we find in that case, when 

 the anode is to the left, that E' is galvanometrically positive, 

 while the excitatory change makes it galvanometrically 

 negative. This means that the excitatory response takes 

 place by the so-called polarisation decrement. When the 

 anode again is to the right, the galvanometrically positive E 

 ends by excitation to become galvanometrically negative. 

 This will be clearly understood from the arrows which 

 accompany the diagram, in figs. 351, 352. The inner and thin 

 arrows represent the direction of the polarising current, and 

 the thick outer arrows the responsive current. These results 

 are tantamount to an example of the so-called polarisation- 

 decrement In order to show, however, that the same 

 excitatory reaction might appear as a polarisation-increment, 

 I shall describe another experiment. 



The experimental tissue is here the isolated nerve of fern, 

 and the method employed is again that of the Conductivity 

 Balance. In this case, however, it will be noticed (figs. 354, 

 355), that the galvanometer is included in series with the 

 polarising E.M.F., instead of being placed as a shunt, as in the 

 last case. The stimulator was first adjusted at balance. The 

 left electrode was now made kathode, the right being anode, 

 the E.M.F. employed being '2 volt (fig. 354). On account 



