588 COMPARATIVE ELECTRO-PHYSIOLOGY 



conditions to which these differences may be due : what it 

 is that so determines the tone, that the excitability of a tissue 

 is made to undergo a profound change during the action of 

 a particular stimulus or on its cessation ; and what finally 

 causes the fact that one identical stimulus, say that of 

 tetanising shocks, will sometimes act to exalt, and at others 

 to depress, the excitability of the same tissue. It is the 

 caprice which has seemed to preside over these phenomena 

 that has forced observers upon the postulation of a hyper- 

 physical 'vital force.' In the course of the present work, 

 however, it has been shown that not only the simple pheno- 

 mena of response, but all their complex variations also, are 

 to be met with in the inorganic as in living matter, and that 

 their explanation, therefore, must be sought for in the nature 

 of antecedent molecular changes. As in the inorganic, the 

 conditions of investigation are less complex than in living 

 tissues, it follows that the study of molecular transformations 

 and their after-effects there, is likely to throw much light 

 upon that phenomenon of response which we have thus seen 

 to be universal. 



Taking first the response of living tissues, we find that 

 the responsive change is of two kinds. This may be illus- 

 trated by the following experiment, carried out on the 

 pulvinus of Erythrina indica during the season of its greatest 

 sensitiveness. The stimulus employed was that of a con- 

 stant electrical current. When the upper half of the pul- 

 vinus was made anode, response was found to take place by 

 local expansion. This is seen in the up-record of figure 360. 

 On the break of the anode, we observe a movement of 

 recovery in the opposite direction. The pulvinus was next 

 subjected to kathode- make, and we observe a responsive 

 contraction. At kathode-break, however, we have a recovery 

 by expansion. We have thus observed two opposite re- 

 sponsive effects, according to the different polarities of the 

 stimulating agent namely, expansion at the make of anode, 

 and contraction at that of kathode. 



Since responsive effects must be due to molecular upset, 



