RESPONSE BY POSITIVE VARIATION 



165 



found that the so-called ' current of injury' had in this case, 



owing to some hitherto unknown cause, undergone a reversal, 



and was now from the living to the dead, the latter being 



galvanometrically positive to the former, to the extent of 



08 volt. The abnormality of the response lay, then, in this 



fact, that the current of reference had become reversed, and 



that the responsive current, due to excitation, was now con- 



cordant with it, instead of antagonistic, thus constituting a 



positive variation (fig. 112). Later 



on, I discovered many instances in 



which the killed end was positive 



to the unkilled. Since, then, it is 



possible for the current of reference 



itself to undergo such obscure and 



spontaneous reversals, from un- 



known causes, it is easy to see 



how uncertain the study of re- 



sponsive phenomena must become, 



if we are to depend upon the 



negative variation as our only 



reliable means for their investiga- 



tion. I next, therefore, turned my 



attention towards an inquiry into 

 the causes of these anomalous 

 reversals. 



The subject therefore resolved itself into an investigation 

 as to what conditions determined the negativity or positivity 

 of a tissue at the onset of death. My first attempt, then, was 

 to study a case in which the approach of death was natural, 

 and not the result of any sudden or violent change, such as 

 might conceivably give rise to abnormal reactions. And in 

 my search for suitable specimens, I noticed that often, owing 

 to local mal-nutrition or other causes, the leaves of plants 

 exhibited spots or areas, from which, as centres, death pro- 

 ceeded in constantly widening circles. Thus, in the leaves 

 of Colocasia, for example, we find such dead and dying areas 

 in otherwise fairly healthy leaves. The innermost of these 



FIG. 112. Response by Positive 

 Variation of Resting Current 



reversed, the killed end having 

 bec^galvanometricallyposi- 



