EXCITABILITY UNDER CHEMICAL REAGENTS 133 



this point clear. Taking a carrot as a specimen, I found its 

 resistance plus the resistance of the non-polarisable electrodes 

 to be 20,000 ohms. The application of a chemical reagent 

 reduced this to 19,000 ohms. The resistance of the galva- 

 nometer used was 1,000 ohms, and the high constant external 

 resistance interposed was I million ohms. The variation of 

 resistance induced in the circuit by the application of the 

 reagent was thus 1,000 in 1,020,000, or less than one part in 

 a thousand. 



In studying the variation of excitability in animal tissues, 

 the method of negative variation is employed. But I may 

 here draw attention to the advantage which is afforded by 

 the employment of the Method of Block instead. For, 

 in the method of negative variation, one contact being 

 injured, the chemical reagents act on injured and uninjured 

 unequally. It thus happens that by this unequal action the 

 resting difference of potential is indefinitely altered. But the 

 intensity of response in this method of injury may to a 

 certain extent be dependent on the resting difference. It is 

 thus seen that, when this method is employed, a factor is 

 introduced which may give rise to complications. 



According to the Block Method, however, the two contacts 

 are made with uninjured surfaces, and the effect of the 

 reagents on both is similar. Thus no advantage is given to 

 either contact over the other. The changes now detected in 

 the response are therefore due to no adventitious circum- 

 stance, but to the reagent itself. If further proof be desired 

 of the effect ascribed to the action of the reagent, we can 

 now obtain it by the alternate stimulation of the two ends 

 A and B. I give below (fig. 92) a record of responses 

 obtained in this way from the petiole of turnip. This petiole 

 was somewhat conical in form, and owing to this difference 

 between the A and B ends, the responses given by one were 

 slightly smaller than those given by the other, though the 

 stimuli were equal in the two cases. A few drops of a 

 10 per cent, solution of NaOH were applied at" both ends. 

 The record shows how quickly this reagent abolished the 



