7 6 RESPONSE IN THE LINING AND NON-LIVING 



variation. We read this effect by means of galvano- 

 metric deflections. And if the resistance of the circuit 

 remained constant, then an increase of galvanometer 

 deflection would accurately indicate a heightened or 

 depressed E.M. response, due 'to greater or less excita- 

 bility of tissue caused by the reagent. But, by the 

 introduction of the chemical reagent, the resistance of 

 the tissue may undergo change, and owing to this 

 cause, modification of response as read by the galvano- 

 meter may be produced without any E.M. variation. 

 The observed variation of response may thus be partly 

 owing to some unknown change of resistance, as well as 

 to that of the E.M. variation in response to stimulus. 



We may however discriminate as to how much of 

 the observed change is due to variation of resistance by 

 comparing the deflections produced in the galvanometer 

 by the action of a definite small E.M.F. before and after 

 the introduction of the reagent. If the deflections be 

 the same in both cases, we know that the resistance has 

 not varied. If there have been any change, the 

 variation of deflection will show the amount, and we 

 can make allowance accordingly. 



I have however adopted another method, by which 

 all necessity of correction is obviated, and the galvano- 

 metric deflections simply give E.M. variations, unaffected 

 by any change in the resistance of the tissue. This is 

 done by interposing a very large and constant resistance 

 in the external circuit and thereby making other 

 resistances negligible. An example will make this 

 point clear. Taking a carrot as the vegetable tissue, 

 I found its resistance plus the resistance of the non- 



