480 COMPARATIVE ELECTRO-PHYSIOLOGY 



have seen, for example, that a rise of temperature, by in- 

 creasing molecular mobility, enhances conductivity. But 

 this increase of molecular mobility and internal energy also 

 goes to augment the force of recovery, and, owing to this, 

 the amplitude of excitatory response may be decreased. 

 Thus, while a rise of temperature increases conductivity, it 

 may appear to decrease responsive excitability. So much 

 for the necessity of a distinction between conductivity and 

 responsivity. The term ' excitability ' is commonly used for 

 receptivity and responsivity indifferently. But I shall show 

 in the course of the present chapter that it is important to 

 make a distinction between these, since the same external 

 agent may effect the two differently. 1 In the following 

 investigation, receptive excitability, or receptivity, will be 

 represented by R, conductivity by C, and responsive ex- 

 citability, or responsivity, by E. 



In determining the effect of any external condition such 

 as the application of a chemical reagent on responsive ex- 

 citability, in the case of animal nerve, it is usual to take a 

 series of normal responses, and then to record the modified 

 responses after the application of the reagent. By com- 

 paring a number of such series of records, representing the 

 action of various reagents on different specimens, the relative 

 effect of each chemical may be inferred. The drawback to 

 this method lies, first, in the fact that by the addition of the 

 chemical reagent the resistance of the electrical circuit 

 undergoes an unknown change, thus inducing a variation in 

 the amplitude of response, which is not necessarily due to the 

 excitatory electromotive change per se. It is true that this 

 difficulty may to a greater or less extent be obviated by 

 interposing a high external resistance in the circuit, but this, 

 by reducing the deflection, necessarily reduces the sensitive- 

 ness of the method also. Different specimens again cannot 

 but be characterised by slight individual peculiarities, and the 

 experimental arrangements therefore can only be considered 

 to be perfect when we are able to compare the effects of two 



1 See also Bose, Plant Response, pp. 215-230. 



