654 COMPARATIVE ELECTRO-PHYSIOLOGY 



His previous habits and prepossessions all contribute, as is 

 well known, to modify it more or less permanently. The 

 principal reason for the constancy of the records made by 

 the physical indicators lies in the greater or less constancy 

 of the properties of those elements of which they are com- 

 posed. Even here there is a fluctuation of sensitiveness, 

 owing to changes in the properties of the material. But 

 these changes are neither so rapid nor so considerable as 

 in the neural apparatus, whose excitability is extremely 

 susceptible of modification under the influence of fatigue, 

 and of such varying factors as health and tonicity, as well as 

 by the action of the stimulus itself. 



Looking now at the whole range of impulses generated 

 in the nerves under increasing stimulus, we shall see that 

 the positive effect is gradually augmented till it reaches a 

 maximum. It then undergoes a decline, passing into a 

 resultant negative. This resultant negative response con- 

 tains, as we know, a masked positive component, which 

 can be separated and exclusively demonstrated by appro- 

 priate methods. With increasing stimulus the negative 

 response undergoes an enhancement till a limit is reached. 

 Expressed in terms of sensation, then, the effect perceived 

 is at first of positive tone, and this, growing in intensity, is 

 pleasurable. This positive tone, however, afterwards under- 

 goes a diminution, and finally passes over the zero -line ; 

 this constitutes the commencement of a somewhat extended 

 range, in which the resultant negative, with its masked 

 positive, undergoes increase. Referring to the curve at the 

 crossing of the zero-line, it must be said that we. have here 

 a neutral point. This is not, however, the same as 

 that absolute zero at which the curve of sensation was 

 initiated, for here the neutrality is not due to absence of 

 effect, but to the fluctuating equilibrium of two opposite 

 effects, one positive and the other negative. It is thus to 

 be understood that, while in the positive region the tone is 

 simple, in the region of the resultant negative it is complex, 

 the sensation here being compounded of positive and nega- 



