290 THE GROWTH OF THE BRAIX. 



but the condition of the nervous system on which these 

 stimuli play is ever changing and never capable of any 

 but the most general expression ; hence the difficulty of 

 predicting the reactions of the organism as a whole. It 

 is possible, however, to study some of the general 

 changes which occur in this background, and it was 

 as a foundation for this study that the observations 

 bearing on the diffusion of the nerve impulse have been 

 introduced. 



Granting that the central system responds throughout 

 a large extent to all the impulses acting upon it, and 

 that by virtue of these responses the background varies, 

 it remains to be determined how far the law and order 

 in these changes can be formulated. There are some 

 observations on the reinforcement of the knee-kick I 

 which are important in this place. In these experiments 

 it was found that, if a voluntary contraction of the hand 

 was made by the subject about the time that his tendon 

 was tapped, the extent of the knee-kick was thereby' 

 influenced. The modification thus introduced by the 

 additional activity had the effect of increasing or 

 decreasing the height of the kick according to the time 

 relations of the two events. The following diagram 

 expresses the results of varying this relation (Fig. 68). 



This curve is typical, and although there is some 

 variation in the reaction according to the nature of the 

 stimulus and the peculiarities of the subject, yet in 

 general the cord reacts more vigorously just after the 

 reinforcement. 



The reactions obtainable by the application of a given 

 stimulus depend on the other stimuli with which the 

 new one is competing. Cold water does not feel cold 

 after ice ; a black line on a grey surface has a value 



1 Bowditch and Warren,/0r;z. of Physiology, 1890. 



