284 THE GROWTH OF THE BRAIN. 



of it. In thus picturing the entire nervous system as a,, 

 sensitive mechanism, it is evident that it must respond 

 to the surrounding stimuli as does the water of a lake 

 to the breeze ; and such is the relation between the 

 central system and its environment that the breeze is 

 always blowing and the waves of change always chasing 

 one another among the responsive elements. If there 

 are no waves then the cells are dead. The breeze still 

 blows, but it falls on a frozen surface, on cells chilled 

 and rigid beyond the power of response. 



The ceaselessness of this stimulation cannot be pre- 

 sented too strongly, because those stimuli which do not 

 come clearly into consciousness are but too readily neg- 

 lected. Yet the responses of this ever sensitive system 

 reacting to never-ending stimuli are by no means always 

 part of our conscious life, and hence these changes must 

 be indirectly studied if they are to be recognised at all. 

 That such variations may be due either to changes in 

 the exciting stimuli, or to different degrees of respon- 

 siveness on the part of the central system, is self-evident. 

 The stimuli during the day are many and strong, but 

 few and weak at night, different according to the seasons 

 of the year, and dependent on changes both without and 

 within the body, changes involving not only alterations 

 in those forms of energy for which special sense organs 

 exist, and which produce the sensations of light, sound, 

 taste, odour, and touch, but also in those for which there 

 are no such organs, as humidity, electric tension, atmo- 

 spheric pressure, and the like. Yet to all these stimuli 

 responses are made, and they are never twice the same. 



The changes in the organism as a whole, for the most 

 part unrecognised, are well illustrated by the variations 

 in the capacity for the performance of voluntary mus- 

 cular work. It has been found by Lombard * that this 

 1 Lombard, Journ. of Physiol., 1892. 



