PHYSIOLOGICAL CHANGES IN THE CENTRAL SYSTEM. 285 



power was decreased by a decrease in the atmospheric 

 pressure, while in summer, several days of high tem- 

 perature, especially with great humidity, were followed 

 by a distinct loss. The variation in the capacity for 

 muscular work is mainly due to changes in the nervous 

 system, and hence may be taken as an example of the 

 susceptibility of this system to influences for which we 

 possess no special organ of sense. Such general stimuli 

 as have just been mentioned, humidity and pressure, 

 affect all parts of the central system at the same time, 

 yet the power of causing a general effect is common to 

 all stimuli, be they never so localised at their origin. 



On the wide diffusion of impulses depends the fact 

 that a slight stimulation of one part of the system is 

 favourable to the reaction following the stimulation of 

 another. Indeed a condition of diffused stimulation is 

 an essential of the waking state, and if it is not for- 

 gotten that stimuli which when of low intensity can 

 thus reinforce one another, may become antagonistic 

 when their intensity is raised, no confusion need be 

 introduced by the observations to be cited later. This 

 reinforcing influence of two stimuli can be illustrated 

 by the studies which have been made on the knee- 

 kick. 



If the thigh be supported, so that the leg can swing 

 freely, and the tendon below the knee-pan be sharply 

 struck, the leg will be kicked out. This reaction is 

 brought about by the contraction of extensor muscles 

 in the thigh, and requires for its normal occurrence both 

 the nervous connections of these muscles with the cord, 

 and also a healthy condition of the cord. The study of 

 the normal knee-kick has shown that it differs widely in 

 different persons ; that in the same person it varies from 

 day to day ; that during sleep it may disappear, while in 

 the waking state it may show not only rhythmic varia- 



