FUNCTIONS 02' THE BRAIN 947 



by stimulation of the anterior spinal roots. When the hand is opened 

 the muscles that open it are excited, and those which close it are in- 

 hibited from the cortex. 



Reaction Time. Just as in a reflex act a certain measureable 

 time (reflex time) is taken up by the changes that occur in the lower 

 nervous centres, so we may assume that in all psychical processes 

 the element of time is involved. And, indeed, when the interval 

 that elapses between the application of a stimulus and the signal 

 which announces that it has been felt (reaction time) is measured, 

 it is found that for the cerebral processes associated with the per- 

 ception of the simplest sensation and the production of the simplest 

 voluntary contraction it is longer than the time which the spinal 

 centres require for the elaboration of even complex and co-ordinated 

 reflex movements. Suppose, e.g., that the stimulus is an induction 

 shock applied to a given point of the skin, and that the signal is the 

 closing of the circuit of an electro-magnet, then, if both events are 

 automatically recorded on a revolving drum, the interval can be 

 readily determined. It is evident that this includes, not only the 

 time actually consumed in the central processes, but also the time 

 required for the afferent impulse to reach the brain, and the efferent 

 impulse the hand, along with the latent period of the muscles. The 

 time taken up in these three events can be approximately calculated, 

 and when it is subtracted, the remainder represents the reduced or 

 corrected reaction time that is, the interval actually spent in the 

 centres themselves. This is by no means a constant. It is in- 

 fluenced not only by the degree of complexity of the psychical acts 

 involved, and the mental attitude of the person (whether he expects 

 the stimulus or is taken by surprise, whether he has to choose 

 between several possible kinds of stimuli and respond to only one, 

 etc.), but it varies also for different kinds of sensation, for the same 

 sensation at different times, and, as is recognized in the personal 

 equation of astronomers, in different individuals. For sensations 

 of touch and pain it may be taken as one-ninth to one-fifth, for 

 hearing one-eighth to one-sixth, and for sight one-eighth to one- 

 fifth of a second. So that the proverbial quickness of thought is 

 by no means great, even in comparison with that of such a gross 

 process as the contraction of a muscle (one-tenth of a second). 

 Nor is it the case that the man ' of quick apprehension ' has always 

 a short reaction time, or the dullard always a long one, although in 

 all kinds of persons practice will reduce it. 



SECTION XI. FATIGUE AND SLEEP HYPNOSIS. 



Sleep and Fatigue. Certain gland-cells, certain muscular fibres, 

 and the epithelial cells of ciliated membranes, never rest, and 

 perhaps hardly ever even slacken their activity. But in most 



