FATIGUE AND SLEEP HYPNOSIS 983 



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 

 organs periods of action alternate at more or less frequent inter- 

 vals with periods of relative repose. In all the higher animals 

 the central nervous system enters once at least in the twenty-four 

 hours into the condition of rest which we call sleep. What the 

 cause of this regular periodicity is we do not know. It is 

 accompanied by changes in the microscopical appearance of the 

 nerve-cells. Thus, Hodge found differences between the cells of 

 certain portions of the cerebral cortex in birds, and of certain 

 ganglia in the honey-bee after a long day of work and after a 

 night's rest. Mann, Lugaro, and other observers, found similar 

 differences in the cells of the cerebral cortex and the anterior 

 horn, and Dolley in the Purkinje's cells of the cerebellum in 

 dogs fatigued by muscular exercise as compared with rested dogs 

 (Fig. 398). 



