THE CENTRAL NERVOUS SYSTEM 873 



approximately calculated, and when it is subtracted, the re- 

 mainder 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 influenced 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 recognised 

 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. 



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 

 intervals 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. 366). 



According to Dolley, who has made the most recent observations 

 on this subject, there is, as a result of continued activity, at first a 

 steady increase of the basic chromatic material. This increase 

 affects first the extra-nuclear chromatin, the Nissl substance, which, 

 according to the most modern view, is really nuclear substance 

 distributed through the cytoplasm, and functions as such (Gold- 

 schmidt) . The size and number of the granules are increased, and 

 some of the chromatic material is diffused throughout the cytoplasm, 

 as indicated by diffuse staining. Then the intranuclear chromatin 

 also undergoes an increase, and the size of the cell is increased too. 



