THE CENTRAL NERVOUS SYSTEM 761 



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 pro- 

 cesses, 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 cal- 

 culated, and when it is subtracted, the remainder repre- 

 sents 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 psychicai 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 sensa- 

 tion, 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. 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 microsco- 

 pical 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; and Mann and other observers in the cells of 

 the cerebral cortex and the anterior horn in dogs fatigued by mus- 

 cular exercise as compared with rested dogs (Fig. 272). The most 



