CENTRAL NERVOUS SYSTEM. 291 



Time Taken in Central Processes. — All processes in the nervous system 

 take time, and are for the most part easy to measure. The rate of the nerve- 

 impulse has already been given. When, however, it passes from one element 

 to another, the delay is even more marked, and it is plausible to assume that 

 this detention occurs at the juncture of the elements. Thus in those parts of 

 the central system where the cell-elements and also the cell-junctions are most 

 numerous, the time taken is longest. 



Fig. 122 shows this very well. Between the middle of the cerebral hem- 

 isphere and the optic lobes, although the distance is short, the impulse takes 

 twice as long to travel as between the bulb and the lumbar enlargement. 

 When this time is measured in the conscious individual it is, of course, open 

 to a long series of modifying conditions, and these appear to be in part the 

 same conditions which modify the muscular endurance of the individual at 

 different portions of the day. Thus it has been determined that the speed 

 with which reactions can be made as indicated by the reaction time, is subject 

 to variations, and does not steadily decrease from the morning to the evening. 



Fig. 122.— To show the rate at which impulses pass through the nervous system of a frog. At the 

 extreme left the vertical has the value of 0.5 second and the other verticals are compared with it; thus 

 between the cerebrum and the optic lobe requires about 0.25 second ; between the bulb and the lumbar 

 enlargement a greater distance— only about half the time ; and for the still greater distance represented 

 by the length of the sciatic nerve even less time is needed (Exner). 



It has been the purpose of the paragraphs just preceding to indicate that 

 through the day it is not possible to demonstrate a steady decline of power in 

 the nervous system. We begin the morning, to be sure, feeling fresh, and 

 are fagged in the evening, but the course by which this condition has been 

 attained is not a simple or direct one. 



D. Sleep. 



Conditions Favoring Sleep. — To recover from fatigue sleep is required. 

 The prime condition favoring sleep is the diminution of nerve-impulses pass- 

 ing through the central system. This is accomplished in two ways. In the 

 first instance it is usual to reduce all incoming stimuli to a minimum. This 

 is most directly under our own control. On the other hand, the permeability of 

 the nervous system and the intensity with which it responds arc decreased as 

 the result of the beginning fatigue. I low these conditions arc brought aboul 

 has been a matter of much speculation and some experiment. 



The parts played by the sensory and that by the central cells vary some- 

 what at different times of life, for impulses arc much less widely diffused in 

 early years than at maturity. Moreover, in childhood the amount of stored 

 material is small, large at maturity, ami small again in old age, and this holde 



