CENTRAL NERVOUS SYSTEM. 739 



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. 



0.5 See. 



FIG. 210. 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). 



Figure 210 shows this very well. Between the middle of the cerebral 

 hemisphere and the optic lobe, 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 condi- 

 tions which modify the muscular endurance of the individual at different por- 

 tions 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 varia- 

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



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 1 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 are decreased as 

 the result of the beginning fatigue. How these conditions are brought about 

 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 are much less widely diffused in 

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

 stored material is small, large at maturity, and small again in old age, and 

 this holds true for all the groups of cells. Hence the cells would, by reason 

 of this fact, have the greatest capability for work in the middle period. 



