FATIGUE. 



315 



the latter have in some way become incapable of 

 reconstruction. With this change in the nerve elements 

 goes the remarkable power in the blood vessels to vary 

 their diameter, and thus control the supply of blood. 

 The limits of this variation probably increase to 

 maturity, and in old age are greatly diminished, so that 

 the calibre becomes more constant. The degrees to 

 which the blood supply can be controlled, and the 



HOURS <X5 LO I.S .0 ZS 3.0 3.5 40 4.5 SO 5.5 6.0 



7.0 7.5 7.8 



FIG. 72. Curve illustrating the strength of an auditory stimulus 

 (a ball falling from a height) necessary to waken a sleeping 

 person. The hours are marked below. The tests were made 

 at half-hour intervals. The curve indicates that the distance 

 through which the ball required to be dropped increased 

 during the first hour and then diminished, at first very rapidly, 

 then slowly. (Kohlschiitter.) 



amount of substance capable of yielding energy at 

 various periods of life are so different, that considering 

 these factors alone, though there are probably others, it 

 may be easily appreciated that the sleep of childhood, 

 maturity, and old age should be quite different. 



During sleep stimuli can always produce an effect, 

 otherwise a person could not be wakened. When 



