SLEEP 623 



It may safely be assumed that processes analogous to those just 

 described take place in the brain. By absence of voluntary effort, the 

 muscles have time for rest and for the repair of physiological waste, 

 and their action is for the time suspended. As the activity of the brain 

 involves consciousness, volition, the generation of thought, and, in short, 

 the mental condition observed while awake, complete repose of the brain 

 is characterized by the opposite conditions. It is true that the brain 

 may be rested without sleep, by abstaining from mental effort, by the 

 gratification of certain of the senses and by mental distraction of various 

 kinds, and that the mind may work to some extent during sleep ; but 

 during the period of complete repose a condition necessary to perfect 

 health and full mental vigor consciousness and volition are lost, there 

 is no thought, and the brain, which does not receive blood enough 

 to stimulate it to action, is simply occupied in the insensible repair of 

 its substance and is preparing itself for renewed work. Exhaustion of 

 the muscles produces a sense of fatigue of the muscular system, indis- 

 position to muscular exertion, and a desire for rest, not necessarily 

 involving drowsiness. Fatigue of the brain is manifested by indisposi- 

 tion to mental exertion, dulness of the special senses and a desire for sleep. 

 Simple repose will relieve physiological fatigue of muscles ; and when 

 a particular set of muscles has been used, the fatigue often disappears 

 when these muscles alone are at rest, although others be brought into 

 action. Sleep, and sleep alone, relieves fatigue of the brain. 



During sleep nearly all the physiological processes, except those 

 directly under the control of the sympathetic nervous system, are dimin- 

 ished in activity. The circulation is slower, and the pulsations of the 

 heart are less frequent, as well as the respiratory movements. These 

 points have already been considered in connection with the physiology 

 of circulation and respiration. Physiologists have little positive infor- 

 mation in regard to the relative activity of the processes of digestion, 

 absorption and secretion during sleep. The drowsiness which many 

 persons experience after a full meal may be due in part to a determi- 

 nation of blood to the alimentary canal and a consequent diminution in 

 the supply to the brain. 



It must be that fatigue, as well as sleep, has a physical basis. While 

 fatigue may be in part explained by the assumption that during exer- 

 cise the reparative processes are for the time unequal to the wear or 

 waste, this does not fully account for the nervous phenomena observed. 

 It has been shown, indeed, by Ranke, and later by Mosso, that the 

 blood of a fatigued animal injected into an animal at rest will produce 

 promptly all the phenomena of fatigue. This is explained by the 

 assumption that exercise involves the production of what may be called 



