264 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



tion of the brain during sleep, and some physiologists have be- 

 lieved that this condition precedes and causes the state of sleep, 

 while others take the opposite view that it follows and is merely 

 one result of sleep. On the basis of the plethysmographic ex- 

 periments mentioned above the author* has proposed a theory 

 of sleep in which the diminished flow of blood to the brain is ex- 

 plained and is assumed to be the chief factor in bringing on sleep. 

 The theory assumes that the periodicity of sleep is dependent 

 mainly upon a rhythmical loss of tone in the vasomotor center in 

 the medulla in consequence of fatigue from continued activity 

 during the waking hours. That is, the vasomotor center is in 

 constant action during this period; the continued flow of sensory 

 stimuli and the constant activity of the brain act reflexly on this 

 center and through it cause a constriction of the blood-vessels of 

 the body, particularly of the skin, by means of which the blood- 

 flow through the brain is maintained with an adequate velocity. 

 In consequence of this varying but constant activity the center 

 undergoes fatigue; stronger and stronger stimulation is necessary 

 to maintain its normal tone, and eventually its effect on the blood- 

 pressure becomes insufficient to maintain an adequate flow through 

 the brain and unconsciousness or sleep results, even against one's 

 desires, as is shown by the experience of those who have attempted 

 to keep awake much beyond the habitual period. Ordinarily, 

 however, this fatigue of the vasomotor center and its resulting 

 tendency to a cessation of activity is favored by our voluntary 

 withdrawal of stimulation. Our preparations for sleep, closure of 

 eyes, darkened and if possible quiet room, cessation from disturbing 

 thoughts, result in a diminution of the sensory and mental stimuli 

 that normally play upon the vasomotor center. The cessation 

 of such stimuli may, indeed, at any time be all that is necessary 

 to bring about a partial loss of activity in this center, a les- 

 sened flow of blood through the brain, and a period of sleep which, 

 however, is usually short. If, however, the vasomotor center has 

 been previously fatigued, as may be supposed to be the case at the 

 end of the day, the withdrawal of these stimuli permits it to fall 

 into a more complete state of inactivity, and the diminution of 

 blood-flow to the brain and the state of unconsciousness is longer 

 lasting, — lasts indeed, according to the curves of which an example 

 is given in Fig. 116, until the gradual resumption of activity in the 

 vasomotor center brings about a constriction of the blood-vessels 

 of the body and thus drives enough blood through the brain to 

 cause spontaneous awakening. A third factor which must aid in 

 the production of unconsciousness as a result of the lessened flow 

 of blood, and in the return of consciousness in connection with 

 * Howell, "Journal of Experimental Medicine," 2, 313, 1897. 



