THE PHYSIOLOGY OF SLEEP. 247 



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. 109, 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 

 the increased flow of blood, is the greater or less fatigue of the 

 cortical cells themselves after a day's activity, and their greater 

 irritability after a night's rest. Many factors, therefore, co-oper- 

 ate in the development of the normal state of sleep lasting for 

 six to eight hours out of twenty-four, but the central factor which 

 explains its rapid onset, involving nearly simultaneously all the 

 conscious areas of the brain, whether previously fatigued or not, 

 and the equally sudden restoration to consciousness of the entire 

 cortex, is to be found in the amount of blood-flow to the brain. 

 Under normal conditions this is the factor that stands in most 

 immediate relation to that appearance and disappearance of full 

 consciousness which mark for us the limits of sleep. A similar 

 view is advocated by Hill,* who believes, however, that the regu- 

 lation of the blood-flow through the brain is effected through the 

 vasomotor control of the splanchnic area, whereas the author's 

 view is that the regulation is effected mainly through variations 

 in the cutaneous circulation, that is, for the normal occurrence 

 of sleep. The drowsiness that follows a heavy meal is probably due 

 mainly to the mechanical effect of a dilatation of the blood-vessels of 



* Hill, "The Physiology and Pathology of the Cerebral Circulation," Lon- 

 don, 1896. 



