THE PHYSIOLOGY OF SLEEP. 265 



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

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

 imtability 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 folloAvs a heavy meal is probably due 

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

 the viscera and the consequent diminution in the blood-flow 

 through the brain; but the sleep that occurs at the end of the day 

 is associated with a dilatation of the blood-vessels of the skin of 

 the trunk and extremities. What the condition in the visceral 

 organs may be at such times we have at present no means of 

 knowing. It should be stated that in opposition to these views 

 regarding an anemic condition of the brain during sleep, Shepardf 

 reports, as the result of an extensive series of plethysmographic 

 observations made upon two individuals with defects in the skull, 

 that during sleep the volume of the brain is increased. He 

 believes, therefore, that during this period there is a vascular 

 dilatation in the brain and an increased circulation, that the brain, 

 in fact, receives its greatest supply of blood during its period of 

 least activity. This conclusion is at variance with what we 

 should expect from observations on the blood-flow in other organs 

 during their periods of maximum and minimum functional activity. 

 The author concludes also from his observations that the brain 

 must possess an efficient supply of vasomotor nerves, which during 

 the waking hours are in tonic activity, but in sleep suffer a dim- 

 inution in tone that leads to a local dilatation. Evidence from 

 other sources (p. 624) , on the contrary, makes it rather improbable 

 that the brain possesses an efficient vasomotor system. 



Hjrpnotic Sleep. — The sleep that can be produced by so-called 



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

 London, 1896. 



t Shepard, loc. cit. 



