434 POPULAR SCIENCE MONTHLY. 



favored by the removal of all that may excite either the attention or 

 the reflexes. Darkness, quiet, bodily comfort and mental serenity are 

 therefore sought. Sleep may be prevented by any of the contrary 

 conditions — light, noise, pain or anxiety. When it is necessary to 

 contend against drowsiness, one instinctively seeks objects for atten- 

 tion or sensory stimulation, such as may be secured by taking a 

 slightly uncomfortable position. Evidently sleep presupposes a release 

 of the brain from many stimuli and may be warded off by seeing to 

 it that no such release is granted. There is on record the case of an 

 unfortunate boy who had no cutaneous sensibility, was blind in one 

 eye and deaf in one ear. His mentality was of a low order. To cause 

 him to sleep it was only necessary to cover the serviceable eye and 

 ear for a few moments. Here the waking condition was clearly de- 

 pendent on an unceasing flow of sensory impulses into the brain. A 

 person of higher intelligence, similarly afflicted, would doubtless sleep 

 much less readil}', for trains of thought might keep him awake in 

 default of external stimuli. 



The approach of sleep is accompanied by distinct vascular changes. 

 The blood stream is shifting its bed. A most imperious summons to 

 sleep comes from the dryness of the eyes, the sign, probably, of a 

 lessened blood-flow through the tear glands. At the same time the 

 temperature of the skin rises, possibly excepting that of the extremi- 

 ties. There is evidence then of a dilatation of the cutaneous vessels 

 as sleep comes on, and the final passage into unconsciousness is accom- 

 panied by a considerable further dilatation. These vascular changes 

 have been nicely gauged by what is known as the plethysmographic 

 method, where the subject lay with one hand and forearm fixed in a 

 glass cylinder filled with water. An increase of blood in the arm 

 displaced water from the cylinder and a delicate recording apparatus 

 showed how this dilatation came on with sleep and passed oS with 

 waking. An account of such experiments, of more than technical 

 interest, is that contributed by Dr. W. H. Howell to The Journal of 

 Experimental Medicine (Vol. II.). 



It is generally inferred that the cutaneous dilatation at once re- 

 duces the general blood-pressure and the quantity of blood flowing 

 through the brain, by diverting a large share to the skin. The lower- 

 ing of pressure has been demonstrated by Brush and Fayerweather ; 

 the fact that there is anemia of the brain during sleep has been estab- 

 lished by direct observation. An English physiologist. Hill, has been 

 led to believe that the dilatation of blood-vessels that relieves the brain 

 in sleep is not limited to the skin, but shared by the arteries of the 

 digestive tract. That this is so is difficult to prove, but it is suggestive 

 that a heavy meal is followed by a long sleep in the case of the lower 

 animals and often with us by a hard struggle with drowsiness. 



