610 SLEEP. 



through accident, pressure upon the brain with the hand at once 

 sends the person to sleep. A full meal causes drowsiness as long 

 as the food is in the stomach, perhaps from the great activity of 

 the organ, so that, from the general sympathy, the brain among 

 the rest is drained of its power : some ascribe a little to the 

 more difficult expansion of the chest, and consequent accumula- 

 tion of blood in the head. Fat and plethoric people are drowsy, 

 and in them there is excessive fulness of the blood-vessels of the 

 head from plethora and from the obstruction produced by dif- 

 ficult respiration. 



The causes of waking are the opposite of those of sleeping. 

 The accumulation of vigour gradually proceeds while sleep lasts, 

 till the brain is spontaneously active again. But, before this, we 

 may awake from an external excitant, to whatever sense it 

 may be applied ; from any internal causes of feeling, mental, or 

 in the body at large ; or from excitement having been so strong 

 before sleeping that the brain will not remain torpid ; from ex- 

 citement of the nutritive functions of the brain, its circulation, 

 evolution of heat, &c. ; from the agency of certain substances 

 which possess the property of keeping the mind active, as tea, 

 coffee, which may prevent sleep altogether or cause it to be 

 short. 



'The proximate cause of sleep or the condition of the brain in 

 it has been variously viewed. Some have fancied the brain 

 compressed, and compression will disqualify the brain for its 

 functions and cause sleep, even coma, apoplexy, and death : but 

 there is no proof or even probability of this in ordinary sleep. 

 Blumenbach says, he thinks that sleep " probably consists in a 

 diminished or impeded flow of oxygenated (arterial) blood to 

 the brain ; for that fluid is of the highest importance, during the 

 waking state, to the re-action of the sensorium upon the functions 

 of the senses and upon the voluntary motions. r 



* " Those who wish to know and compare other opinions upon the causes of 

 sleep, may consult, 



M. de Grimaud, MJmoire sur la Nutrition. Petersb. 1789. 4to. p. 194. 



H. Nudow, Versuch einer Theorie des Schlafs. Kbningsberg. 1791. 8vo. 



Steph. Gallini at the end of his Saggio d' Osservazioni sui nuovi progressi della 

 Fisica del Corpo Umano. Padua. 1792. 8vo. 



Mauduit, in Fourcroy, in the Medecine Eclairfa, $c. t. iv. p. 273. 



T. Chr. Reil, Functiones Organo Animce Peciitiares. Hal. 1749. 8vo. 

 p. 108. 



