PHYSIOLOGY OF SLEEP LEGENDRE. 595 



retraction; if the blood retracts completely it means death." More 

 recently many have likewise held that sleep is due to cerebral anaemia. 

 It is true others have presented a contrary hypothesis, namely, that 

 sleep is due to an influx of blood to the brain, wliile others still, dis- 

 countenancing these contradictory statements or basing their opin- 

 ions upon other experiments, have reached the conclusion, which 

 seems to be the best, that neither sleep nor wakefulness depend upon 

 cerebral circulation. 



Sensibility is preserved during sleep, since we can then hear, feel, 

 and smell. The working of the sense organs seems, however, to be 

 modified. The eyes are generally shut tight; the tears less abundant, 

 and by this diminution of lachrymal secretion is explained the pricking 

 sensation which precedes the desire to sleep. Under the closed eyelids 

 the eyes are directed upward and diverge, according to certain 

 writers; and the pupils are contracted, being dilated again just at 

 the moment of awaking. The sensitiveness of other sense organs is 

 diminished, but it is hard to tell whether this reduction is due to the 

 organs themselves or to the quieting of the nerve centers. 



The muscles are generally relaxed. One may move very frequently 

 while asleep, and we have read of persons sleeping on horseback or 

 while walking, by which certain muscles are necessarily contracted. 

 Galien, who heard such a statement made not long since, put no faith 

 in it; but he was compelled to believe it when one night that he was 

 obliged to walk constantly he fell asleep, and wliile going the distance 

 of a furlong was distracted by a dream. The nerve centers also 

 undergo modifications. The reflex movements dependent upon them 

 increase a little before sleep, diminish, then disappear. So the subject 

 who falls asleep and is tickled by merely touching the nose, hand, or 

 foot responds less and less to these irritations and reacts only to those 

 that are made more and more intense. 



The cells of the nerve centers of animals killed wliile awake or asleep 

 have likewise been examined to ascertain if the cells present any vari- 

 ations in appearance, and if the difference noted can be considered a 

 histological cause of sleep. Stefanowska observed nothing in the case 

 of sleeping mice, which was not astonishing, for in killing them they 

 woke up. But others have believed they could discern something, 

 and Mathias Duval has explained sleep as well as all other cerebral 

 phenomena, by an ingenious hypothesis, which though without 

 foundation has had the good fortune to rapidly become classic on 

 account of its simplicity, and of which you have surely been informed. 

 The nerve cells lengthen or contract and so permit or prevent com- 

 munications between the centers; sleep would be simply due to their 

 contraction. 



Such are the physiological phenomena that have been observed 

 when man is asleep. The simple observation of slumber has enabled 



