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 anemia. 
It is true others have presented a contrary hypothesis, namely, that 
sleep is due to an influx of blood to the brain, while 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 
~ obhged to walk constantly he fell asleep, and while 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 while 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 betweep 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 
