NOURISHMENT OP THE BRAIN 677 



According to some experimental determinations the carbon dioxide elimi- 

 nation reaches its minimum in the second hour of sleep, and this probably 

 constitutes to a certain extent the expression of the deepest sleep in a sleeping 

 period of perhaps six to eight hours. 



The following peculiarities have also been observed in sleep. The eyes with 

 pupils contracted are turned inward and somewhat upward. The respiratory 

 movements are less frequent than in the waking condition, and even in' the man 

 are mainly of the costal type. The respirations are also sometimes periodically 

 suspended. The heart action is retarded; the, vascular tone decreases in the 

 cutaneous vessels and probably also in the visceral vessels, and as a consequence 

 the blood pressure falls. This in its turn is said by many authors to cut down 

 the supply of blood to the brain, to produce in short a condition of cerebral 

 anemia. 



Howell has observed the volumetric variations of the hand and the lower 

 part of the forearm in sleep by means of the plethysmograph, and has found 



FIG. 299. Curve representing the depth of sleep, after Piesbergen. The abscissae represent 



hours. 



that the amount of blood in the part increases gradually from the beginning of 

 sleep and reaches its maximum within one to one and three-quarter hours. It 

 remains at this level until about three-quarters of an hour before awakening 

 and then falls rather rapidly to the end of sleep. 



The inception of sleep is favored by cutting off the sensory stimuli, espe- 

 cially if the attention be not kept aroused by any active mental processes. 

 Striimpell has reported a case in which the patient became blind in one eye 

 and deaf in one ear merely by stopping all cutaneous sensations. As soon as 

 the good eye was closed and the functional ear was stopped he fell asleep. 



Sleep does not depend entirely upon processes going on in the cerebral cor- 

 tex, for as mentioned at page 623 a change from the sleeping to the waking 

 condition and vice versa can be observed on decerebrated animals. 



[Perhaps the most satisfactory theory which has yet been given to explain 

 the cause of sleep, is that of Howell. The dilatation of the cutaneous vessels 

 during sleep observed by this author, taken in conjunction with many other 

 observations that there is during sleep a reduction of the general blood pressure, 

 and that there is at the same time a diminished blood flow to the brain, sug- 

 gested the idea that the depression of the psychical activities below the threshold 

 of consciousness is due primarily to ana?mia of the brain (cf. page 240). To 



