744 FUNCTIONS OF THE CEREBRUM [CH. L. 



Sleep and Narcosis. 



The conditions that favour sleep are : 



(1) A diminution of the impulses entering the central nervous 

 system by the afferent channels. This is under our voluntary 

 control, as, for instance, in closing the eyes, or retiring to a quiet 

 room. 



(2) Fatigue. This diminishes the readiness of the central 

 nervous system to respond to stimuli. 



The first two hours of sleep are always the most profound ; later 

 on, relatively weak stimuli will cause awakening. Of the parts of 

 the central nervous system, the spinal cord is always less profoundly 

 affected than the brain, but even the brain is never entirely irrespon- 

 sive, and unless slumber is very profound, dreams are the subjective 

 result of external stimuli. Sensations of sound appear to be the 

 last to disappear as sleep comes on, and the first to be realised 

 on awakening. 



Sleep has been attributed by some to changes in the blood-supply 

 of the brain, and ultimately referred to fatigue of the vaso-motor 

 centres. The existence of an effective vaso-motor mechanism in the 

 cerebral blood-vessels themselves is problematical (see p. 313); so 

 that if changes occur in the cerebral blood-pressure or rate of flow, 

 they are mainly secondary to those which are produced in other 

 parts of the body. Plethysmographic records from the arm of a 

 sleeping man show a diminution in its volume every time he is 

 disturbed, even though the disturbance may not be sufficient to 

 awaken him. This is interpreted as meaning a diminution in the 

 blood of the body, and a corresponding increase in the blood-flow 

 through the brain. It is, however, quite possible that the vascular 

 condition is rather the concomitant or consequence of sleep than its 

 cause. Howell among others believes it to be the cause, and attributes 

 the sleepiness that follows a heavy meal to the mechanical effect of a 

 dilatation of the abdominal vessels in producing a diminished blood- 

 flow through the brain ; but the sleep that normally comes on at the 

 end of the day, he believes to be produced by cerebral anaemia follow- 

 ing dilatation of the blood-vessels of the skin, such dilatation being 

 due to vaso-motor fatigue. 



Some of the theories to account for sleep have been chemical. 

 Thus certain observers have considered that sleep is the result of the 

 action of chemical materials produced during waking hours, which 

 have a soporific effect on the brain; according to this theory 

 awakening from sleep is due to the action of certain other materials 

 produced during rest, which have the opposite effect. Obersteiner 

 has gone so far as to consider that the soporific substances are 

 acid in nature, and others regard them as alkaloidal. These theories 



