ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. 707 



ie eyeball is turned upward and inward; at the same time hearing 

 disappears and consciousness vanishes. During sleep the metabolic 

 processes of nutrition are slowed, and there is a diminution of the 

 heart-beats, of the arterial tension, and of the movements of respira- 

 tion. Sleep is deepest during the first one and one-half hours ; after 

 that its depth greatly diminishes. Durham was the first to show that 

 during sleep the brain is anaemic, but it is only an epiphenomenon, 

 and not the cause of sleep. Plethysmographic tests of the arm in 

 a sleeping person show a decrease of volume whenever the subject 

 is disturbed, although the noise may not be sufficient to wake him. 



Fig. 285. Curve of the Depth of Sleep. ( PIESBEKGEN. ) (From 

 Tigerstedt's "Human Physiology," copyright, 1906, by D. Appleton and 

 Company. ) 



Read from left to right. 



This means that the brain is anasmic during sleep, and that the 

 blood-supply of the brain is increased upon waking. 



The histological theory of Demoor is that during sleep the den- 

 Irons are retracted and break the connections between the dendrons 

 id arborizations which are necessary for the action of the nerve- 

 centers. Demoor found that in deep anaesthesia there were moniliform 

 varicosities on the dendrons. The chemical theory is that during 

 rakefulness certai^i fatigue-products (lactic acid, etc.) are generated, 

 rhich have a somnolent effect upon the brain. If the blood of an 

 diausted dog is transfused into a dog awftke, it will cause him to 

 fatigued. It is probable that the fatigue of the brain-cells, the 

 law of periodicity of the action of the nerve centers, and a decrease of 

 external stimuli are the main causes of sleep. The intimate cause 

 not known. 



