THE BRAIN 249 



The interruption of psychical functions by sleep can be- 

 accounted for by the rest of the nerve cells of the cerebral 

 cortex. How this rest is brought about is not known. The 

 supposition that cessation in the activity of the cells is due 

 to fatigue or lack of blood in the cells of the brain does not 

 explain all the phenomena of sleep. Sleep also depends 

 upon the stimulation of the sense organs. A person can 

 be made to sleep by withdrawing the stimulation of the 

 senses as far as possible. Customary sensations do not dis- 

 turb sleep, strange sensations do. Sometimes the cessation 

 of customary sensations awakes the sleeper. (The awaking 

 of the miller when the mill stops.) 



During sleep only the functions of the cerebral hemispheres 

 cease ; the other centres of the central nervous system (reflex and 

 coordinated centres) may remain active. The eyelids are closed 

 during sleep, the eyes are turned inward and upward, the pupils 

 are contracted, respiration is slower. Metabolism is less during 

 sleep than during waking hours. 



Dreams are due to less profound sleep. Somnambulism and hyp- 

 notism are abnormal conditions of partial sleep. 



Chemical composition and metabolism of the central nervous 

 organs. The white substance of the central nervous system con- 

 tains 31$ solids, including proteid and collagen 8$, lecithin 3$, 

 cholesterin and fat 15$, protagon 3$; besides these, some substances 

 containing nitrogen and phosphorus insoluble in ether (nuclein, 

 neuro-keratin, jecorin) 1.5$; salts 0.2$. 



The gray substance contains 18$ solids, including proteid and 

 collagen 10$, lecithin 3^, cholesterin and fat 3.5$, cerebrin and 

 substances insoluble in ether i$, salts 0.5$. 



Nothing is known concerning the metabolism in the spinal cord 

 and brain. Metabolism is not increased to an appreciable extent 

 by mental work. The abundance of blood in the brain and the 

 fact that stoppage of blood supply paralyzes the nerve cells in a few 

 minutes, indicates that the metabolism is very energetic. 



The cerebro -spinal fluid which surrounds the central nervous sys- 

 tem and fills its cavities has a specific gravity of 1.005. ^ con- 

 tains 11.5$ solids, in which proteids are either absent or only 

 present in traces. In it has been found a substance which reduces 

 cupric oxide and appears to be pyrocatechin. 



