302 THE NERVOUS SYSTEM 



this organ. The brain works longer, and must, therefore, 

 rest longer at a time than most of the other organs of the 

 body. True, so far as the voluntary muscles are concerned 

 they rest best probably when the brain is resting, but the lat- 

 ter condition is not a necessary one for the maintenance of 

 their physiological integrity. This repose of the brain this 

 temporary abolition of the cerebral functions is sleep. 

 While, of course, the activity of that organ during wakeful- 

 ness may be increased or diminished by volition, and it may, 

 therefore rest from a comparative standpoint as when one 

 ceases to think actively upon a subject and becomes men- 

 tally listless still the brain can never, under such circum- 

 stances, rest properly, and sleep finally becomes imperative. 



Vascular Phenomena of Sleep. Coma is analogous to 

 sleep in that consciousness is lost ; but in this case the brain 

 is congested and the condition is unnatural. It was long 

 supposed that this was the vascular condition during natural 

 sleep, but application of the physiological principles prevail- 

 ing in other parts of the body would rather presuppose a 

 condition of cerebral anemia; for the brain receives blood 

 for two purposes first, to supply nutrition to the nervous 

 substance, and second, to bring supplies which, by the ac- 

 tion of the brain cells, may be converted into nerve force 

 and during sleep only the first of these purposes is to be 

 served. This is true in case of glands, muscles, etc., during 

 their intervals of repose. As a matter of fact, the cerebral 

 vessels are contracted and there is much less blood in the 

 brain during sleep than during consciousness. 



Dreams. In explanation of the phenomena of dreams and 

 somnambulism, it is said that what we call sleep may occur in 

 one part of the brain and not in another, or in different de- 

 grees in different parts of the nervous centers. "In the 

 former case [dreams] the cerebrum is still partially active ; 

 but the mind products of its action are no longer corrected 

 by the reception, on the part of the sleeping sensorium, of 

 impressions of objects belonging to the outer world ; neither 



