THE SYMPATHETIC SYSTEM 301 



Now we may say that it is the function of the brain to furnish 

 consciousness if we can allow that consciousness embraces all 

 the various manifestations of nerve force peculiar to the brain. 

 For the brain to suspend this function at frequent intervals like 

 the heart (e. g.) would be manifestly impossible if one is to do 

 any consecutive work depending upon 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 volun- 

 tary muscles are concerned they rest best probably when the 

 brain is resting, but the latter 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 

 wakefulness 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 mentally list- 

 less still the brain can never, under such circumstances, 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 con- 

 gested and the condition is unnatural. It was long supposed that 

 this was the vascular condition during natural sleep, but appli- 

 cation of the physiological principles prevailing 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 action of the brain cells, may be con- 

 verted 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 



