466 STUDIES IN ADVANCED PHYSIOLOGY. 



of these centers suspended for a moment. In the higher 

 conscious centers of the brain there is, however, a marked 

 exception. At certain periods there supervenes what we 

 familiarly designate as "sleep." This common phenomenon 

 so easily experienced is nevertheless one of the most difficult 

 of explanation, and we are at present entirely at a loss to 

 understand what the exact nature of sleep is. We know 

 that it concerns only the higher conscious centers. The 

 reflex centers below are in their regular activity, unless we 

 should modify this by the statement that during sleep the 

 activities here are sometimes reduced, but never suspended. 



Going to sleep is a sudden thing, although it is preceded 

 by a short period in which the sensations become gradually 

 dimmed. Waking up, too, is a somewhat sudden event. 

 What an interesting question it would be to determine, if 

 possible, just what occurred when sleep suddenly super- 

 venes or when later on with similar suddenness conscious- 

 ness returns. It is held by some physiologists that sleep 

 results when no impressions reach the brain. This, of 

 course, is at once faulty in its general application, because 

 it is possible to go to sleep even amidst a confusion of noises 

 and sensations. On the other hand, however, experiments 

 have been made with animals with paralyzed sensory nerves 

 and which were in addition to this blind and deaf on one 

 side. When under such circumstances the only remaining 

 sources of sensation to the brain, that is, the opposite eye 

 and ear were closed, the animal at once dropped to sleep 

 and awakened as soon as these avenues of impression were 

 opened. 



It has been suggested, but only as a suggestion and not 

 a scientific fact, or even theory, that sleep may result from 

 the slight withdrawal of the dendrons surrounding the cere- 

 bral cells. It is, of course, conceivable that these dendrons 

 might separate to a slight extent, possibly separate so far as 

 to make the transmission of an impulse more difficult, and 

 that this separation, like the opening of the switch on 

 a switch board would produce a cessation of the flow of 



