590 THE NERVOUS SYSTEM 



The association centers are the highest coordinating regions of the nervous 

 system. They are to the sensory and motor centers what these latter are to 

 the reflex centers of the cord, the difference being one of degree and not of 

 kind. Further, they are probably set into activity by the complex of in- 

 flowing or afferent impulses in just the same sense that the spinal reflex cen- 

 ters are set in activity by sensory or afferent stimuli ; the condition is, of course, 

 a thousand times more complex. 



THE PHYSIOLOGY OF SLEEP. 



All parts of the body which are the seat of active change require periods of 

 rest. The alternation of work and rest is a necessary condition of their main- 

 tenance and of the healthy performance of their functions. These alternating 

 periods, however, differ much in duration in different cases; but, for any in- 

 dividual instance, they preserve a general and rather close uniformity. Thus, 

 the periods of rest and work mentioned, in the case of the heart, occupy, each 

 of them, about half a second; in the case of the ordinary respiratory muscles 

 the periods are about four or five times as long. In many cases (as of the 

 voluntary muscles during violent exercise), while the periods during active 

 exertion alternate very frequently, yet the expenditure goes far ahead of the 

 repair, and, to compensate for this, an after-repose of some hours becomes 

 necessary, the rhythm being less perfect as to time than in the case of the 

 muscles concerned in circulation and respiration. 



Obviously, it would be impossible that, in the case of the brain, there 

 should be short periods of activity and repose, or, in other words, of conscious- 

 ness and unconsciousness. The repose must occur at long intervals and must 

 be proportionately long. Hence the necessity for that condition which we call 

 Sleep; a condition which, seeming at first sight exceptional, is only an unusually 

 perfect example of what occurs, at varying intervals, in every actively working 

 portion of our bodies. 



By exposing, at a circumscribed spot, the surface of the brain of a living 

 animal, and protecting the exposed part by a watch-glass, Durham was able 

 to prove that the brain becomes visibly paler, anemic, during sleep; and the 

 anemia of the optic disc during sleep, observed by Hughlings Jackson, may 

 be taken as a strong confirmation, by analogy, of the same fact. 



The Circulation During Sleep. Blood is supplied to the brain in 

 four distinct but anastomosing arteries. This efficient anatomical arrange- 

 ment is obviously all the more important when it is remembered that the cir- 

 culation in the brain has no local device for regulating the blood- flow, but that 

 it must depend on the variations in general blood pressure. Any variation in 

 the flow of blood in the brain depends on changes in general blood pressure; 

 changes which are themselves dependent on variations in the activity of the 

 heart, the caliber of the blood-vessels, etc., discussed on page 186. 



