THE CIRCULATION DURING SLEEP 623 



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 

 maintenance and of the healthy performance of their functions. These alter- 

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

 any individual instance, they preserve a general and rather close uniformity. 

 Thus, the periods of rest and work mentioned, in the case of the heart, oc- 

 cupy, each of them, about half a second; in the case of the ordinary respira- 

 tory 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 be- 

 comes 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 in the case of the brain, that 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 proive that the brain becomes visibly paler, anemic, during sleep; and the 

 anema 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 only an inefficient local device for regulating the 

 blood-flow, and that the circulation of the brain is constantly influenced by 

 the variations in general blood pressure. 



Howell and others have studied the circulation by the plethysmographic 

 method during sleep. The results show that with the loss of consciousness, 

 and immediately following, there is a sharp dilatation of the blood vessels of 

 the arm, probably chiefly of the skin, as shown by the increase in volume. 

 The vessels remain dilated until the individual begins to awaken, when there 

 is a rapid constriction with decrease of volume of the organ. 



The dilatation of the general blood vessels draws off the supply of blood 

 from the brain, and the resulting partial anemia contributes to loss of con- 

 sciousness. The blood supply is ample for growth and repair and rest of the 

 nervous system. How efficient this rest period is for the rejuvenation of the 



