THE PHYSIOLOGY OF SLEEP 673 



sympathetic ganglia, and pass through the rami communicantes, to end by 

 terminal arborizations in the spinal root ganglia, chiefly around cells of 

 the Dogiel type. The number and significance of this class of afferent 

 neurones remain as yet uncertain. Though their spinal relations suggest 

 some association with the phenomenon of referred pain. 



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 alternating periods, however, differ much in duration in different 

 organs or regions of the body. But, for any individual instance they pre- 

 serve 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 neces- 

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

 muscles concerned in circulation and respiration. 



Obviously, short periods of activity and repose, or in other words, of con- 

 sciousness and unconsciousness would be impossible in the case of the brain. 

 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, at first sight seeming exceptional, is only an unusually perfect exam- 

 ple of what occurs at varying intervals in every actively working portion 

 of our bodies. 



By exposing the surface of the brain of a living animal at a circum- 

 scribed spot, 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 Hugh- 

 lings Jackson, may by analogy be taken as a strong confirmation 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 

 circulation in the brain has only an inefficient local device for regulating 

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

 enced by the variations in general blood pressure, see page 242. 



Howell and others have studied the circulation by the plethysmographic 

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



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