

THE NERVOUS SYSTEM. 135 



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 condi- 

 tion of their maintenance and of the healthy performance of their func- 

 tions. These alternating periods, however, differ much in duration in 

 different cases; but, for any individual instance, they preserve a general 

 and rather close uniformity. Thus, as before mentioned, the periods of 

 rest and work, 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, again (as of the volun- 

 tary 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 con- 

 sciousness and unconsciousness. The repose must occur at long inter- 

 vals; and it must therefore 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. 



A temporary abrogation of the functions of the cerebrum imitating 

 sleep, may occur, in the case of injury or disease, as the consequence of 

 two apparently widely different conditions. Insensibility is equally pro- 

 duced by a deficient and an excessive quantity of blood within the cranium, 

 (coma); but it was once supposed that the latter offered the truest anal- 

 ogy to the normal condition of the brain in sleep, and in the absence of 

 any proof to the contrary, the brain was said to be during sleep con- 

 gested. Direct experimental enquiry has led, however, to the opposite 

 conclusion. 



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

 living animals, and protecting the exposed part by a watch-glass, Dur- 

 ham was able to prove that the brain becomes visibly paler (anaemic) 

 during sleep; and the anaemia of the optic disc during sleep, observed by 

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

 of the same fact. 



A very little consideration will show that these experimental results, 

 correspond exactly with what might have been foretold from the analogy 

 of other physiological conditions. Blood is supplied to the brain for two 

 partly distinct purposes. (1.) It is supplied for mere nutrition's sake. 

 (2.) It is necessary for bringing supplies of potential or active energy, 



