THE NERVOUS SYSTEM. 637 



charging its functions. The remaining hemisphere was, in these cases, 

 adequate to the functions generally discharged by both; but the mind 

 does not seem in any of these cases to have been tested in very high 

 intellectual exercises ; so that it is not certain that one hemisphere will 

 suffice for these. In general, the brain combines, as one sensation, the 

 impressions which it derives from one object through both hemispheres, 

 and the ideas to which the two such impressions give rise are single. In 

 relation to common sensation and the efforts of the will, it must always 

 be remembered that the impressions to and from the hemispheres of the 

 brain are carried across the middle line ; so that in destruction or com- 

 pression of either hemisphere, whatever effects are produced in loss of 

 sensation or voluntary motion, are observed on the side of the body 

 opposite to that on which the brain is injured. 



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 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 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 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 appar- 

 ently widely different conditions. Insensibility is equally produced by a 

 deficient and an excessive quantity of blood within the cranium (coma) ; but it 

 was once supposed that the latter offered the truest analogy to the normal con- 

 dition of the brain in sleep, and in the absence of any proof to the contrary, 

 the brain was said to be during sleep congested. Direct experimental inquiry 

 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, Durham 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 corre- 

 spond exactly with what might have been foretold from the analogy of other 



