THE CEREBRO- SPINAL NERVOUS SYSTEM. 515 



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 in the cranium 

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

 analogy 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 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 Hugh- 

 lings 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 u. e., combustible matter or heat) which may be transformed by 

 the cerebral corpuscles into the various manifestations of nerve-force. 

 During sleep, blood is requisite for only the first of these purposes; and 

 its supply in greater quantity would be not only useless, but, by supply- 

 ing an excitement to work, when rest is needed, would be positively 

 harmful. In this respect the varying circulation of blood in the brain 

 exactly resembles that which occurs in all other energy transforming 

 parts of the body; e. g., glands or muscles. 



At the same time, it is necessary to remember that the normal anaemia 

 of the brain which accompanies sleep is probably a result, and not a 

 cause of the quiescence of the cerebral functions. What the immediate 

 cause of this periodical partial abrogation of function is, however, we do 

 not know, 



Somnambulism and Dreams. What we term sleep occurs often in 

 very different degrees in different parts of the nervous system; and in 

 some parts the expression cannot be used in the ordinary sense. 



The phenomena of dreams and somnambulism are examples of differ- 

 ing degrees of sleep in different parts of the cerebro-spiual nervous 

 system. In the former case the cerebrum is still partially active; but 

 the mind-products of its action are no longer corrected by the reception, 

 on the part of the sleeping sensorinm, of impressions of objects belong- 

 ing to the outer world; neither can the cerebrum, in this half-awake 

 condition, act on the centres of reflex action of the voluntary muscles, 

 so as to cause the latter to contract a fact within the painful experience 

 of all who have suffered from nightmare. 



In somnambulism the cerebrum is capable of exciting that train of 

 reflex nervous action which is necessary for progression, while the nerve- 

 centre of muscular sense (in the cerebellum ?) is, presumably, fulJy 

 awake; but the sensorium is still asleep, and impressions made on it are 



