THEO&IES OF SLEEP. 457 



supply of blood, here, as in other parts, under varied physio- 

 logical conditions. The vessels of the brain are provided 

 with vaso-motor nerves, and it is sufficient to have noted 

 that the arteries are contracted during sleep, the mechanism 

 of this action being well established by observations upon 

 other parts of the circulatory system. Contraction of the 

 vessels of the pia mater has been observed by Kathnagel 

 and others, though there is some discussion with regard to 

 its exciting cause. 1 



It must be acknowledged that we know but little of the 

 intimate nature of the processes of nutrition of the brain 

 during its functional activity and in repose ; but there can 

 be no doubt of the fact that there is more or less cerebral 

 action at all times when we are awake. Though the mental 

 processes are much less active during sleep, even at this time, 

 the operations of the brain are not always suspended. It is 

 equally well established, that exercise of the brain is attended 

 with physiological waste of nervous substance, and, like other 

 parts of the organism, its tissue requires periodic repose to 

 allow of the regeneration of the substance consumed. Analo- 

 gies to this are to be found in parts that are more easily 

 subjected to direct observation. The muscles require repose 

 after exertion, and the glands, when not actively engaged in 

 discharging their secretions, present intervals of rest. 3 As 

 regards the glands, during the intervals of repose, the supply 

 of blood to their tissue is very much diminished. It is prob- 

 able, also, that the muscles in action receive more blood than 

 during rest ; but it is mainly when these parts are not active, 

 and when the supply of blood is smallest, that the processes 

 of regeneration of tissue seem to be most efficient. As a 

 rule, the functional activity of parts, while it is attended 



1 A. reference to these experiments is to be found in the Journal of Anatomy 

 and Physiology, Cambridge and London, 1871, vol. v., p. 401. 



2 Luys has compared the condition of repose of the brain, with its diminished 

 supply of blood, to the period of inactivity of the glands (Recherches sur k 

 systhae nerveux, Paris, 1865, p. 450). 



