CONDITION OF THE BRAIN DURING SLEEP. 651 



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. 

 As regards the glands, during the intervals of repose the supply of blood to 

 their tissue is much diminished. It is probable, 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 activity of 

 parts, while it is attended with an increased supply of blood, is a condition 

 more or less opposed to the processes of repair, the hyperaemia being, appar- 

 ently, a necessity for the marked and powerful manifestations of their pecul- 

 iar action. When the parts are active, the blood seems to be required to keep 

 at the proper standard the so-called irritability of the tissues and to increase 

 their power of action under proper stimulus. Exercise increases the power 

 of regeneration and favors full development in the repose which follows ; but 

 during rest, the tissues have time to appropriate new matter, and this does 

 not seem to involve a large supply of blood. A muscle is exhausted by pro- 

 longed exertion ; and the large quantity of blood passing through the tissue 

 carries away carbon dioxide and other products of disassimilation, which are 

 increased in quantity, until it gradually uses up its capacity for work. Then 

 follows repose ; the supply of blood is reduced, but under normal condi- 

 tions, the tissue repairs the waste which has been excited by action, the blood 

 furnishing nutritive matter and carrying away a comparatively small quan- 

 tity of effete products. 



It may safely be assumed that processes analogous to those just described 

 take place in the brain. By absence of voluntary effort, the muscles have 

 time for rest and for the repair of physiological waste, and their action is for 

 the time suspended. As the activity of the brain involves consciousness, 

 volition, the generation of thought, and, in short, the mental condition ob- 

 served while awake, complete repose of the brain is characterized by the 

 opposite conditions. It is true that the brain may be rested without sleep, 

 by abstaining from mental effort, by the gratification of certain of the senses, 

 and by mental distraction of various kinds, and that the mind may work 

 to some extent during sleep ; but during the period bf complete repose that 

 condition which is so necessary to perfect health and full mental vigor con- 

 sciousness and volition are lost, there is no thought, and the brain, which 

 does not receive blood enough to stimulate it to action, is simply occupied in 

 the insensible repair of its substance and is preparing itself for renewed work. 

 The exhaustion of the muscles produces a sense of fatigue of the muscular 

 system, indisposition to muscular exertion, and a desire for rest, not neces- 

 sarily involving drowsiness. Fatigue of the brain is manifested by indisposi- 

 tion to mental exertion, dullness of the special senses and a desire for sleep. 

 Simple repose will relieve physiological fatigue of muscles ; and when a par- 

 ticular set of muscles has been used, the fatigue often disappears when these 

 muscles alone are at rest, though others be brought into action. Sleep, and 

 sleep alone, relieves fatigue of the brain. 



