748 NERVOUS SYSTEM. 



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 sup- 

 ply of blood to their tissue is very much diminished. It is probable, also, that the mus- 

 cles 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 with an increased supply of blood, is a condition more or less opposed to the 

 process of repair, the hyperremia being, apparently, a necessity for the marked and 

 powerful manifestations of their peculiar functions. When the parts are in active func- 

 tion, the blood seems to be required to keep at the proper standard the so-called irri- 

 tability 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 it carries away carbonic 

 acid, urea, and other products of disassimilation, which are all increased in amount, until 

 it gradually uses up its capacity for work. Then follows repose ; the supply of blood is 

 reduced, but, under normal conditions, the tissue repairs the waste which has been 

 excited by action, the blood furnishing nutritive matter and carrying away a compara- 

 tively small amount of effete products. 



"VVe may safely assume that processes analogous to those just decribed take place in 

 the brain. By absence of voluntary effort, we allow the muscles time for rest and for the 

 repair of physiological waste, and their active function is for the time suspended. As 

 the activity of the brain involves consciousness, volition, the generation of thought, and, 

 in short, the mental condition observed while awake, complete repose of the brain is 

 characterized by the opposite conditions. It is true that we rest the brain without sleep, 

 by abstaining from mental effort, by the gratification of certain of the senses, and by men- 

 tal distraction of various kinds, and that the mind may work to some degree during sleep ; 

 but, during the period of complete repose that condition which is so necessary to perfect 

 health and full mental vigor we lose consciousness and volition, 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 future work. 

 The exhaustion of the muscles produces a sense of fatigue of the muscular system, indis- 

 position to muscular exertion, and a desire for rest, not necessarily involving drowsiness. 

 Fatigue of the brain is manifested by indisposition to mental exertion, dulness of the 

 special senses, and a desire for sleep. Simple repose will relieve physiological fatigue of 

 muscles; and, when a particular set of muscles has been used, the fatigue disappears 

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

 sleep alone, relieves fatigue of the brain. When the sleep has continued long enough for 

 the rest of the brain and the repair of its tissue, we awake, prepared for new effort. 



We have now only to refer to a new theory of sleep, proposed by Sommer. Taking 

 as a basis the researches of Pettenkofer and Voit upon respiration, Sommer advances the 

 idea that, when the brain is active, or while we are awake, the system appropriates but 

 a small quantity of oxygen in respiration and eliminates a relatively large proportion of 

 carbonic acid; after a time, the oxygen thus appropriated is consumed, and the system 

 demands a new supply ; and, during sleep, the organism appropriates oxygen largely and 

 eliminates a relatively small amount of carbonic acid. When the elimination of carbonic 

 acid at the expense of the oxygen stored up reaches a certain point, the necessity for a 

 farther supply of oxygen induces sleep ; and when, during sleep, oxygen has been appro- 

 priated in sufficient quantity, the system awakes, prepared for a new period of activity 

 of the animal functions. 



By reference to the researches of Pettenkofer and Voit, we find that these observers, 

 in experiments upon a man confined in a chamber in which the interchanges of gases in 



