458 NERVOUS SYSTEM. 



with an increased supply of blood, is a condition more or 

 less opposed to the process of repair, the hypersemia being, 

 apparently, a necessity for the marked and powerful mani- 

 festations of their peculiar functions. When the parts are 

 in active function, 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 prolonged 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 iises up its capa- 

 city 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 comparatively small 

 amount of effete products. 



We may safely assume that processes analogous to those 

 just described take place in the brain. By absence of vol- 

 untary 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 condi- 

 tions. 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 mental 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, volition, there is no thought, and the 

 brain, which does not receive blood enough to stimulate it 



