622 NERVOUS SYSTEM 



requires sleep, the vessels of the brain contract in obedience to a stimu- 

 lus received through the sympathetic system of nerves, diminishing the 

 supply of blood here as in other parts under varied physiological con- 

 ditions. The vessels of the brain are provided with vasomotor nerves, 

 and it is sufficient to have noted that the arteries are contracted during 

 sleep, the mechanism of this action being well established by observa- 

 tions on other parts of the circulatory system. 



Little is known of the intimate nature of the processes of nutrition 

 of the brain during its 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 one is awake. Although 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 wear of nervous tissue, and like 

 other parts of the organism, its tissue requires periodic repose for regen- 

 eration of the substance consumed. Analogies to this are to be found 

 in parts that are more easily subjected to direct observation. The mus- 

 cles require repose after exertion, and the glands, when not actively 

 engaged in discharging their secretions, present intervals of so-called 

 rest. As regards the glands, during the intervals of rest the supply of 

 blood to their tissue is much diminished. It is probable, also, that 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 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 hyperemia being, apparently, a necessity for 

 the marked and powerful manifestations of their peculiar 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 that 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 pass- 

 ing through the tissue carries away carbon dioxide and other products of 

 katabolism, 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 conditions, the tissue repairs the waste 

 which has been excited by action, the blood furnishing nutritive 

 matter and carrying away a comparatively small quantity of effete 

 products. 



