354 PHYSIOLOGY AND HEALTH. 



which we direct by our volition. The movements of the 

 limbs by the contraction of the muscles, the circulation of 

 the blood, digestion, nutrition, respiration, the development 

 of heat, the secretion of all the fluids, depend upon the brain. 

 The mentaj and moral actions, the reception of knowledge 

 through the organs of sense, thought, volition, the feelings, 

 and the passions, are also connected with the brain. Every 

 living action depends on the life of this organ, and every 

 correct action depends upon its soundness. This organ and 

 its powers are placed* partially under our control ; we think, 

 we feel, we indulge in passion through it, and we direct its 

 energies to muscular action. But it gives its energies to the 

 involuntary actions without our will. We are responsible for 

 the u^e of its power in all voluntary actions, and we may so 

 use it as to interfere with its control of the involuntary 

 actions. 



814. The brain performs all these offices well when it is 

 fresh and vigorous ; but, whenever the nervous energies are 

 exhausted or reduced, the brain works languidly, and all the 

 functions of the other organs also languish. When we are 

 fatigued with muscular effort, digestion is feeble. ( 162, p. 

 77.) When the powers of the nervous system are diminished 

 by excessive mental labor, the animal heat is sparingly gen- 

 erated, ( 434, p. 186;) or in whatever way the sustaining 

 power of the brain is reduced, it is less able to support any 

 of the bodily or mental operations, until it shall renew its 

 power by sufficient rest. 



815. The brain sustains and superintends some of the 

 voluntary, and all the involuntary, operations at the same time. 

 We can walk, and breathe, and think, at the same moment, 

 when neither of these requires any great effort or attention of 

 the mind. But the brain cannot concentrate its power, so as 

 to make any unusual exertion, upon more than one thing at 

 a time. If, therefore, any one of these operations becomes 

 difficult, or if we perform it with unusual energy, and conse- 

 quently demand of the brain extraordinary power of direction 

 or exertion, it can give but sparing energy and power to the 



