BRAIN AND NERVOUS SYSTEM. 345 



the brain upon their proposed labor, accomplish their work. 

 This is frequently done, and with temporary success ; but it 

 is at the cost of the permanent power of the brain, as well as 

 of the general health of the body. Naturally, the brain has 

 the greatest power of labor, and the mind the greatest energy 

 of thought, in the early part of the day, and less in the latter 

 part, and least in the night; and consequently all mental 

 labor exhausts the brain more in the evening and night than 

 in the morning and the bright hours of day. 



792. The brain gains strength by moderate and appropriate 

 exercise, when this is interchanged with rest; and, if fre- 

 quently called upon to exert itself, it acts with greater en- 

 ergy, just as the muscles become strong with use, or the skin 

 hardy with exposure. It is easy to see the difference of men- 

 tal power in the laborer, who uses his muscles, and not 

 his brain, and works only under the direction of another, 

 and in the employer, whose brain is ever active with his 

 plans of business. But excessive mental toil exhausts the 

 power of the brain. Long-protracted labor of the brain, with 

 insufficient or no intervals of rest, waste and weaken it ; and 

 any over-exertion, for even short periods, is injurious. The 

 brain can bear excess of action no better than the muscles. 

 The blacksmith or the stone-cutter can as safely do two days' 

 work every Saturday afternoon and night, as the clergyman 

 can write his sermons in the same time. 



793. Although the brain is placed above and over all the 

 other organs of the body, to give them life, and energy, and 

 direction, yet it is dependent upon them for its own health 

 and power. It has intimate connections and sympathies 

 with each one of the others ; it is strong with their strength, 

 and weak with their weakness ; it enjoys their pleasures, and 

 suffers with their pains. 



794. The brain is constantly connected with, and imme- 

 diately dependent on, the heart. It must have a large and un- 

 failing supply of blood at any time. If the heart is dis- 

 eased, and cannot admit the return of the venous blood, the 

 brain is crowded with it ; then pain and confusion, and even 



