246 THE REASON WHY. 



" Love not sleep lest tliou come to poverty : open thine eyes, and thou shalt b 

 satisfied with bread." PHOV. xx. 



promotes perspiration, by which, through the millions of pores 

 of the skin, much of the fluid of the body is changed and purified. 

 And it induces that genial and diffused warmth, which is one of tho 

 chief conditions of a high degree of vitality. 



1017. Why do we feel fatigue ? 



Because those organs which stimulate the mechanism of the 

 Dody to act, themselves require rest and repair. When the brain 

 and nerves arrive at that state, they make their condition known to 

 the system generally, by indications which we denominate fatigue. 



1018. Why, after rest, do we return invigorated to our 

 labours ? 



Because the nervous system has accumulated, during the hours 

 of rest, a fresh amount of that vital force which we call the nervous 

 fluid, aud by which the various organs of the body are excited to 

 perfora the duties assigned to them. 



1019. What is sleep ? 



Sleep is understood to be that state of the body in which the 

 relation of the brain to some parts of the body is temporarily 

 suspended. 



There are some parts of the body that never sleep : such are the 

 heart, the lungs, the organs of circulation, and those parts of the 

 nervous system that direct their operations. 



But when sleep overtakes the system, it seems as if the relations 

 of those parts under the controul of the will were temporarily 

 suspended ; as if, for instance, those nerves which move the arms, 

 the legs, the eyes, the tongue, &c., were all at once unfastened, 

 just as the strings of an instrument are relaxed by the turning of 

 a key, or the throwing down of a bridge over which they were 

 stretched. 



"What is meant by the temporary suspension of the relation of the brain to 

 some parts of the body, may be thus explained. Notice a man when ho sits 

 dosing in a chair : at first his head is held up, the brain controlling the muscles 

 of the neck, and keeping the head erect. But drowsiness comes on, the brain 

 begins to withdraw its influence, and the muscles of the neck becoming as 

 it were "unstrung," the head drops down upon the breast. But the sleep is 

 unsound, and disturbed by surrounding noises. The brain is therefore fre- 

 quently excited to return its influence to tho muscles, and draw up the head of 



