130 



HUMAN BIOLOGY 



pure, nutritious blood. Nearly one fifth of the blood goes 

 to the brain. It is clear that the brain cannot give out 

 energy until it has first received it; the blood supplies 

 energy to the brain. The blood in turn receives the nour- 

 ishment from food and pure 

 air. A rested cell is full of 

 nourishment ; a tired cell is 

 shriveled (see Fig. 117). 



Sleep. During waking 

 hours energy is used up 

 faster than it is stored in 

 the cells, and protoplasm is 

 oxidized faster than the 

 cells can replace it. Dur- 

 ing sleep the opposite is 

 true ; repair is more rapid 

 than waste. During sleep 

 the muscles are strength- 

 ened, the breathing is less, 

 the heart beats more slowly, 

 less heat is produced, diges- 

 tion is slower, less blood goes to the brain. Why is it 

 necessary to be more warmly protected by clothing or bed 

 covering when asleep than when awake ? Above all, the 

 nervous system has an opportunity to recuperate from the 

 constant activity of waking hours. The eye and the ear 

 are rested by darkness and silence. Sleep caused by 

 morphine or other drug is not normal sleep and brings 

 little refreshment 



Practical Suggestions. Sleep is deepest during the second hour 

 after going to sleep, and a greater shock is given to the nervous system 

 by waking a sleeper during that hour than at another time. An alarm 

 clock is a very unhealthful device. One who cannot trust to nature 



FIG. 117. EFFECTS OF FATIGUE ON 

 NERVE CELLS. 



A, resting cell, B, fatigued cell, with its 

 body and nucleus shrunken. 



