128 APPLIED PHYSIOLOGY 



must keep acting all the time. The spinal cord must keep 

 sending orders to the cells to eat and grow, and the me- 

 dulla must send orders for us to keep breathing. But they 

 send an order and rest a second, and then send another. 

 Like the heart, they rest half the time. 



When the thought cells rest, we do not know anything, 

 but are asleep. During sleep they regain strength and 

 grow like a resting muscle. 



245. Worry. We can do a great amount of hard brain- 

 work if we can only sleep. It is doubtful if any one can 

 overwork the brain if he gets rest in sleep. He cannot 

 help sleeping when his brain gets tired, and when he 

 wakes he will be ready for work again. But sometimes 

 a person is troubled. This keeps his mind in action just 

 enough to prevent his resting. Then he feels tired, even 

 if he does not work, for he gets no rest. 



246. How much sleep ? A child needs at least ten 

 hours of sleep each. day up to the age of twelve years. By 

 the time he is eighteen, he needs only eight hours. By 

 the time he is thirty, six or seven hours of good sleep will 

 be enough. When he becomes old and feeble, he will need 

 more again. 



The time of sleeping is of less importance than that this 

 time should be regular. A short nap in the middle of the 

 day is very helpful. 



247. Habit. When the cells of the brain have done a 

 thing a few times, they want to do it again, and will often 

 act without our knowledge. So we can form a habit of 

 doing a thing. All of us have habits, and are forming 

 new ones. We may swear, or drink, or be dishonest once 

 or twice, and not be so again. But if the temptation comes 

 again we shall yield more easily for having yielded once, 

 and after a few times we shall yield even if we do not 



