BIOLOGY APPLIED TO HEALTHFUL LIVING 537 



does far more damage than starvation. Men have volun- 

 tarily fasted 30, 40 and even 50 days, and afterward quickly 

 regained their normal weight and health; but it is certain 

 that a normal healthy man could not go without sleep so 

 long. Sleep is a period of rest, repair, and growth, and is 

 especially important for growing children. Much of the 

 physical harm to children in crowded tenements is due to a 

 combination of late retiring to rest, uncomfortable bedroom, 

 disturbed sleep, and early rising. Many a child in the poorer 

 regions of great cities is anaemic, nervous, haggard in face, 

 listless in school, sluggish at play; and the real trouble may 

 be that he is not- getting enough good sound sleep. This is 

 likewise true of some adults, who should average seven to 

 nine hours of sleep in twenty-four. As far as possible, this 

 should be taken during the quietest hours (10 P.M. to 6 A.M.) 

 when there are the fewest external stimuli tending to cause 

 awakening. 



MUSCULAR SYSTEM 



461. Exercise for Health. Those whose business or 

 pleasure leads them to sedentary habits of life need to con- 

 sider most seriously the question of physical exercise. The 

 scientific reason for exercise is to be found in the coordina- 

 tion of muscular activity with all the other organs of the 

 body, rather than in the development of the muscular system 

 itself. In short, most people should exercise primarily in 

 order to get reactions of the digestive, circulatory, respira- 

 tory, and nervous organs, while secondarily and incidentally 

 they may develop their muscular systems. However, it is 

 very doubtful in the opinion of many qualified physiologists 

 whether excessive development of the muscular system is 

 best for the general health of those who are not professional 

 athletes or laborers. The well-known aphorism, " Mens 

 sana in corpore sano " (a sound mind in a sound body), 

 means that giant intellects ought to be located in healthy 



