246 HUMAN PHYSIOLOGY CHAP. 



Physicians tell us that grief, anxiety, and worry are 

 common causes of a nervous breakdown. A few years 

 ago there was formed in this country the " Don't Worry 

 Society," which now numbers in its membership hundreds 

 of brain workers from all professions. One of the very 

 best ways to prevent worrying is to keep the brain and 

 the body in general in a healthy working order. Have 

 pleasant and interesting work to do ; take a delight in 

 doing whatever falls to your lot ; and find a few min- 

 utes each day for quiet and perfect relaxation of every 

 muscle. 



168. Rest and Sleep. While exercise is necessary to 

 the growth and development of the brain, it must be 

 followed by periods of rest and sleep. By rest we may 

 mean either complete relaxation of mind and body from 

 all forms of mental and physical exertion, or simply a 

 change of employment. The latter is by far the more 

 important. After one kind of physical labor another 

 may be restful, and after mental work physical exercise 

 usually proves very refreshing. This is because when 

 certain brain centers have been active for a time they 

 may be rested by calling other centers into activity. 

 But the waste products formed by the working cells go 

 on accumulating in the blood and must in time give rise 

 to fatigue and complete exhaustion. 



The wastes in the blood, together with the change in 

 the circulation to the brain after a period of exercise, 

 are the causes of natural sleep. Sleep is the great 

 reparative agent for the tired and fatigued brain cells. 

 During sleep respiration is decreased, the heart beat is 



