THE MUSCLES AND HOW TO USE THEM 



55 



the voluntary muscles become more vigorous through exer- 

 cise. When we exercise the heart beats more vigorously 

 and carries more blood to the tissues. Exercise causes 

 the lungs to draw in an extra 

 amount of fresh air and to get 

 rid of more impure air. 



Again, exercise stimulates the 

 organs of digestion, giving a 

 good appetite by creating a de- 

 mand for food. Hence the skin, 

 the lungs, the kidneys, and the 

 intestines have to do more work 

 to get rid of the increased waste 

 products. 



81. Amount of Physical Exer- 

 cise. The amount of exercise 

 which is necessary to keep the 

 body in the best condition is a 

 most important and practical 

 question. Too much exercise, 

 as well as too little, is a fruitful 

 cause of ill health. 



Exercise beyond the point of 

 fatigue only does harm, while 

 judicious exercise with suitable 

 rest is of real benefit. 



It may be laid down as a safe 

 rule that a person of average 

 height and weight, engaged in 

 study or any other indoor or in- 

 active business, should have an Fl T G ' 45 ' /\ e , of , the 



Layers of Muscles of the 



amount of exercise equivalent to Nec k and the Back. 



