MANUAL OF NATURE STUDY. 
29 
Now what happens to the air in the lungs when 
they are pressed together by the ribs? What hap¬ 
pens to the blood pump? What effect would that 
have upon the supply of blood to the system? 
Why do we need the blood? Why do we need air? 
Teachers must explain that the blood must be 
purified and that our breathing helps to do that 
thing. That there are blood tubes and air pas¬ 
sages, capillaries and millions of air cells in the 
lungs, and that the blood trades off its impurities 
for the oxygen of the air. The lungs may be com¬ 
pared to a market house in that it is a place for 
trading. Which makes the best trade, the blood 
or the air? 
When does a gardner find it hard to get rid of 
his cabbage or radishes? When the market house 
is already full of such vegetables. When does 
the blood find it difficult to get rid of its impurities? 
When the air in the lungs is already full of im¬ 
purities. 
Why is plenty of pure air then necessary? How 
does stooping over while sitting at the desk affect 
the amount of pure air in the lungs? How does 
smoking cigarettes or tobacco of any kind affect the 
purity of air we breathe ? How do close, ill-ven¬ 
tilated school rooms affect the purity of air? Then 
how would that affect the purity of the blood ? 
