the heart more than the rest. It often makes the heart
beat slowly at one time and fast at another. It weakens the heart and
keeps it from working harder when the working cells need more food. A
smoker gets out of breath quickly. He cannot run far or work very
hard. Chewing is a still more harmful form of using tobacco. When men
train for a game or a race they never use tobacco.

Boys are not so strong as men, and so tobacco is more hurtful to them.
Boys are harmed by tobacco far more than men. Cigarette smoke harms
their stomachs and keeps food from their blood. If boys smoke, they
become pale and weak. The poisonous smoke weakens the heart, and they
cannot run or work so hard as they should. Even if a father uses
tobacco, he should not allow his boys to use it.

WHAT WE HAVE LEARNED

1. Blood is a liquid. It contains many round red cells and a few
white cells.

2. Blood contains all kinds of food for the cells of the body.

3. The blood is kept moving by the heart.

4. The heart pumps or beats about seventy times a minute.

5. The blood flows through arteries to all parts of the body.

6. The arteries open into the capillaries. Capillaries make a
network around each cell of the body.

7. Some of the liquid parts of the blood go out through the sides
of the capillaries and become food for the cells of the
body.

8. From the capillaries the blood flows into the veins and back
to the heart.

9. Bleeding can be stopped by holding the cut tightly between the
hands.

10. The white blood cells grow into the sides of cuts, and so
heal them. They also guard the body against the seeds of
many diseases.

11. The red blood cells carry air to the cells of the body.

12. Alcohol weakens the heart and arteries.

13. Tobacco harms the heart.

CHAPTER X

BREATHING, HEAT, AND CLOTHING

=100. The lungs.=--Our food becomes blood and feeds the cells of our
body, but we grow only a little heavier. What becomes of the food?

[Illustration: =The air tubes and lung.=

_a_ larynx or voice box.
_b_ trachea or windpipe.
_d_ air sacs, each like a tiny frog's lung.]

Besides food, air is always getting into our bodies. In breathing, air
passes through the nose into a tube in the neck. This tube is called
the _windpipe_. You can feel it as a pile of hard rings in