body?

7. How does alcohol in the blood affect the brain?

8. When does the heart rest?

9. How does exercise in the fresh air help the
heart?

10. What harm does alcohol do to the heart?

CHAPTER XV.

THE LUNGS.

[Illustration: T]HE blood flows all through the body, carrying good food
to every part. It also gathers up from every part the worn-out matter
that can no longer be used. By the time it is ready to be sent back by
the veins, the blood is no longer pure and red. It is dull and bluish in
color, because it is full of impurities.

If you look at the veins in your wrist, you will see that they look
blue.

If all this bad blood goes back to the heart, will the heart have to
pump out bad blood next time? No, for the heart has neighbors very near
at hand, ready to change the bad blood to pure, red blood again.

THE LUNGS.

These neighbors are the lungs. They are in the chest on each side of
the heart. When you breathe, their little air-cells swell out, or
expand, to take in the air. Then they contract again, and the air passes
out through your mouth or nose. The lungs must have plenty of fresh air,
and plenty of room to work in.

[Illustration: _The lungs, heart, and air-passages._]

If your clothes are too tight and the lungs do not have room to expand,
they can not take in so much air as they should. Then the blood can not
be made pure, and the whole body will suffer.

For every good breath of fresh air, the lungs take in, they send out one
of impure air.

In this way, by taking out what is bad, they prepare the blood to go
back to the heart pure and red, and to be pumped out through the body
again.

How the lungs can use the fresh air for doing this good work, you can
not yet understand. By and by, when you are older, you will learn more
about it.

CARE OF THE LUNGS.

Do the lungs ever rest?

You never stop breathing, not even in the night. But if you watch your
own breathing you will notice a little pause between the breaths. Each
pause is a rest. But the lungs are very steady workers, both by night
and by day. The least we can do for them, is to give them fresh air and
plenty of room to work in.

You may say: "We can't give them more room than they have. They are
shut up in our chests."

I have seen people who wore such tight clothes that their lungs did not
have room to take a full breath. If any part of the lungs c