into my lungs, the dark blood in them is not changed into pure red blood,
and goes back through my body dark blood, which cannot keep me alive.

5. If I would have healthy lungs,
I must breathe pure air,
I must live in the sunlight,
I must keep my body clean,
I must wear loose clothing,
I must wear clean clothing,
I must sit and stand erect,
I must keep all parts of my body warm,
I must not change my winter clothing too early in the spring,
I must avoid draughts of cool air,
I must not rush into the cold when I am in a perspiration,
I must not poison my lungs with alcohol or tobacco.

* * * * *

QUESTIONS FOR THE FORMULA.

1. What are the lungs?

2. Describe the lungs.

3. How do you breathe?

4. Why can you not live without breathing?

5. What is necessary if you would have healthy lungs?

* * * * *

THE AIR AND THE LUNGS.

The air which enters through the nose and mouth passes into a tube of
muscles and ring-like pieces of cartilage. The upper part of this tube is
the voice-box or _larynx_, covered by a spoon-shaped lid which closes when
we swallow; the lower part is the _trachea_, and the two parts are the
windpipe. The trachea divides into two branches, _the bronchial tubes_, one
for each lung. These tubes divide again and again like the branches of a
tree, and end in exceedingly small sacs or bags. The air in these sacs, or
air-cells, gives _oxygen_ to the blood in the tiny blood-vessels of the
lungs and takes from them the poison, _carbonic-acid gas_, water, and
impurities, which it carries back through the windpipe into the outside
air.

* * * * *

QUESTIONS ON THE LUNGS AND RESPIRATION.

Of what are the lungs composed?--"Of a soft, fleshy substance, full of
small air-cells and tubes."

Of what use are the lungs?--"They are the breathing machines of the body."

How do the lungs appear when healthy?--"Porous and spongy."

How does the air get into the lungs?--"The air flows through the nose and
mouth, into the windpipe and along the air-tubes, into the air-cells of the
lungs."

What does the air do in the lungs?--"It swells the lungs and causes the
chest to expand."

What do you mean by expand?--"To increase in size."

How is the air expelled from the lungs?--"The chest contracts and sends
the impure air through the tubes and windpipe, the nose and