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.

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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?

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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