ur thumb and is named the
_larynx_, or _voice box_. The bump on its front part forms the lump in
the throat called the _Adam's Apple_.

From the voice box extends the _windpipe_ called _trachea_, down to
the lungs. The windpipe divides at its lower end between the lungs
into two branches. One of these enters each lung.

=The Air Tubes in the Lungs.=--As the branch of the windpipe enters
each lung it divides into smaller branches just like the limbs of a
tree. These divide into still smaller tubes, which branch again and
again until they are as small as a hair. These hairlike tubes have
swollen ends called _air sacs_. The walls of the air sacs are much
thinner than tissue paper.

[Illustration: FIG. 57.--A bunch of air cells at the end of an air
tube in the lungs, showing the blood vessels which receive the oxygen
and give out the carbon dioxide.]

=How the Blood trades Waste for Oxygen in the Lungs.=--The blood,
which is constantly running from all parts of the body to the lungs,
collects waste formed from the burnt food and dying parts of the
organs. When the blood comes to the lungs, it is full of this waste,
called carbon dioxide. The blood tubes divide into fine branches with
very thin walls and form a rich network over the air sacs. This allows
the carbon dioxide and water to pass out of the blood tubes into the
air sacs, while the oxygen at the same time goes through into the
blood. More than a pint of water is given off in the breath daily.

=How we Breathe.=--The bottom of the chest cavity is formed by an
upward arching sheet of muscle called the _diaphragm_. This is
fastened to the lower ribs. The ribs at rest slant downward and
inward. When the ribs are pulled up or the arch of the diaphragm down,
the cavity of the chest becomes larger. The air then runs into the
lungs and swells them out. When the ribs are let drop or the arch of
the diaphragm goes up, the air is pushed out of the lungs.

Without thinking, we work the muscles to draw up the ribs about
eighteen times every minute, because all parts of the body are calling
for oxygen. The harder we work the oftener we breathe because the
muscles need more oxygen to make them go.

=Why we should breathe through the Nose.=--Most persons find it easy
to breathe through the nose. In some, however, the passages in the
nose are too small to carry the air without effort. On this account
they let the mouth hang open and breathe through it.

[Illustration: FIG