the winter in the same place.
They used no strong drink at any time and as a consequence all but two
of them were reported well and strong in the following spring.

CHAPTER XIV

BREATHING

=The Lungs.=--The lungs are two light spongy bodies filling up the
greater part of the chest. The heart lies between the lungs. The
lungs are formed largely of thousands of thin-walled sacs and two sets
of tubes. One set of tubes carries air into and out of the lungs, and
the other set is filled with blood. These sacs and tubes are held in
place by a loose meshwork of tissue.

[Illustration: FIG. 54.--The ribs and front wall of the chest cut away
to show the lungs. A piece of one lung is cut off to show the heart.
_A_ and _E_, parts of the breastbone; _F_, diaphragm.]

=Why we Breathe.=--Breathing means taking air into the lungs and
forcing it out. The air is made to go into the lungs in order that a
part of it called oxygen may get into the blood. The blood then
carries the oxygen to all parts of the body where it can help the
organs do their work.

[Illustration: FIG. 55.--Photograph of a salamander, showing the gills
on either side of the head, which are used in breathing.]

The air which comes out of the lungs is not the same as that which
goes in. Some of the oxygen has been used up and in its place is a
heavier gas named _carbon dioxide_, which has been given out by the
body. This carbon dioxide is part of the waste formed in every part of
the body from the used-up food and dying parts of the body. We breathe
therefore to get oxygen into the body and to take out some of the
waste matter.

All animals must breathe. If our breath is shut off only four or five
minutes, death results. In the earthworm the oxygen goes right through
the skin into the blood. Bugs and flies have several little openings
along the sides of the body which lead into tubes branching throughout
the body to carry air. A fish gets air through its gills lying under a
bony flap on each side of the head.

[Illustration: FIG. 56.--The windpipe and lungs viewed from in front.
On the right, the tissue is removed to show the air tubes.]

=How the Air passes into the Lungs.=--The outer openings of the nose
are called nostrils. From here two channels lead back through the nose
to the throat. The cavity of the throat behind the nose and tongue is
the _pharynx_. At the bottom of the pharynx is a tube made mostly of
gristle. This tube is larger than yo