[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 your 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.]