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.
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.