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.
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.