=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. 58.--Face cut through the middle to show how the adenoids stop the air from passing through the nose. Arrows show the course which the air should take.] The air should pass only through the nose because it is lined with hairs and tiny waving threads which catch the dust. In this way germs and dirt are prevented from getting into the throat and lungs, and in winter the cold air is warmed. =Why Some Children cannot breathe through the Nose.=--When one has a cold, the lining of the nose becomes swollen and gives out a white substance called _mucus_. The swelling of the lining and the mucus fill up the passages. The nose should be kept clean by using a handkerchief and blowing out the mucus into it. _Never put the finger into the nose._ Disease germs often get on the fingers from things touched. Children who have the habit of breathing through the mouth should be examined by a physician. He will, in most cases, find soft spongy growths called _adenoids_ in the back part of the nose. They should always be removed as soon as possible. They may cause disease or deafness and may even injure the mind. [Illustration: FIG. 59.--A view of the voice box from the top.]