If the dust is too irritating, or the air is foul and contains disease germs, these set up an inflammation in the nose, and we "catch cold," as we say. If we keep on breathing bad or dusty air, the walls of the nasal passages become permanently thickened and swollen; the mucus, instead of being thin and clear, becomes thick and sticky and yellowish, and we have a catarrh. Catarrh is the result of a succession of neglected "bad colds," caused, not by fresh, cold air, but by hot, stuffy, foul air containing dust and germs. The best and only sure way to avoid catarrh is by breathing nothing but fresh, pure air, day and night, keeping your skin clean and vigorous by cool bathing every day, and taking plenty of play in the open air. So perfect is this heating, warming, and dust-cleansing apparatus in the nose, that by the time quite cold air has passed through the nostrils, and got down into the back of the throat, it has been warmed almost to the temperature of the body, or blood-heat, and has been moistened and purified of three-fourths of its dust or disease germs. When you go out of doors on a cold, frosty morning, your nose is very likely to block up, because so much hot blood is pumped into these little steam-coils of blood vessels, in order to warm the air properly, that they swell until they almost block up the nostrils. The Sense of Smell. The lower three-fourths of the nasal passages have nothing whatever to do with the sense of smell; this is found only in the highest, or third, division of the passages, right up under the root of the nose, where odors can readily rise to it. Here can be found a little patch of mucous membrane of a deep yellowish color, which is very sensitive to smells, and from which a number of tiny little nerve twigs run up to form the nerve of smell (_olfactory nerve_), which goes directly to the brain. The position of the smell area at the highest and narrowest part of the nose passage explains why when you have a very bad cold, you almost lose your sense of smell; the lining of the lower part of the nose has become so inflamed and swollen as to block up the way to the highest part where the smelling is done. [Illustration: ADENOIDS A section through the nose and mouth: _A_, adenoid growth; _P_, soft palate; _T_, right tonsil.]