Our noses are nine-tenths for breathing, and only about one-tenth for smelling; so that by far the greater part of the nose is built on breathing lines. But the smelling part of it, though small, is very important, because it now has to decide, not merely upon the goodness or badness of the food, but also upon the purity or foulness of the air we breathe. The _nostrils_ lie, as you can see, side by side, separated from each other by a thin, straight plate of gristle and bone known as the _septum_. This should be perfectly straight and flat; but very often when the nose does not grow properly in childhood, it becomes crumpled upon itself, or bulged over to one side or the other, and so blocks up one of the nostrils. This is a very common cause of catarrh, and requires, for its cure, a slight operation, a cutting away of the bulging or projecting part of the septum. The rims of the openings of the nose, known as the _wings_, have little muscles fastened to them which pull them upward and backward, thus widening the air openings or, as we say, dilating the nostrils. If you will watch any one who has been running fast, or a horse that has been galloping, you will see that his nostrils enlarge with every breath; and these same movements occur in sick people who are suffering from disease of the lungs or the heart, which makes it difficult for them to get breath enough. Each nostril opens into a short and rather narrow, but high, passage, known as the _nasal passage_, through which the air pours into the back of the throat, or _pharynx_, and so down into the windpipe and lungs. Instead of having smooth walls, however, the passage is divided into three almost separate tubes, by little shelves of bone that stick out from the outer wall. These are covered with thick coils of tiny blood vessels, through which hot blood is being constantly pumped, like steam through the coils of a radiator, so that the air, as it is being drawn into the lungs, is warmed and moistened. The passage is lined with a soft, moist "skin," called mucous membrane, very much like that which lines the stomach and bowels, except that it is covered with tiny little microscopic hairs, called _cilia_, and that its glands pour out a thin, sticky _mucus_, instead of a digestive juice. This thick network of blood vessels just under the thin mucous "skin" is easily scratched into or broken, and then we have "nose-bleed." The purpose of this mucus is to catch and hold, just as flypaper catches flies, all specks of dust, lint, or germs that may be floating in the air we breathe, and to keep them from going on into the lungs. As these are caught upon the lining of the nose, they are washed down by the flow of mucus or wafted by the movement of the tiny hairs back into the throat, and swallowed into the stomach, where they are digested. Or, if they are very irritating, they are blown out of the nostrils, or sneezed out, and in that way got rid of.