From the root of the hairs, two or three little bundles of muscle run up toward the surface of the skin. When these contract, they pull the root of the hair up toward the surface, causing the hair to stand erect, or "bristle," as we say. This is what makes the hair on a dog's or a cat's back stand up when he is angry; but the commonest use of the movement is, when animals are cold, to make their coats stand out so as to hold more air and retain the body-heat better. We have lost most of our hairy coating, but whenever we get chilly, whether from cold or from fright, these little muscles of our hair bulbs contract and pull the hair glands of our skin up toward the surface, so that it looks all "pimply" or "goose-skinned." Each hair pouch has sprouted out from its sides a pair of tiny pouches, which form _oil glands_ to lubricate the hair and keep it sleek and flexible. It is hard to beat nature at her own game, and her method of oiling the hair is far superior to any hair oil that can be put on from the outside. Keep your hair well brushed and washed, and nature will oil it for you much better than any hair oil or scalp reviver ever invented.[19] THE NAILS How the Nails are Made. Another "trade," which our wonderful skin has literally "at its fingers' ends," is that of making nails. Indeed, every kind of scale, armor, fur, feather, and leather coating possessed by bird, beast, or fish was made by, and out of, the skin. Nail-making, however, is one of its simplest feats, as it is carried out merely by turning a little patch, or area, of itself into a horn-like substance. This, the skin of insects, of fishes, of crocodiles, etc., does all over the surface of their bodies; but in animals and birds only a number of little patches at the tips of the toes harden up in this way, to form the claws or nails; and in birds, the beak; and in some animals, the horns. So it is quite correct to call the substance of our nails "horn-like." In some animals and birds, these little horny patches at the ends of the toes grow out into long, curved hooks, or broad, digging chisels and scoops; but on our own fingers, they simply make a little mould over the finger-tip. If, however, they are protected from being broken off, they will grow four or five inches long; in fact, they are carefully trained to do this by some of the upper classes in China, merely for the purpose of showing that they have never been obliged to degrade themselves, as they foolishly regard it, by working with their hands. You can easily prove that the nails do grow constantly from the root or base, out toward the tip, by watching, some time when you have pounded one of your nails, how the black or discolored patch in it will grow steadily outward toward the tip, where it will be broken off and shed.