The Two Layers which Make Up the Skin. Like our "internal skin," the mucous membrane, which lines our stomach and bowels, the skin is made up of two layers--a deeper, or basement, sheet, woven out of tough strands of fibrous stuff (_derma_); and a surface layer (_epidermis_) composed of cells lying side by side like the bricks in a pavement, or the tiles on a floor, and hence called "pavement" (_epithelial_) cells. These pavement cells are fastened on the basement membrane much as the kernels of corn grow on a cob; only, instead of there being but one layer, as on a cob of corn, there are a dozen or fifteen of them, one above the other, each one dovetailing into the row below it, as the corn kernels do into the surface of the cob. As they grow up toward the surface from the bottom, they become flatter and flatter, and drier, until the outer surface layer becomes thin, fine, dry, slightly greasy scales, like fish-scales, of about the thickness of the very finest and driest bran. We are continually Shedding our Skin. One way in which the skin keeps itself so wonderfully clean and fresh is by continually shedding from its surface showers of these fine, dry, scaly cells, which drop, or are rubbed off, as they dry. This is the reason why no mark, not even a stain or dye, upon the skin, will stay there long; for no matter how deeply it may have soaked into the layers of the pavement-cells, every cell touched by it will ultimately grow up to the surface, dry up, and fall off, carrying the stain with it. If you want to make a mark on the skin that will be permanent, you have to prick the colors into it so deeply that they will go through the basement layer and reach cells which will not grow toward the surface. This "pricking-in" operation is known as _tattooing_; and it is as foolish as it is painful, for blood-poisoning and other diseases may be carried into the system in the process. [Illustration: THE LAYERS OF THE SKIN _E_, epidermis; _C_, capillaries; _D_, dermis; _F_, fat globules and connecting fibres.] Perhaps you will wonder why, if you are shedding these scales from all over your surface every day, you don't see them. This is simply because they are so exceedingly small, thin, and delicate, that you cannot see them unless you get a large number of them together; and when you are changing your clothing, bathing, etc., they are rubbed off and float away. If a part of the body has been shut in--as when a broken arm, for instance, is in a cast, which cannot be changed for several weeks--when finally you take off the bandage, you will find inside it spoonfuls--I had almost said handfuls--of fine scales, which have been shed from the skin and held in by the wrappings. THE GLANDS IN THE SKIN