Page:Popular Science Monthly Volume 4.djvu/710

690 with hair, claws, or finger-nails, and is only modified skin. When a section through a piece of skin is examined with the aid of a moderately-powerful microscope, the lower or internal surface is seen to be made up of little, irregularly-rounded cells, or bags, with soft semi-fluid contents; and, while the animal is alive, new cells are constantly forming under the old ones, which are pushed outward and crowded together, and gradually lose their soft contents, and are flattened out into very small scales. The outer layer of the skin is made up of

these scales, which are fastened to each other firmly enough to be separated from the living layer below in a thin sheet, as happens when a blister is raised on the hand by unusual work, but in most parts of the body they are slowly rubbed off as new ones grow; but at the tips of the fingers they are so firmly united that they form horny plates, or "nails," which are pushed forward as new cells form at the root.

In the skin of a bird where a new feather is to grow there is a little pit, and, at the bottom of this, an elevation or pyramid; extending up one side of this pyramid is a groove, or furrow, deepest at the base, and gradually growing shallower until it disappears near the top; from each side of this furrow a great many smaller grooves extend around to the other side of the pyramid, and these also decrease in depth, and at last disappear just as they are about to meet on the side opposite the large furrow. The whole pyramid is covered with skin, and the surface is made of the same scales, or flattened cells, that are found over the rest of the surface of the body; but, instead of falling off when they are pushed out by the new ones below them, they become united or welded to each other, so as to form a horny coat over the surface of the pyramid, with ridges on its lower or inner surface, corresponding to the grooves on the pyramid; and, as new cells grow at the base, this coat or cast of the surface is pushed upward till it breaks at its thinnest part, which is, of course, the smooth part without ridges opposite the large furrow; and then, as it is