You know that the whole of the external surfaces of your body are covered with a thin coat or covering-the skin. You know, too, what a protection this thin covering is, because you have sometimes fallen down and grazed it, and the bare exposed place has been very sore.
Thin as this skin is, it may be separated into two distinct layers. You have all seen the skin of the hand or the foot raised up in a blister. The raised blister is the upper layer of the skin separated from the lower one. This outer layer is so hard, tough, and horny that it may be cut or pricked without causing the least pain. You may run a needle through it without feeling it, and without drawing blood. That being so, we know there can be neither nerves nor blood-vessels of any kind in this part of the skin.
Beneath the tough, outer layer is another-the trueskin. The proper name for this is the dermis. The outer layer is called the epi-dermis because it lies on the dermis.
The under part of the dermis-that next the flesh-consists of fibers closely interwoven together. The rest of the layer consists of soft little bags or cells, too small to be seen without the aid of the microscope. The cells areclosely packed together, and surrounded on all sides with nerves and blood-vessels. We could not prick the dermis anywhere with the finest needle without drawing blood and causing pain.
Fresh dermic cells are being constantly formed, and as these form, they push the old ones up to the surface, where they become flattened into tough horny scales- like the scales of a fish. These scales form the outer layer-the epi-dermis. They are really dead flattened cells. They are being constantly rubbed off the surface of the body, and constantly renewed from below.
But the skin is not merely a protecting coat for the body. It is an important organ for cleansing the blood of its impurities. Our earlier lessons have shown us the perspiration or sweat, oozing out through the pores of the skin after work or exercise. This perspiration is really waste matter, which has been drained out of the blood, by little coiled-up tubes-called sweat glands-in the underlayer of the skin.
The bodies of all animals which have an internal skeleton are covered with a double skin, similar in structure to that of man.
Fishes, frogs, and reptiles, being cold-blooded animals, have naked skins. They have no outer clothing, or protection, other than the skin itself, for the simple reason that there is no need to preserve the bodily heat-there is, in fact, no bodily heat to preserve.
In mammals and birds, whose blood is warm, we find the skin clothed with a more or less thick and warm covering. In birds this outer coat takes the form of feathers, in most mammals the covering is hair. We have already had occasion to notice the special fitness of feathers as a clothing for birds, both as regards warmth and lightness, and as an aid to locomotion through the element in which they live.
Most mammals, as we said, are clothed with hair, although there are various modifications of this kind of covering. The cat, rabbit, squirrel, and mole, for instance, have a very smooth, soft coat, which we call fur. But what is fur? It is in reality fine, soft, silky hair, growing very close and thick on the skin. Then, too, the sheep has a coat of wool. But wool is only fine, long, curly hair set very close and thick on the skin.
The hog has a covering of bristles, and bristles are merely thick, strong, stiff hairs. Some animals, such as the hedgehog and porcupine, have these same stiff, strong hairs, only much stiffer and stronger still. We then call them spines.
These outer coverings of feathers, or hair in its various forms, are all modifications of the horny matter of which the epidermis of the skin consists. It becomes in certain parts very hard and horny indeed. It forms nails on our hands and feet, and claws on the toes of various animals. The hoofs and horns of other animals are all formed of the same hard, horny development of the epidermis of the skin.
We tan the skins of many of our largemammals into leather, but it is only the under layer, or dermis, which is used; the epidermis is all scraped away.
It is worth noticing that the Cetacea (or whale-like animals) have smooth, naked skins to assist in their water locomotion; but, being warm-blooded mammals, their bodily heat must be kept in. This is accomplished by a thick under-coat of fat or blubber beneath the skin. Wool, fur, or feathers would be out of the question.
Lesson 54