What the Skin Is. The skin is the most wonderful and one of the most important structures in the body. We are prone to think lightly of it because it lies on the surface, and to speak of it as a mere coating, or covering—a sort of body husk; but it is very much more than this. Not only is it waterproof against wet, a fur overcoat against cold, and a water jacket against heat, all in one, but it is also a very important member of the "look-out department," being the principal organ of one of our senses, that of touch.

The eyes in the beginning were simply little colored patches of the skin, sunk into the head for the purpose of specializing on the light-rays. The smelling areas of the nose also were pieces of the skin, as were also the ears. Not only so, but—although it is a little hard for you to understand how this could have happened—the whole brain and nervous system is made up of folds of the skin tucked in from the surface of the back; so that we can say that the skin, with the organs that belong to it and have grown from it—the eyes, nose, ears, brain, and nerves—forms the most wonderful part of the body. Everything that we know of the world outside of us is told us by the skin and the look-out organs that have grown out of it. The skin is not only the surface part and coating of the body, far superior to any six different kinds of clothing which have yet been invented, but it is related to, and assists in, the work of nearly half the organs in the body. Not only all that we learn by touch and pressure, but everything that we know of heat and cold, of moisture and dryness, and most of pain, comes to us through our skin, through the little bulbs on the ends of the nerve twigs in it. It also helps the lungs to breathe, the kidneys to purify the blood, and the heart to control the flow of blood through the body.

A healthy skin is of very great importance; and part of this health we can secure directly, by washing and bathing, scrubbing and kneading and rubbing, because the skin lies right on the surface, where we can readily get at it. But, on the other hand, no amount of attention from the outside alone will keep it healthy. All the organs inside the body must be kept healthy if the skin is to be kept in good condition. Although the external washing and cleaning are very important, the greater part of the work of developing a healthy skin and a good complexion must be done from the inside.

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.

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

Sweat Glands. Like all the pavement (epithelial) surfaces of the body, inside and out, the skin has the power of making glands by dipping down little pouches or pockets into the layers below. In the skin, these little gland-pockets are of two kinds, the sweat glands and the hair glands.

The sweat glands are tiny tubes which go twisting down through the different pavement layers, through the basement layer, and right into the coat of fat, which lies just under the skin. The tube of the sweat gland soaks, or picks, out of the blood some of the waste-stuff—just as the kidney tube does in the kidney,—together with a good deal of water and a small amount of delicate oil, and pours them out on the surface of the body in the form of the "sweat," or perspiration.

As you will remember, when the muscles work hard and pour more waste into the blood, then the heart pumps larger amounts of blood out into the skin; and this causes it to redden. The sweat glands work harder to purify this extra blood, and they pour out the waste and oil and water on the surface. As soon as this water gets upon our hot skin, it begins to evaporate and cool us off, as well as to carry off some of the waste in the form of gas. The trace of oil in the perspiration helps to lubricate the skin and keep it soft; but when too much of it is poured out we have that greasy feeling, which we have all felt after perspiring freely.

From all this cooling and breathing and blood-purifying work going on upon the surface of our skin, you can easily see why it is so important that all our clothing should be loose and porous and that next the skin easily washed; else it will very soon become clogged up and greasy, and shut off the breathing and blood-purifying work of the skin and make it dirty and unhealthy. This continual mist of water, rising and bubbling up through our skin like springs out of a hillside, is another of nature's wonderful ways of cleansing the skin and of preventing any kind of dirt from permanently sticking to or lodging in it. Remember, you do not need to dig below the surface when you wash.

Hair Glands. The other kind of skin glands, the hair glands, are also pouches growing out from the deepest part of the stem of the hair, known as the root, or hair bulb.

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.

You cannot see the softest and youngest row, or layer, of the nail cells at the base, because a fold of skin, the nail fold, has been doubled, or folded, over them to protect them while they are young and soft. It is not best to push this fold of skin back too much, as, by so doing, you may uncover the young nail cells while they are soft and tender, and expose them to injury. The reason why there is a little whitish crescent at the base of the nail is that the cells of the nail do not grow hard and horn-like and transparent until they have grown out a quarter of an inch or so from under the fold, but at first look whitish, or opaque, like the rest of the skin.

Health Shown by the Color of the Nails. Your nails and your lips are not really any redder, or pinker, than the rest of your skin; but the cells forming them are clear and transparent and allow the red blood to show through. This is why we often look at the nails and lips to see what the color of the blood is like, and how well or badly it is circulating. If the blood is anemic, or thin, then both lips and nails are pale and dull. If the blood is healthy and the circulation good, then the nails are pink, and the lips clear red. If, on the other hand, the circulation is bad, as in some forms of lung disease and heart disease, so that the blood is loaded with carbonic acid until it is blue and dark, then the lips may become purplish or dark blue, and the finger nails nearly the same color.

THE BLOOD-MESH OF THE SKIN

The Blood Vessels under the Skin. Not merely the nails and the lips, but the whole surface of the skin is underlaid with a thick mat, or network, of blood vessels. These vessels are all quite small, so that a cut has to go down completely through the skin, and generally well down into the muscles, before it will reach any blood vessel which will bleed at a dangerous rate. But there are so many of them, and they cover such a wide surface throughout the body, that they are actually capable of holding, at one time, nearly one-tenth of all the blood in the body.

This "water-jacket" coat of tiny blood vessels all over our body has some very important uses: It allows the heart to pump large amounts of blood out to the surface to be purified by the sweat glands, and to breathe out a little of its carbon dioxid and other gas-poisons.

The Skin as a Heat Regulator. Heat, as well as waste, is given off by the blood when it is poured out to the surface; so another most important use of the skin is as a heat regulator. As we have already seen, every movement which we make with our muscles, whether of arms and limbs, heart, or food tube, causes heat to be given off. We very well know, when we work hard at anything, we are likely to "get warmed up." Although a certain amount of this heat is necessary to our bodily health, too much of it is very dangerous.

Just as it is best for the temperature, or heat, of a room to be at about a certain level, somewhere from 60° to 70° F., so it is best for the interior of our bodies to be kept at about a certain heat. This, as we can show by putting a little glass thermometer under the tongue, or in the armpit, and holding it there for a few minutes, is a little over 98° F. (98.4° to be exact); and this we call "body heat," or "blood heat," or "normal temperature." Our body cells are, in one way, a very delicate and sensitive sort of hot-house plants, though tough enough in other respects. Whenever our body heat goes down more than five or six degrees, or up more than two or three degrees, then trouble at once begins. If our temperature goes down, as from cold or starvation, we begin to be drowsy and weak, and finally die. If, on the other hand, our temperature climbs up two, three, or four degrees, then we begin to be dizzy and suffer from headache and say we have "a fever."

A fever, or rise of temperature, that can be noted with a thermometer, is usually due to disease germs of some sort in the body; and most of the discomfort that we suffer is really due more to the poisons (toxins) of the germs than to the mere increase of heat, though this alone will finally work serious damage. However, as we well know from repeated experience, we need only to run or work hard in the sun for a comparatively short time to make ourselves quite hot enough to be very uncomfortable; and if we had no way to relieve ourselves by getting rid of some of this heat, we should either have to stop work at once, or become seriously ill. This relief, however, is just what nature has provided for in this thick coat of blood vessels in our skin; it enables us to throw great quantities of blood out to the surface where it can get rid of, or, as the scientists say, "radiate," its heat. This cooling process is hastened by the evaporation of the perspiration poured out at the same time, as we have seen.

One of the chief things in training for athletics is teaching our skin and heart together to get rid of the heat made by our muscles, as fast, or nearly as fast, as we make it, thus enabling us to keep on running, or working, without discomfort. As soon as we stop running, or working, the heart begins to slow down, the blood vessels in the skin contract and diminish in size, the flush fades, and we begin to cool off. We are not making either as much heat or as much waste as we were, and hence do not need to get rid of so much through our skins.

When we feel cold, just the opposite kinds of change occur in the skin. The blood vessels in the skin contract so as to keep as much of our warm blood as possible in the deeper parts of our body, and prevent its losing heat. As blood showing through the pavement-layer of the skin is what gives us our color, or complexion, our skin becomes pale and pasty-looking; and if all the blood is driven in from the surface, our lips and finger nails will become blue with cold. Here again, by changes in the skin, nature is simply trying to protect herself from the loss of too much heat.

If we exercise briskly, or eat a good warm meal, and thus make more heat inside of our body, then there is no longer any need to save its surface loss in this way; and the blood vessels in our skin fill up, the heart pumps harder, and the warm, rich color comes back to our faces and lips and finger nails.

So perfectly and wonderfully does this skin mesh of ours work, by increasing or preventing the loss of heat, that it is almost impossible to put a healthy man under conditions that will raise or lower his temperature more than about a degree, that is to say, about one per cent above, or below, its healthful level. Men studying this power of the skin have shut themselves into chambers, or little rooms, built like ovens, with a fire in the wall or under the floor, and found that if they had plenty of water to drink and perspired freely, they could stand a temperature of over 150° F. without great discomfort and without raising the temperature of their own bodies more than about one degree. If, however, the air in the chamber was moistened with the vapor of water, or steam, so that the perspiration could no longer evaporate freely from the surface of their bodies, then they could not stand a temperature much above 108° or 110° without discomfort.

Other men, who were trained athletes, have been put to work in a closed chamber, at very vigorous muscular exercise, so as to make them perspire freely. But while a thermometer placed in that chamber showed that the men were giving off enormous amounts of heat to the air around them, another thermometer placed under their tongues showed that they were raising the temperature of their own bodies only about half a degree. One man, however, happened to try this test one morning when he was not feeling very well, and didn't perspire properly, and the thermometer under his tongue went up nearly four degrees.

THE NERVES IN THE SKIN

How We Tell Things from Touch, and Feel Heat and Cold and Pain. Last of all, the skin is the principal organ of the sense of touch, and also of the "temperature sense"—the sense of heat and cold—and of the sense that feels pain. All these feelings are attended to by little bulbs lying in the deeper part of the skin and forming the tips of tiny nerve twigs,[20] which run inward to join larger nerve branches and finally reach the spinal cord. There are millions of these little bulbs scattered all over the surface of the skin, but they are very much thicker and more numerous in some parts than in others; and that is why, as you have often noticed, certain parts of the skin are more sensitive than others. They are thickest, for instance, on the tips of our fingers and on our lips, and fewest over the back of the neck and shoulders, and across the lower part of the hips.[21]

For a long time, it was supposed that all these little nerve-bulbs in the skin did the same kind of work, because they looked, under the microscope, exactly alike; but it was found that they divide the work up among them, so that some of them give their entire attention to heat, and others to cold, others to touch, and others again to pain. So carefully has the work been mapped out among them that they report to different centres in the brain and spinal cord, so that we now understand why, in diseases which happen to attack one or other of these centres, we may lose our sense of heat and cold, as in that terrible disease, leprosy; or our sense of touch, as in paralysis; or we may even, in some very rare cases, lose our sense of pain, and yet have all our other senses perfect.