It has been estimated that the external skin of an ordinary adult is equal to an area of about twelve square feet, and that in a tall man it may be as much as eighteen square feet. There is a considerable difference between twelve square feet and twelve feet square, and it is well to mention the fact in order that there may be no confusion. From this large surface alone, therefore, it is quite easy to see that the skin requires to have some attention paid to it. But it is really far more important than even its extensive surface would be likely to indicate, for it fulfils no less than seven different duties. In the first place it serves as an external covering to the body, and, as we shall see also, the internal skin acts as a support to the internal organs. Secondly, it is endowed with an extensive system of nerves, which give rise to the sensations of touch, of temperature, of pressure, and of pain. In this way we can tell whether a substance is rough or smooth, and whether it is hot or cold; we recognise, moreover, the difference between a gentle pressure of the hand and one so forcible as to cause pain. Thirdly, the skin, as we shall find farther on, contains thousands of small tubes for the purposes of perspiration, and besides this, there are other tubes secreting an oily substance. Fourthly, the skin plays an important part in regulating the temperature of the body. Thus in a warm atmosphere the skin becomes reddened and moist, and much heat is lost; on the other hand, when the air is colder the skin becomes pale, cool, and dry, thus conserving the body heat. Fifthly, the respiratory action of the skin must not be forgotten, although it is nothing like so great as that of the lungs. Nevertheless quite an appreciable amount of oxygen is absorbed through the skin, and beyond all question carbonic acid is exhaled from it. Sixthly, it is an absorbent; that is to say, the skin is capable of absorbing into the body certain substances applied to it. In this way remedies are often introduced into the system by what is known as inunction. And lastly, the skin is a great emunctory, and carries off waste matters from the body. Accordingly it acts as a purifier of the blood, in which it assists the kidneys, intestines, and the lungs. And more than this, it often happens that the turning point in any disease is announced by a sudden, profuse, and markedly offensive perspiration, as if a considerable amount of deleterious and noxious matter was suddenly expelled from the system.
From the foregoing it is evident that the skin has many varied and important duties to perform. As we might expect, moreover, an organ with such functions is of complicated structure. Its component parts, therefore, deserve to have some little attention paid to them, since the importance of the skin from a health point of view will then be all the more appreciated. The skin is most conveniently considered under three divisions - the skin itself; the glands, producing perspiration, oil, and hair, which are found within it; and the appendages belonging to it, the hair and the nails. The skin itself may be described as the soft and elastic tissue which invests the whole of the surface of the body, and consists of two layers, the outer or scarf skin, and the deeper or true skin. The interior of the body is likewise lined with a covering, which is termed mucous membrane, from the fact that from its surface, or from certain special glands within it, or from both, there is constantly being secreted a thin semi-transparent fluid called mucus. At the various openings of the body, as the mouth, the nostrils, and other parts, the external and internal skins are continuous with one another. Indeed, at these apertures the mucous membrane, or internal skin, takes leave of absence from the world to line the cavities within the body. So that, as Professor Huxley expresses it, "every part of the body might be said to be contained within the walls of a double bag, formed by the skin which invests the outside of the body, and the mucous membrane, its continuation, which lines the internal cavities."
The use of the scarf skin is manifestly to protect the more delicate true skin, while at the same time it allows the waste products and used-up material to escape from the body. In the substance of the true skin are thousands of minute little bodies called papillae, which are specially concerned in the sense of touch, for the vast majority of these papillae contain the end of a small nerve. The numberless fine ridges seen on the palmar surface of the hands and fingers, and on the soles of the feet, are really rows of these papillae, covered of course by the layers of the outer skin. The supply of blood to the skin is also very plenteous, each of its innumerable papillae being abundantly supplied in this respect. As a proof of the amount of blood circulating within the skin, and of its extensive nerve supply, it is only necessary to mention the fact that the finest needle cannot be passed into it without drawing blood and inflicting pain.
In addition to the foregoing the skin also contains a countless number of very fine tubes, which penetrate through its layers and open on its surfaces by minute openings called pores. There are altogether three different varieties of these tubes distributed throughout the skin, namely, those intended for perspiration; secondly, those which lead from the oil glands; and lastly, those which enclose each hair of the body. The first of these, which carry away the perspiration from the body, are very fine, the end away from the surface being coiled up in such a way as to form a ball or oval-shaped body, constituting the perspiration gland. The tube itself is also twisted like a corkscrew, and widens at its mouth. It is estimated that there are between 2,000 and 3,000 of these perspiration tubes in every square inch of the skin. Now, as we have already seen, the external skin of an ordinary adult is equal to an area of about twelve square feet, and in a tall person it may be as much as eighteen square feet. The number of these tubes, therefore, in the whole body will be many hundreds of thousands, so that it will readily be seen how exceedingly important it is that they should be kept in thorough working order by cleanliness. The two great purposes fulfilled by the perspiration are the removal by its means of worn-out or effete material which is injurious to the system, and the regulation of the heat of the body by its influence. When it is stopped by any reason, such as catarrh or disease, the skin fails in its work, and the noxious matters, instead of being expelled from the body, are thrown back into the system. Hence there is a good deal of truth in the belief that a freely acting skin is always a safeguard against disease.