英文科學(xué)讀本 第五冊(cè)·Lesson 21 The Skin

Lesson 21 The Skin

In all mammals, indeed, in all warm-blooded animals, the skin has practically the same structure and the same functions. It will be well, therefore, to consider now the structure of our own skin, as typical of the rest, especially as it will lead to some very practical results, so far as we ourselves are concerned. We can then proceed to examine the variety of coverings with which Nature clothes her creatures to suit the varying conditions of their lives and habits.

The skin, although merely a thin covering for the body, is in reality a very complex organ, both in its structure and functions. The anatomist can easily separate it into two distinct layers or skins, one underlying the other.

The outermost or top skin is called the cuticle. It is a thin, horny, almost transparent layer, having neither blood-vessels nor nerves. It has, therefore, no sensibility to pain, and does not bleed if cut. It forms a protection to the more sensitive layer below. You may run a needle through the skin on the palm of the hand without making it bleed, or causing pain, because, in doing so, you pierce this outer layer only, in which there are neither blood-vessels nor nerves.

The true skin lies below the cuticle, and is known as the cutis or dermis. As the cuticle lies on the dermis, it sometimes gets the name epidermis—epidermis meaning simply on the dermis.

The true skin is a closely interwoven mass of fibrous tissue, crossed and re-crossed with blood-vessels and nerves. If, instead of running the needle through the epidermis, we prick the skin with it, we draw blood, and at the same time cause a little sharp twinge of pain. This tells us, first, that the blood-vessels form so close a network everywhere through the true skin, that it is impossible to prick it without piercing some of them, and causing blood to flow; and secondly, that the nerve-fibers are as abundantly distributed as the blood-vessels, or we should not feel pain from the prick of the needle.

The two layers of the skin lie naturally close to each other, so that it is impossible to make a fold of the skin without taking up both. When, however, we scald or burn the skin, and a blister is formed, it is the cuticle or epidermis only that rises. The irritation of the burn causes a watery fluid to exude from the under surface of the cuticle. This fluid, not being able to escape, collects there, and forces asunder and separates the cuticle from the true skin below, and so a blister is formed.

We have thus far regarded the skin as merely a double protecting coat for the body. Now let us look at it from another point of view. What happens to our skin when we have been undergoing any violent exertion, or when we sit for some time in a very hot room? We find it covered with round drops of liquid. This liquid we call sweat or perspiration. It oozes out from the skin. Let us ascertain what it is, and how and why it is thrown off by the skin in this way.

If a piece of the skin from any part of the body were examined under a microscope, it would be found to be perforated with a great number of tiny holes. These holes are the pores of the skin. In the palm of the hand there are about 2500 on every square inch of the skin; and there are from three to six millions of them on the entire surface of the body.

The pores are the openings of little tubes which extend inwards from the surface. They are about a quarter of an inch long, and the inner extremity is coiled up into a sort of ball. We call them the perspiration or sweat glands.

By the term gland we mean an organ whose business it is to be constantly separating or taking away certain fluids from the blood, some to be thrown off from the system as injurious, others to be used in the work of the body.

Our lessons on digestion showed you certain little glands in the mouth and under the tongue, whose duty it is to separate the fluid, called saliva, which is so necessary in the work of masticating the food.

The sweat, which these sweat-glands pour out over the surface of the skin, is a poisonous fluid, which must be thrown off from the system if the body is to be kept in health. We must take an early opportunity of learning how this is done.