The Skin of the Hand
By the aid of the accompanying drawings you will be able to form some idea of the structure of the skin of the hand.
One of them (fig. 70) represents a section of the skin, made perpendicular to the surface, as seen under the microscope. It is from the end of the thumb, and includes three of those delicate lines, or ridges that are found there.
The superficial, or uppermost strata (a and b), are the “Cuticle” or “false skin.” The outer layer (a) is hard, horny, and dry. It is composed of numerous fine scales laid upon one another, like the tiles upon the roof of a house, but adhering more closely together, so as to form one continuous sheet extending all over the body. The outermost of these scales are continually being shed, peeling off as scurf, or being rubbed off; and fresh ones are supplied by the next layer (b), which is a softer material and lies immediately upon the surface of the “cutis” or “true skin.”
This softer layer (b) is often called the “Rete Mucosum.” It is made up of minute bags or bladders, named “cells” by anatomists, which grow and propagate upon the exterior of the true skin, being nourished by the blood in the skin. Those which lie nearest the cutis are the youngest and the softest. Gradually they are pushed outwards by their successors or offspring; and, as they approach the surface, they become flatter and drier and more adherent to one another, and are finally converted into the thin scales of the cuticle. Thus, there is no real line of division between the cuticle and the rete mucosum; but the cells of the latter are gradually transformed into the scales of the former.
The rete mucosum is thicker in the Negro than in the white man, and contributes somewhat to the softness of his skin. It contains also the colouring matter in the form of minute black particles diffused among its cells (fig. 72). These particles disappear, more or less, as the cells become changed into scales; hence the outer part of the cuticle of the Negro is not so dark as the rete mucosum, but, as it is transparent, or nearly so, it allows the dark colour of the rete to show through it.
Persons commonly speak of the cuticle as if it were the whole thickness of the skin. Thus, when a blister has drawn, they say the skin is raised; whereas it is only the cuticle. This is forced off from the skin by the fluid effused into its softer layer—i. e. into the rete—in consequence of the irritating influence of the blister.
The cuticle has no nerves, and, therefore, no feeling. It may be cut or torn without pain. The snipping of a blister with the scissors is not felt, because the cuticle only is touched. It forms a covering to the whole surface of the body, and is invaluable as a means of preventing too great evaporation. Without it we should be dried up, almost mummified, by the end of a summer’s day. It also protects the delicate sensitive skin underneath. How sore is the knuckle when the cuticle has been rubbed off! The cuticle has, moreover, the accommodating property of becoming thickest where it is most wanted, as on the sole of the foot, and on the palms of the hands of blacksmiths, and artizans, and persons who handle the oar. And if any other part of the body be subjected to much friction, for instance, the knees of housemaids, or the shoulders of men who carry packs, the cuticle soon becomes thickened there.
Beneath the cuticle lies the “Cutis” or “True Skin” (c, fig. 70, and c and d, fig. 71). It is a tough structure consisting of interlacing fibrous and fine muscular tissue, and contains the blood-vessels and nerves. The cuticle may be pared off without any bleeding; but directly the skin is wounded the blood flows. The cutis does not present an even surface next the cuticle, but shoots out into a number of little finger-like processes, called “Papillæ,” which project into the contiguous soft stratum of the cuticle, and are embedded in it. Thus the superficies of the skin is increased; and as the blood-vessels and nerves of the cutis are continued into the papillæ, they contribute very greatly to the sensitiveness of the skin. They are most numerous in parts where the sensitiveness of the skin is greatest; for instance, they are more numerous on the palmar, than on the dorsal, surface of the hand. Near the ends of the fingers and thumb they are arranged in a linear manner, forming the delicate ridges that encircle the cones of the pulps. Sections of these ridges are represented in fig. 70.
The superficial or papillary part of the cutis is of finer and more delicate structure than the deeper or fibrous layer, and is, therefore, sometimes described as a separate layer. It is so represented in the accompanying figure (71, c).
As we are upon the subject of the cuticle and the papillæ, I will take the opportunity to say a word respecting two diseases of these structures, in which most of you, probably, have a personal interest. I mean “Warts” and “Corns.”
A Wart (fig. 74) depends chiefly on a diseased state of the papillary stratum of the skin. The papillæ become coarse and grow up beyond the level of the surrounding skin, so as to present an uneven or “warty” surface. They carry a layer of cuticle before them. This layer is usually thin, so that the wart bleeds easily when it is rubbed. Sometimes, however, it is very thick and hard like a piece of horn. We, now and then, hear of a horn growing upon some part of the body, perhaps on the forehead. Such a horn is, usually, nothing more than a conical mass of cuticle formed upon the surface of a large wart. Warts are generally caused by something irritating the skin, as dirt or soot rubbed into the cuticle. For this reason they are more frequent upon the hands than upon other parts of the body.
In a Corn (fig. 73), also, the papillæ are somewhat enlarged; and this accounts in part for the great tenderness of corns. But the primary and essential feature of a corn is a thickened state of the cuticle. This is caused by too great rapidity in its formation, and is, usually, dependent upon pressure, especially if the pressure be combined with some friction. Hence corns are most commonly found upon the foot, and upon the parts of the foot, where the skin is subject to pressure and rubbing against the shoe. The drawing shows the appearance presented by a vertical section through a corn and through a small portion of the skin on either side. The accumulated layers of cuticle are seen, and the enlarged papillæ shooting up into them. I need scarcely add that it is owing to ignorance, or something worse, when corn-cutters talk of curing the malady by taking out the roots; for, corns, evidently, have no roots.
One word of advice about corn-cutting. Most persons have some experience in this art, and some opportunity of practising it on themselves; and many pride themselves on their skill in it. The usual plan is to shave off layer after layer from the whole surface of the corn; and this, by lessening the projection of the corn, may give relief for a few days, though it does not always do that. Soon, however, the distress returns; and the area of the corn increases after each operation. Now, I would have you observe that it is at the middle of the corn that the papillæ are most enlarged; and it is here that the formation of cuticle goes on most quickly, giving rise to the little white cone or cones often seen in a corn and sometimes wrongly called the roots. The proper mode is to confine the cutting to this part, and to remove as much of the thickened cuticle as you can from this spot, digging, as it were, a hole in the middle and leaving the circumference intact. The circumference, which is not usually tender, thus forms a wall round the excavated centre and defends it from pressure; and great relief is experienced. Further benefit will be found from covering the corn with some soft adhesive plaster; and you may sometimes, with advantage, lightly apply common caustic before putting on the plaster. If you follow these directions carefully you may be your own chiropodists, and almost defy your bootmakers.
If, in cutting a corn, you go too deeply, you will wound the tops of the papillæ and cause some bleeding; this is not however usually followed by any ill consequences.