The Principles of Leather Manufacture :: CHAPTER VII. STRUCTURE AND GROWTH OF SKIN.

CHAPTER VII. STRUCTURE AND GROWTH OF SKIN.

Although, at first sight, the skins of different animals appear to have little in common, a closer examination shows that all the Mammalia possess skins which have the same general structure, and thus an anatomical description of the skin of an ox applies almost equally to that of a sheep, goat, or calf, though on account of the difference in texture and thickness the practical uses of these various materials may differ widely. The skins of lizards, alligators, fishes and serpents differ from those of the higher animals, chiefly in having considerable modifications in the epidermis, so that it becomes harder and forms “scales,” and the arrangement of the fibres presents considerable difference. In many fish-skins for instance, the fibres are in successive layers, at right angles to each other and diagonal to the skin, but not interlaced.

In its natural condition, the skin is not merely a covering for the animal, but at the same time an organ of sense and of secretion, and hence its structure is somewhat complicated. It consists of two principal layers, the epidermis (epithelium, cuticle) and the corium (derma, cutis or true skin). These are totally distinct, not only in structure and functions, but in their origin. In the egg of a bird and the ovum of a higher animal, the living germ consists of a single cell, which, as soon as fertilised, begins to multiply by repeated division. The mass of cells thus formed early differentiates into three distinct layers, from the upper of which the epithelium arises, while the true skin, together with the bones and cartilages, is derived from the middle one.

This distinction of origin corresponds with a wide difference of both anatomical and chemical characteristics. A diagrammatic section of calf-skin is shown in Fig. 9, and a more correct representation of its actual appearance is given in Plate I. (Frontispiece). The epidermis is very thin as compared with the true skin which it covers, and is entirely removed preparatory to tanning; it nevertheless possesses important functions. It is shown in Fig. 10 at a and b, more highly magnified. Its inner mucous layer b, the rete malpighi, which rests upon the true skin c, is soft, and composed of living nucleated cells, which multiply by division and form cell-walls of keratin. These are elongated in the deeper layers, and gradually become flattened as they approach the surface, where they dry up, and form the horny layer a. This last is being constantly worn away, thrown off as dead scales of skin, and as constantly renewed from below, by the multiplication of the cells. It is from the epithelial layer that the hair, as well as the sweat and fat-glands, are developed.

Fig. 9.—Vertical section of calf-skin, magnified about 50 diameters. a, epidermis; b, grain or papillary layer; c, fibrous layer of skin; d, hairs; e, fat-glands; f, sweat-glands; g, opening of ducts of sweat-glands; h, hair-muscles.

Each hair is surrounded by a sheath which is continuous with the epidermis, and into which the young hair usually grows as the old one falls out. The hair itself is covered with a layer of overlapping scales, like the slates on a roof, but of irregular form. These give it a serrated outline at the sides, and when strongly developed as in wool and some furs, confer the property of felting. Within these scales, which are called the “hair cuticle,” is a fibrous substance which forms the body of the hair; and sometimes but not always, there is also a central and cellular pith, which under the microscope frequently appears black and opaque, from the optical effect of imprisoned air. On boiling or long soaking in water, alcohol, or turpentine, the air-spaces become saturated with the liquid, and then appear transparent.

Fig. 10.—Epidermis layer.

The fibrous part of the hair is made up of long spindle-shaped cells, and contains the pigment which gives the hair its colour. The hair of the deer differs from that of most other animals in being almost wholly formed of polygonal cells, which, in white hairs, are usually filled with air. In dark hairs, both the hair and sheath are strongly pigmented, but the hair is much the most so, and hence the bulb has usually a distinct dark form. The dark-haired portions of a hide from which the hair has been removed by liming still remain coloured by the pigmented cells of the hair-sheaths, which can only be completely removed by “bating and scudding.”

Fig. 11.—a, sebaceous gland; b, hair; c, erector muscle. Mag. 200.

Fig. 12.—a, hair; b, hair cuticle; c, inner root-sheath; d, outer root-sheath; e, dermic coat of hair-sheath; f, origin of inner sheath; g, bulb; h, hair-papilla.

Near the opening of the hair-sheath to the surface of the skin the ducts of the sebaceous or fat-glands pass into the sheath and secrete a sort of oil to lubricate the hair. The glands themselves are formed of large nucleated cells arranged somewhat like a bunch of grapes; the upper and more central ones being highly charged with fatty matter. Their appearance is shown in Fig. 11. The base of the hair is a bulb, enclosing the hair papilla h (Fig. 12), which is a projecting knob of the true skin and which by means of the blood-vessels contained in it supplies nourishment to the hair. The hair-bulb is composed of round soft cells, which multiply rapidly, and pressing upwards through the hair-sheath, become hardened, thus increasing the length of the hair.

The cells outside the bulb, shown at f in Fig. 12, pass upwards as they grow, and form a coating around the hair, known as the “inner root-sheath.”

In embryonic development, a small knob of cells forms on the under side of the epidermis, over a knot of capillary blood-vessels in the corium, and enlarges and sinks deeper into the latter, while the root-bulb of the young hair is formed within it, surrounding the capillaries from which it derives nourishment, and which form the hair-papilla, Fig. 13. In the renewal of hair in the adult animal the process is very similar. The bulb of the old hair withers, and the hair falls out, and in the meantime a thickening takes place in the epidermal coating of the bottom of the sheath, and the young hair is formed below, and usually to one side of the old one, growing into the sheath, and taking the place of the old hair. This is one cause of the difficulty of removing ground-hairs in the process of unhairing, since they are not only short, but deeper seated than the old ones.

The process of development of the sudoriferous or sweat-glands is very similar to that of the hairs. They consist of more or less convoluted tubes with walls formed of longitudinal fibres of connective tissue of the corium, lined with a single layer of large nucleated cells, which secrete the perspiration. The ducts, which are exceedingly narrow, and with walls of nucleated cells like those of the outer hair-sheaths, sometimes open directly through the epidermis, but more frequently into the orifice of a hair-sheath, just at the surface of the skin. Each hair is provided with a slanting muscle called the arrector or erector pili (see Fig. 11), which is contracted by cold or fear, and causes the hair to “bristle,” or stand on end; by forcing up the attached skin, it produces the effect known as “goose-skin.” The muscle, which is of the unstriped or involuntary kind, passes from near the hair-bulb to the epidermis, and just under the sebaceous glands, which it compresses when it contracts.

Beside the hair, and hair-sheaths, and the sebaceous and sudoriferous glands, the epidermis layer produces other structures of a horny character, including horns, hoofs, claws and finger-nails; which both chemically and anatomically are analogous to exaggerated hairs, such as the quills of the porcupine.

The whole of the epidermis, together with the hairs, is separated from the corium by an exceedingly fine membrane, called the hyaline or glassy layer. This forms the very thin buff-coloured “grain-” surface of tanned leather, which is evidently of different structure from the rest of the corium, since, if it gets scraped off before tanning, the exposed portion of the underlying skin remains nearly white, instead of colouring. The whole of the hair-sheath is enclosed in a coating of elastic and connective-tissue fibres, which are supplied with nerves and blood-vessels, and form part of the corium.

Fig. 13.—Development of young hair.

Fig. 14.—Connective-tissue fibres. (Ranvier.)

The structure of the corium or true skin is quite different from that of the epidermis which has just been described, as it is principally composed of interlacing bundles of white fibres, of the kind known as “connective tissue” (see Fig. 14); these are composed of fibrils of extreme fineness, cemented together by a substance somewhat more soluble than the fibres themselves. The fibres are not themselves living cells, but are apparently produced by narrow spindle-shaped cells lying against them. The felted fibre-bundles are more loosely interwoven in the middle portion of the skin, but become compacter again near the flesh. In the case of sheep-skins this is especially marked, the middle part being full of fat-cells and very loose. Any ill treatment of the pelt during the wet-work is liable to still further loosen this middle layer so that grain and flesh may sometimes be torn apart. The flesh-splits of sheep-skins must have this loose fatty layer frized off before chamoising, and American “waxed fleshes” from ox-hides are levelled by splitting away this portion, and finished on the flesh. The outermost layer, just beneath the epidermis, is exceedingly close and compact, the fibre-bundles that run into it being separated into their elementary fibrils, which are so interlaced that they can scarcely be recognised. This is the pars papillaris, and forms the lighter-coloured layer, called (together with its very fine outer coating) the “grain” of leather. It is in this part that the fat-glands are embedded, while the hair-roots and sweat-glands pass through it into the looser tissue beneath. It receives its name from the small projections or papillÆ, with which its outer surface is studded, and which form the characteristic grain of the various kinds of skin.[20] (See Fig. 9 and Plate I.)

[20] It will be noted that the word “grain” is used by the tanner in at least three different senses, which are productive of much confusion. The extremely thin hyaline layer forms a natural glaze to the skin, and might well be spoken of as such; the form and arrangement of the papillÆ and hair-pores might be called the “pattern” of the grain, leaving the use of the word “grain” itself restricted to the pars papillaris.

Fig. 15.—Fat-cells in connective tissue. a, fat-globule; p, protoplasm; n, nucleus; m, cell-wall. (Ranvier.)

The study of the structure of the grain, and especially of the arrangement of the hair-pores is very important, as it is usually the readiest means of identifying the kind of skin of which a leather is made, which in finished skins with artificially printed grain is often very difficult. (Plate II.) The examination is facilitated by wetting and stretching the skin, and by the use of a good lens, or a low power of the microscope.[21]

[21] Under the microscope, the skin is of course lighted from above by direct light from a window, or by that of a lamp concentrated by a “bullseye” condenser. The reversal of the image in the microscope often causes a pseudoscopic effect very puzzling to the beginner, prominences appearing as hollows, and vice versa till the real direction of the lighting is considered.

PLATE II.

Photo-micrographs of Grain of various Skins (A. Seymour-Jones).

1. Cow-hide; 2. Calf-skin; 3. East India Goat; 4. Pig-skin; 5. East India Sheep; 6. Welsh Sheep.

[Face p. 52.

Fig. 16.—Striped, or voluntary muscular fibre. (Ranvier.)

As stated above, the surface of skin which is next to the flesh is firmer than that in the centre, and as the fibres run nearly parallel with the surface it has a more or less membranous character. The skin is united to the body of the animal by a network of connective tissue (panniculus adiposus), which is frequently full of fat-cells and is then called adipose tissue. This constitutes the whitish layer which is removed, together with portions of actual flesh, in the operation of “fleshing.” If a minute portion of adipose tissue be examined microscopically, it will appear to consist of a mere mass of fat-globules entangled in connective tissue. If, however, it be stained with carmine or logwood it may be at once observed that each globule is contained in a cell, of which the nucleated protoplasm, by which the fat was secreted, is pressed closely against the wall (Fig. 15). Similar cells are contained in considerable quantities throughout the hide, and especially in the loose tissue of the central part; hence in leather manufacture it is impossible to expel or wash out the fat until the cells have been broken down by “liming” or in some other way.

Many animals (ox, horse, etc.) possess a thin layer of voluntary muscle (red flesh) spread over the inner side of the skin, and used for twitching to drive off flies. In rough fleshing this is sometimes left on and may be a cause of dark flesh in sole leather. Even in the finished leather its striped structure may be detected microscopically (Fig. 16).

Besides the connective-tissue fibres, the skin contains a small proportion of fine yellow “elastic” fibres. If a thin section of hide be soaked for a few minutes in a mixture of equal parts of water, glycerine, and strong acetic acid, and then examined under the microscope, the white connective-tissue fibres become swollen and transparent, and the yellow “elastic” fibres may be seen, as they are scarcely affected by the acid. The hair-bulbs and sweat- and fat-glands are also rendered distinctly visible by this treatment. On the other hand, the white gelatinous fibres are most easily seen by examining the section in a strong solution of common salt, or in one of ammonium sulphate; or by staining with some aniline dyes such as safranine. Sections are most readily cut for these purposes by the use of the freezing microtome, or after previous hardening in alcohol. For further details see L.I.L.B., p. 254.

Ordinarily in the production of leather only the corium, or true skin is used, and in order to obtain it in a suitable condition for the various tanning processes, the hair or wool, together with the epithelium, must be completely removed without damaging the skin itself; and especial care must be taken that the grain, or portion next to the epidermis, does not suffer any injury during the treatment. All the methods employed depend upon the fact that the epidermis cells, especially the soft growing ones next to the corium, and those of the epidermis layer which surround the hair-roots, are more easily destroyed than the corium itself owing to their different chemical character. The “unhairing” process consists essentially in breaking down these cells by chemical or putrefactive agents, and removing the hair together with the rest of the epidermis by mechanical means. Of the various substances which may be used for this purpose, lime is one of the most convenient, as its solubility in water is so slight, that a solution of such a strength as to injure the hide cannot be easily made. Caustic alkalies, on the other hand, are much more soluble, and unless care be taken to use only the proper quantity, a dangerously strong solution may be made with consequent damage to the skin. The addition of small amounts of sulphides to the lime-solution accelerates the unhairing owing to their special solvent action on the epidermis-structures, and also in the case of alkaline sulphides, by the caustic alkali which is produced by their reaction with the lime. Even if used alone, strong solutions of alkaline sulphides rapidly destroy both hair and epidermis, converting them into a mass which may be swept off the skin like wet pulp, and yet they have practically no injurious action on the true skin.

In the “sweating” process the epidermic cells are broken down by putrefactive organisms and their products, so that the hair becomes loose and may then be either rubbed or scraped off. Ammonia, which is produced during the putrefaction, has also an important solvent action, and its presence doubtless tends to quicken the processes both of unhairing and of destruction.

To obtain useful knowledge of the structure of any particular skin, it is not necessary to have a very elaborate or expensive microscope, and it is quite possible to obtain useful information merely by the use of a good pocket lens, as for instance, in the examination of various forms of “grain,” and the embossing of one skin to imitate another.

For further details of the manipulation and selection of the microscope, the reader must consult L.I.L.B., pp. 234 et seq.

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