The Principles of Leather Manufacture

by H. R. Procter

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.”