[CHAPTER I.]
ANATOMICAL STRUCTURE OF HIDE.
Before speaking of actual processes of manufacture, it is necessary to devote some attention to the structure and chemical constitution of hide or skin, which forms the raw material. Although a great variety of skins are employed in tanning, they are all constituted on the same general type, and an anatomical description of the hide of the ox will apply almost equally to those of the calf, sheep, and goat; but from differences in thickness and closeness of texture, their practical uses differ widely. [Fig. 1] shows a section of ox-hide, cut parallel with the hair, magnified about 50 diam.: a, epithelial layer or epidermis, consisting of horny layer above, and rete malpighi below; b, pars papillaris, and c, pars reticularis of corium, derma, or true skin; d, hairs; e, sebaceous or fat-glands; f, sudoriferous or sweat-glands; g, opening of ducts of sweat-glands; h, erectores pili muscles, for erecting the hair.
The fresh hide consists of 2 layers: an outer, the epidermis; and an inner, the true skin. 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. 1] at a, and more highly magnified in [Fig. 2]. Its inner mucous layer b, the rete malpighi, which rests upon the true skin c, is soft, and composed of living nucleated cells, which 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, and thrown off as dead scales of skin; and as constantly renewed from below, by the continued multiplication of the cells. It is from this epithelial layer that the hair, as well as the sweat- and fat-glands, are developed. It will be seen in [Fig. 1] that each hair is surrounded by a sheath, which is continuous with the epidermis. In embryonic development, a small knob of cells forms on the under side of the epidermis, and this enlarges, and sinks deeper into the true skin, while the root of the young hair is formed within it; this is shown in [Fig. 3], a b. Smaller projections also form on the stalk of the knob, and in due time produce the sebaceous glands. The process of development of the sudoriferous glands is very similar to that of the hairs. There is a great analogy between this process and that of the ordinary renewal of hair in the adult animal. At d1 [Fig. 1], is seen an old and worn-out hair. It is shrunken and elongated, and is almost ready to fall out. It will be noticed that its sheath or follicle projects somewhat below the hair to the right. This is the first production of a young hair, and is quite analogous to the knob of epithelium which has been described as forming the starting-point of a hair in embryo. At d2, the same process is seen further advanced, the young hair being already formed, and growing up into the old sheath. At d3, it is complete, the old hair having fallen out, and the young one having taken its place.
Fig. 1.
Fig. 2.