The protective organs are the skin and the horny box or hoof.
The external skin, or hide, covers the entire body; in the feet it covers the bones, tendons, and ligaments, even passing in under the hoof and directly covering the os pedis. This portion of the skin, enclosed by the hoof and therefore invisible, is called the pododerm or foot-skin. In Germany it is called the hoof-skin (huflederhaut), because it is a continuation of the outer visible skin, and because it secretes the hoof,—that is, the hoof is produced by it. That part of the skin which is covered with hair is known as the external or hair-skin.
(a) The hair-skin ([Fig. 27, a]) consists of three superposed layers,—(1) the external superficial layer, or epidermis; (2) the middle layer, derm or leather-skin (so-called because leather is made from it); (3) the internal layer, or subcutaneous connective tissue.
1. The external layer, or epidermis, is composed merely of single flattened, horn-like cells (scales) lying side by side and over one another, and uniting to form one entire structure,—a thin, horn-like layer, without blood-vessels or nerves. It extends over the entire surface of the body, and protects the underlying, very sensitive middle layer from external influences. The oldest cell-layers lie on the outer surface, and are being continuously brushed off in patches or scales, while new ones are constantly being formed on the outer surface of the middle layer.
2. The middle layer, leather-skin or dermis, is composed of solid, fibrous, and elastic tissues, and contains many blood-vessels, small nerves, sweat-and oil-glands, and hair follicles from which the hair grows. The hair upon the posterior surface of the fetlock-joint is usually long and coarse, forming a tuft known as the “foot-lock,” which encloses a horny spur, called the ergot. Common-bred horses have, as a rule, larger and coarser footlocks than thoroughbreds. The derm or leather-skin, which produces the hair and epiderm, is the thickest and most important layer of the skin.
3. The inner layer, or subcutaneous tissue, unites the middle layer with the muscles, tendons, ligaments, bones, or other structures. It is that loose fibrous mesh or net-work through which the butcher cuts in removing the hide from the carcass.
Fig. 27.
Foot from which the horny capsule or hoof, has been removed by prolonged soaking: a, skin; on the left the hair has been rubbed away; b, perioplic band; c, coronary cushion; d, podophyllous tissue (fleshy leaves); at the lower border of the figure can be seen the minute thread-like processes or villi which grow down from the lower end of each fleshy leaf.
(b) The hoof-skin ([Figs. 27] and [28, b, c, d]), or pododerm, is completely enclosed by the hoof. Although it is only an extension of the derm or middle layer of the hair-skin, it differs from the latter in structure and relations.