The Appendages of the Skin

The appendages of the skin are modifications of the epidermis, and comprise the hairs, hoofs, claws, horns, etc.

The hairs (Pili) cover almost the entire surface of the body in the domesticated mammals, and some parts which appear at first sight to be bare are found on close inspection to be provided with sparse and very fine hair. The hairs are constantly being shed and replaced, but at certain periods in the horse, for example, they fall out in great numbers, constituting the shedding of the coat. It is customary to distinguish the ordinary hairs (the coat), which determine the color of the animal, from the special varieties found in certain places. Among the latter are the long tactile hairs about the lips, nostrils, and eyes; the eyelashes or cilia; the tragi of the external ear; and the vibrissæ of the nostrils. Other special features will be noted in the discussion of the skin of the various species. The hairs are directed in such a way as to form more or less definite hair-streams (Flumina pilorum), and at certain points these converge to form vortices (Vortices pilorum).

The part of the hair above the surface of the skin is the shaft (Scapus pili), while the root (Radix pili) is embedded in a depression termed the hair-follicle (Folliculus pili). A vascular papilla (Papilla pili) projects up in the fundus of the follicle and is capped by the expanded end of the root, the bulb of the hair (Bulbus pili). The hair-follicles extend obliquely into the corium to a varying depth; in the case of the long tactile hairs they reach to the underlying muscle. Most of the follicles have attached to them small unstriped muscles known as the arrectores pilorum; these are attached at an acute angle to the under side of the deep part of the follicle, and their contraction causes erection of the hair and compression of the sebaceous glands, one or more of which open into the follicle.

The hairs are composed of epidermal cells, and consist from without inward of three parts. The cuticle is composed of horny, scale-like cells which overlap like slates on a roof. The cortex consists of horny fusiform cells which are packed close together and contain pigment. The medulla is the central core of softer, cubical or polyhedral cells; it contains some pigment and air-spaces.

The hair-follicles, being invaginations of the skin, are composed of a central epidermal part, and a peripheral layer which corresponds in structure to the corium. The follicles of the tactile hairs have remarkably thick walls which contain blood-sinuses between their outer and inner layers; in ungulates the sinuses are crossed by trabeculæ and assume the character of cavernous or erectile tissue.

The hoofs, claws, horns, and other horny structures consist of closely packed epidermal cells which have undergone cornification. In structure they might be compared to hairs matted together by intervening epidermal cells. They cover a specialized corium known as their matrix, from which the stratum germinativum derives its nutrition.