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).
[Illustration:
FIG. 572.—LATERAL VIEW OF HORSE TO SHOW HAIR-STREAMS AND VORTICES.
(After Ellenberger-Baum, Anat. für Künstler.)
]
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.