The =skin= or =common integument= (Integumentum commune) is the
protective covering of the body, and is continuous at the natural
openings with the mucous membranes of the digestive, respiratory, and
urogenital tracts. It contains peripheral ramifications of the sensory
nerves, and is thus an important sense organ. It is the principal factor
in the regulation of the temperature of the body, and by means of its
glands it plays an important part in secretion and excretion. Some of
its special horny modifications or appendages are used as organs of
prehension or as weapons.
The =thickness= of the skin varies in the different species, on
different parts of the body of the same animal, and also with the breed,
sex, and age. The color also varies greatly, but this is masked in most
places by the covering of hair or wool. The skin is in general highly
elastic and resistant.
Permanent =folds= of the skin (Plicæ cutis) occur in certain situations,
and in some places there are cutaneous =pouches= or =diverticula= (Sinus
cutis).
The skin is attached to the underlying parts by the =subcutis= or
=superficial fascia= (Tela subcutanea). This consists of connective
tissue containing elastic fibers and fat. When the fat forms a layer of
considerable thickness, it is termed the =panniculus adiposus=. Over a
considerable part of the body the subcutis contains striped muscle, the
=panniculus carnosus= (M. cutaneus); in some regions the fibers of the
muscle are inserted into the skin, and their contraction twitches the
skin or produces temporary folds.[224] The amount of subcutaneous tissue
varies widely; in some places it is abundant, so that the skin can be
raised considerably; in other situations it is practically absent and
the skin is closely adherent to the subjacent structures. =Subcutaneous
bursæ= often develop over prominent parts of the skeleton where there is
much pressure or friction.
=Structure.=—The integument consists of the cutis or skin proper and its
epidermal appendages, _e. g._, hairs, hoofs, claws, horns, etc. The
cutis consists of two distinct strata, viz., a superficial epithelial
layer, the =epidermis=, and a deep connective-tissue layer, the
=corium=.
The =epidermis= is a non-vascular, stratified epithelium of varying
thickness. It presents the openings of the cutaneous glands and the
hair-follicles, and its deep surface is adapted to the corium.[225] It
is divisible into a superficial, harder, drier part, the =stratum
corneum=, and a deeper, softer, moister part, the =stratum
germinativum=. The cells of the latter contain pigment, and by their
proliferation compensate the loss by desquamation of the superficial
part of the stratum corneum. In many places further subdivision into
strata is evident on properly prepared cross-sections.
The =corium= consists essentially of a feltwork of connective-tissue and
elastic fibers. It is well supplied with vessels and nerves, and
contains the cutaneous glands, the hair-follicles, and unstriped muscle.
The deeper part of the corium, the =tunica propria=, consists of a
relatively loose network of coarse bundles of fibers, and in most places
there is no clear line of demarcation between it and the subcutis. The
superficial part, the =corpus papillare=, is of finer texture and is
free from fat. Its superficial face is thickly beset with blunt conical
prominences, the =papillæ=, which are received into corresponding
depressions of the epidermis. They contain vascular loops and nerves,
or, in certain situations, special nerve-endings.
The papillæ are best developed where the epidermis is thick and hairs
are small or absent. On thickly-haired regions they are small or even
absent. On certain parts of the body (anus, vulva, prepuce, scrotum,
eyelids, etc.) the corium contains pigment in its connective-tissue
cells.
The =glands= of the skin (Glandulæ cutis) are chiefly of two kinds,
sudoriferous and sebaceous.
The =sudoriferous= or =sweat glands= (Glandulæ sudoriferæ) consist of a
tube, the lower, secretory part of which is coiled in the deep part of
the corium or in the subcutis to form a round or oval ball (Corpus
glandulæ sudoriferæ). The excretory duct (Ductus sudoriferus) passes
almost straight up through the corium, but pursues a more or less
flexuous course through the epidermis, and opens into a hair-follicle or
by a funnel-shaped pore (Porus sudoriferus) on the surface of the skin.
The =sebaceous glands= (Glandulæ sebaceæ) are in great part associated
with the hairs, into the follicles of which they open. Their size varies
widely, and is in general in inverse ratio to that of the hair. The
larger ones are easily seen with the naked eye, and appear as small,
pale yellow or brownish bodies. In certain situations (_e. g._, the
labia vulvæ, anus, prepuce) they are independent of the hairs and are
well developed. In form they may be branched alveolar, simple alveolar,
or even tubular in type. They secrete a fatty substance, the =sebum
cutaneum=, which serves as a protective against moisture, and may also
(by its aromatic constituents) play an important part in the sexual life
of animals.
The two kinds of glands described above are those which are most
widely distributed, but many special types occur. Some of these are to
be regarded as modified sweat glands, _e. g._, the naso-labial glands
of the ox, the glands of the snout of the pig, and the glands of the
plantar cushion of the horse. Others, _e. g_., the tarsal (or
Meibomian) glands of the eyelids, are modified sebaceous. Still others
are not yet classified satisfactorily. Some of these special types
have been referred to in previous chapters, and others will receive
attention in the special descriptions which follow. The mammary glands
are highly modified cutaneous glands, which are intimately associated
in function with the genital organs, and have been described with the
latter.
=Vessels and Nerves.=—The =arteries= of the skin enter from the
subcutis, where they communicate freely. In the deeper part of the
corium they form a plexus, and another network is formed under the
papillæ. Small vessels from the deep plexus go to the fat and sweat
glands, and the subpapillary plexus sends fine branches to the papillæ,
hair-follicles, and sebaceous glands. The =veins= form two plexuses, one
beneath the papillæ, and another at the junction of the corium and
subcutis. The =lymph vessels= form subpapillary and subcutaneous
plexuses.
The =nerves= vary widely in number in different parts of the skin. The
terminal fibers either end free in the epidermis and in certain parts of
the corium, or form special microscopic corpuscles of several kinds.