SKIN AND EXOSKELETON, in anatomy. The skin is the covering of the whole body, and is continuous at the different orifices with the mucous membrane. It acts as a protective layer, as a regulator of the temperature, as an excretory organ and as a tactile and sensory organ in which nerves end.
The skin varies in thickness from .5mm. in the eyelids to 4 or more mm. in the palms and soles; it is also very thick over the back of the body. Two main layers are recognized, superficially the epidermis and more deeply the dermis or true skin. The epidermis under the microscope is seen to consist of five layers. On the sur face is the horny layer or stratum corneum (see fig. 1) composed of layers of scale-like cells, the walls of which are turned into the horny substance keratin. Deep to this is a thin layer of scale-like cells without keratin known as the stratum lucidum. Deeper still is a layer, the stratum granulosum, in which the cells are not so flattened and contain granules of a substance known as eleidin. In the fourth layer, stratum mucosum or stratum Malpighii, the cells are polygonal and are connected together by delicate prickle like processes. It is in the deeper layers of these cells that the pigment of the negro's skin is found. The fifth and deepest layer of the epidermis is the stratum germinativum, in which there is only one layer of columnar cells. The whole of the epidermis is non-vascular. The true skin, dermis or corium is composed of a felted network of white fibrous tissue with a small number of yel low elastic fibres interspersed. It is divided into two layers.
The superficial or papillary layer lies next to the epidermis and is raised into a number of papillae or conical projections which fit into corresponding depressions on the deep surface of the epi dermis. In sensitive parts like the palms and soles these papillae are specially prominent and form wavy lines, each of which con sists of a double row between which the ducts of the sweat glands pass on their way to the surface. So large are the papillae in these situations that the epidermis is also raised into ridges, and these in the fingers form the characteristic whorls so valuable for pur poses of identification. The papillae contain leashes of blood-ves
sels, and in some of them are special tactile corpuscles in which the nerves end (see NERVOUS SYSTEM).
In the deeper or reticular layer of the true skin the fibrous feltwork is looser and encloses pellets of fat. It also contains a network of blood-vessels and nerves, and in some places a layer of striped or unstriped muscle. Where hairs are present the hair follicles lie in this deeper layer, which gradually merges with the subcutaneous fatty tissue (see fig. 2).
As appendages of the skin are found the hairs, the nails and the sebaceous and sweat glands.
Hair.—The hairs are found in man on the scalp, eyelids, eye brows, armpits, pubic region, vestibule of the nose, external audi tory meatus, face, ventral surface of the trunk and dorsal sur faces of the leg, forearm and hand; indeed the only places which are quite free from them are the palms of the hands, soles of the feet and the glans penis. In some places, such as the armpits, pubic region and the face of the male they grow to a considerable length at and after puberty. In section it is only the straight hairs which are circular; wavy and curly hairs are oval. In the centre of each hair is the medulla or pith, though this is not always present; it is composed of nucleated cells containing pigment, fat and air spaces. Outside this is the fibrous layer or cortex, also containing pigment and air spaces, while most superficially is the cuticle made up of overlapping scales. The hair grows at its root from a hair follicle (see fig. 2), which is a tubular inpushing of the epidermis into the true skin or, in the case of large hairs, deeper still into the superficial fascia. It is divided into an inner and outer root sheath, the former representing the more superficial layers of the epidermis, the latter the deeper layers. At the bot tom of the follicle the hair enlarges to form the bulb, and into the lower part of this a vascular papilla projects from the true skin. The cells of the hair are derived from, and are continuous at the bulb with those of the outer root sheath, and therefore with the deeper layers of the epidermis.