The EPIDERMIS is the outer skin layer. It is made up of tough, flat, scalelike epithelial cells. Four different sublayers of epidermal cells have been identified. The uppermost is called the horny layer (stratum corneum). It is composed of scaly dead cells that form a protective surface and are gradually sloughed off naturally or by irritation (e.g., sunburn) or abrasion. This scaly layer, if unbroken, can block the passage of almost every known type of germ; however, its protective powers are reduced if the skin is not cleansed regularly. Two middle layers of cells may be present in a particular area of skin, depending on its thickness (the soles of the feet are the thickest skin, the eyelids the thinnest). In the innermost sublayer, the stratum germinativum, new epidermal cells are constantly being produced to replace the sloughed off cells. These newly formed cells push the older cells outward. As they approach the surface, they become drier or more scalelike. Because of this constant activity of the deeper cells of the epidermis, any injury of the outer layer of the skin is repaired in a few days without leaving a scar. Skin pigment, called melanin, which is responsible for skin color, is found here in this deepest sublayer. The color and quantity of the melanin are the chief factors in determining ones complexion. The pigment can be darkened by exposure to the ultraviolet rays of the sun (tanning). Freckles are patches of melanin.
The DERMIS, or true skin, lies below the epidermis and gradually blends into the deeper tissues. It is a wide area of connective tissue that contains blood vessels, hair follicles, nerve endings, smooth muscles, and sweat and oil glands.
The blood vessels of the dermis can dilate to contain a significant portion of the bodys blood supply. This ability, along with the actions of the sweat glands, forms the bodys primary temperature regulating mechanism. The constriction or dilation of these blood vessels also affects blood pressure and the volume of blood available to the internal organs.
The skin contains nerve endings that carry impulses to and from the central nervous system. The nerves are distributed to the smooth muscles in the walls of the arteries in the dermis and to the smooth muscles around the sweat glands and hair roots. Through these nerves, messages about the external environment are carried to the brain.
Smooth involuntary muscles are found in the dermis. They are responsible for controlling the skin surface area. When dilated, these muscles allow for maximum skin surface exposure to aid heat loss. When constricted, the skin surface exposure is decreased, thus impeding heat radiation. Repeated muscle contractions (shivering) are also a rapid means of generating body heat.
The appendages of the skin are the nails, hairs, sebaceous glands, sweat glands, and ceruminous glands.
The NAILS are composed of horny epidermal scales and are found on the dorsal surfaces of the fingers and toes. They protect the many sensitive nerve endings at the ends of these digits. New formation of nail will occur in the epitheliums of the nail bed. As new nail is formed, the whole nail moves forward, becoming longer.
HAIR is an epithelial structure found on almost every part of the surface of the body. Its color depends on the type of melanin present. The hair has two components: the root below the surface and the shaft projecting above the skin. The root is embedded in a pitlike depression called the hair follicle. Hair grows as a result of the division of the cells of the root. A small muscle, the arrector, fastens to the side of the follicle and is responsible for the gooseflesh appearance of the skin as a reaction to cold or fear. Each hair follicle is associated with two or more sebaceous (oil) glands.
SEBACEOUS GLANDS are found in most parts of the skin except in the soles of the feet and the palms of the hand. Their ducts open most frequently into the hair follicles and secrete an oily substance that lubricates the skin and hair, keeping them soft and pliable and preventing bacterial invasion.
SWEAT GLANDS are found in almost every part of the skin. They are control mechanisms to reduce the bodys heat by evaporation of water from its surface. The perspiration secreted is a combination of water, salts, fatty acids, and urea. Normally, about one liter of this fluid is excreted daily. However, the amount varies with atmospheric temperature and humidity and the amount of exercise taken. When the outside temperature is high, or upon exercise, the glands secrete excessive amounts to cool the body through evaporation. When evaporation cannot handle all the sweat that has been excreted, the sweat collects in beads on the surface of the skin.
CERUMINOUS GLANDS are modified sweat glands found only in the auditory canal.