Skin

Skin, tissue that forms a sensitive, elastic, protective, and waterproof covering of the body, together with its specializations (e.g., nails, hair). In the adult human it weighs 6.1 lb (2.75 kg), covers 18.3 sq ft (1.7 sq m), and varies in thickness from .04 in (1mm) in the eyelids to. 12 in (3 mm) in the palms and soles. It consists of 2 layers: the outer, epidermis, and the inner, dermis, or true skin. The outermost part of the epidermis, the stratum corneum, contains a tough protein called keratin. Consequently, it provides protection against mechanical trauma, a barrier against microorganisms, and waterproofing. The epidermis also contains cells that produce the melanin responsible for skin pigmentation and that provide protection against the sun's ultraviolet rays. The unique pattern of skin folding on the soles and palms provides a gripping surface and is the basis of identification by fingerprints. The dermis is usually thicker than the epidermis and contains blood vessels, nerves, and sensory receptors, sweat glands, sebaceous glands, hair follicles, fat cells, and fibers. Temperature regulation of the body is aided by the evaporative cooling of sweat, regulation of the skin blood flow, and the erection of hairs that trap an insulating layer of air next to the skin. The rich nerve supply of the dermis is responsible for the reception of touch, pressure, pain, and temperature stimuli. Leading into the hair follicles are sebaceous glands that produce the antibacterial sebum, a fluid that keeps the hairs oiled and the skin moist. The action of sunlight on the skin initiates the formation of vitamin D, which helps prevent rickets.