Skin
Skin is the largest organ of the body; its main function is to detect sensations, such as pressure and temperature. The skin covers and protects the body from pathogenic organisms, dehydration, and chemical and physical damage.
The outer layer of skin is called the epidermis. It contains melanin, a brown pigment which protects the body from ultraviolet light. The epidermis is involved in the regulation of water and salt concentrations in the body. It varies in thickness and can become hard and thick, as on the soles of feet. This layer also produces keratin, which is a waterproof protein found in fingernails.
The next layer is the dermis which is composed of many different types of tissues and appendages. It is a tough, flexible layer primarily composed of connective tissue. Within the connective tissue lie nerves, blood vessels, lymphatic vessels, and fat cells. The blood vessels in this area are responsible for the skin temperature and the blushing response. The fat cells insulate and protect the body. Skin appendages include hair, sebaceous glands, sweat glands and nails.
Hair is composed of modified epidermis cells with a root embedded within a follicle. It is composed of two layers: dermic and epidermic. The dermic layer is the outer layer. It is composed of three different layers: inner layer, cuticle, and basement membrane. These layers are relatively opaque and serve to protect the hair.
The epidermic layer is closely attached to the root of the hair. This layer contains pigment which determines the color of the hair. The hair is lost and replaced every three to five years; baldness seems to be a genetically inherited trait.
Sebaceous glands, are found in most parts of the body and are especially numerous on the face and scalp. They secrete sebum, an oily substance which protects the skin and hair from drying and cracking. Sweat glands are coiled tubes, whose main function is the secretion of fluid onto the skin, where it evaporates and cools the body temperature.
References
Gray, H. 1977. Gray's Anatomy. Crown Publishers, Inc. New York. 1257 pp.
Romer, A.S. & T.S. Parsons. 1978. The Vertebrate Body. W.B. Saunders Company, Philadelphia. 476 pp.
Van Amerongen, C. The Way Things Work; Book Of The Body. New York: Simon and Schuster, 1979.
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