skin patch, transdermal patch, or transdermal delivery system, adhesive patch used to deliver a controlled dose of a drug through the skin over a period of time. A skin patch uses a special membrane to control the rate at which the liquid drug contained in the reservoir within the patch can pass through the skin and into the bloodstream. Some drugs must be combined with substances, such as alcohol, that increase their ability to penetrate the skin in order to be used in a skin patch. Drugs administered through skin patches include scopolamine (for motion sickness), nicotine (for quitting smoking), estrogen (for menopause and to prevent osteoporosis after menopause), nitroglycerin (for angina), and lidocaine to relieve the pain of shingles (herpes zoster). Molecules of insulin and many other substances, however, are too large to pass through the skin.

skin diving, act of swimming freely underwater. It is done with the aid of a face mask, swimming fins for the feet, and either a snorkel breathing tube or scuba [acronym for self-contained underwater breathing apparatus] gear. The fins increase the propulsion and agility of the swimmer. Skin diving is used in scientific, commercial, and military activities and in recent years has gained enormous popularity as a beach sport. Free underwater swimming is not new, and as long ago as the 8th cent. B.C. Greek divers, unconnected to the surface by air hoses or lines, collected sponges and mollusks in depths as great as 100 ft (30 m). The Greeks and Romans employed underwater warriors, trained to hold their breath for long periods of time, to sabotage enemy fleets. In the Pacific islands natives have long practiced skin diving and spear fishing. Many improvements in skin-diving equipment were made during World War II, and the so-called frogmen of the U.S. and British navies played a vital role in operations. An important development of this period was scuba diving with an Aqua-lung (see diving, deep-sea). The scuba diver, with his greater mobility, has replaced in many areas of underwater activity the conventional sea diver who is encumbered by heavy equipment and limited by a lifeline and air hose. However, it is dangerous for a scuba diver to work below a 130-ft (40-m) depth, and although free dives have been made to more than 300 ft (91 m), conventional dress and equipment are generally used for deep descents.

skin cancer, malignant tumor of the skin. The most common types of skin cancer are basal cell carcinoma, squamous cell carcinoma, and melanoma. Rarer forms include mycosis fungoides (a type of lymphoma) and Kaposi's sarcoma. Overexposure to the sun is the primary cause of the common skin cancers, and the popularity of tanning since the 1930s lies behind the rise in skin cancer rates. The depletion of the earth's protective ozone layer also plays a role. The most effective way of preventing skin cancer is to avoid exposure to the sun's ultraviolet rays by consistently applying effective sunscreens (see sunburn) and wearing protective clothing.

Basal and Squamous Cell Carcinomas

Basal and squamous cell carcinomas are the most common types of cancer. Both arise from epithelial tissue (see epithelium). They are rare in dark-skinned people; light-skinned, blue-eyed people who do not tan well but who have had significant exposure to the rays of the sun are at highest risk. Both types usually occur on the face or other exposed areas.

Basal cell carcinoma typically is seen as a raised, sometimes ulcerous nodule. The nodule may have a pearly appearance. It grows slowly and rarely metastasizes (spreads), but it can be locally destructive and disfiguring. Squamous cell carcinoma typically is seen as a painless lump that grows into a wartlike lesion, or it may arise in patches of red, scaly sun-damaged skin called actinic keratoses. It can metastasize and can lead to death.

Basal and squamous cell carcinomas are easily cured with appropriate treatment. The lesion is usually removed by scalpal excision, curettage, cryosurgery (freezing), or micrographic surgery in which successive thin slices are removed and examined for cancerous cells under a microscope until the samples are clear. If the cancer arises in an area where surgery would be difficult or disfiguring, radiation therapy may be employed. Genetic scientists have discovered a gene that, when mutated, causes basal cell carcinoma.

Melanoma

Melanoma is the most virulent type of skin cancer and the type most likely to be fatal. As with the other common skin cancers, melanoma can be caused by exposure to the sun, and its incidence is increasing around the world. There also appears to be a hereditary factor in some cases. Although light-skinned people are the most susceptible, melanomas are also seen in dark-skinned people. Melanomas arise in melanocytes, the melanin-containing cells of the epidermal layer of the skin. Melanin is the pigment that gives skin color and that helps to protect the skin from sun damage. In light-skinned people, melanomas appear most frequently on the trunk in men and on the arms or legs in women. In blacks melanomas appear most frequently on the hands and feet. It is unknown whether melanoma in blacks is related to sun exposure. It is recommended that people examine themselves regularly for any evidence of the characteristic changes in a mole that could raise a suspicion of melanoma. These include asymmetry of the mole, a mottled appearance (variations in color from shades of brown to a bluish tint), irregular or notched borders, and oozing or bleeding or a change in texture. Surgery performed before the melanoma has spread is the only effective treatment for melanoma.

skin, the flexible tissue (integument) enclosing the body of vertebrate animals. In humans and other mammals, the skin operates a complex organ of numerous structures (sometimes called the integumentary system) serving vital protective and metabolic functions. It contains two main layers of cells: a thin outer layer, the epidermis, and a thicker inner layer, the dermis. Along the internal surface of the epidermis, young cells continuously multiply, pushing the older cells outward. At the outer surface the older cells flatten and overlap to form a tough membrane and gradually shed as calluses or collections of dead skin. Horns, hoofs, hair (fur), feathers, and scales are evolutionary adaptations of the epidermis. Although the epidermis has no blood vessels, its deeper strata contain melanin, the pigment that gives color to the skin. The underlying dermis consists of connective tissue in which are embedded blood vessels, lymph channels, nerve endings, sweat glands, sebaceous glands, fat cells, hair follicles, and muscles. The nerve endings, called receptors, perform an important sensory function. They respond to various stimuli, including contact, heat, and cold. Response to cold activates the erector muscles, causing hair or fur to stand erect; fright also causes this reaction. From the outer surface of the dermis extend numerous projections (papillae) that fit into pits on the inner surface of the epidermis so that the two layers are firmly locked together. In humans, whorls on the fingers show where the epidermis falls between rows of papillae, making the patterns used in fingerprinting. The skin provides a barrier against invasion by outside organisms and protects underlying tissues and organs from abrasion and other injury, and its pigments shield the body from the dangerous ultraviolet rays in sunlight. It also waterproofs the body, preventing excessive loss or gain of bodily moisture. Human skin performs several functions that help maintain normal body temperature: its numerous sweat glands excrete waste products along with salt-laden moisture, the evaporation of which may account, in certain circumstances, for as much as 90% of the cooling of the body; its fat cells act as insulation against cold; and when the body overheats, the skin's extensive small blood vessels carry warm blood near the surface where it is cooled. The skin is lubricated by its own oil glands, which keep both the outside layer of the epidermis and the hair from drying to brittleness. Human skin has remarkable self-healing properties, particularly when only the epidermis is damaged. Even when the injury damages the dermis, healing may still be complete if the wounded area occurs in a part of the body with a rich blood supply. Deeper wounds, penetrating to the underlying tissue, heal by scar formation. Scar tissue lacks the infection-resisting and metabolic functions of healthy skin; hence, sufficiently extensive skin loss by widespread burns or wounds may cause death.

Malignant tumour of the skin, including some of the most common human cancers. Though recognizable at an early stage, it has a significant death rate. Light-skinned people have the highest risk but can reduce it by limiting exposure to sunlight and to ionizing radiation. The most common types arise in the epidermis (outer skin layer) and have become more frequent with the thinning of the atmosphere's ozone layer. The most serious form is melanoma, which is frequently fatal if not treated early with surgery. Cancers arising from the dermis are rare; the best-known is Kaposi sarcoma.

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Surface covering of the body that protects it and receives external sensory stimuli, consisting of an epidermis over a thicker dermis. The epidermis contains cells involved in immune defenses, sensory receptors, pigment cells, and keratin-producing cells. The last harden and migrate to the surface to form a dead, relatively dry outer layer of horny tissue that constantly sloughs away. The dermis contains sensory nerves and blood vessels within connective tissue. Collagen and elastin fibres give skin its tough, elastic quality. Cells scattered through it produce its components and take part in immune and other skin responses. A fat layer under the dermis provides nutritional storage, cushioning, and insulation. Skin disorders range from dermatitis and acne to skin cancer. Changes in skin colour (e.g., jaundice) or texture may be clues to systemic disorders. Seealso dermatology; hair; integument; nail; perspiration; sebaceous gland; sweat gland.

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