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The major function of the skin is to keep the body in homeostasis despite the daily assaults of the environment. It provides boundaries for body fluids while protecting underlying tissues from microorganisms, harmful substances, and radiation. It modulates body temperature and synthesizes vitamin D.

The skin is the heaviest single organ of the body, accounting for approximately 16% of body weight and covering an area of roughly 1.2 to 2.3 meters squared. It contains three layers: the epidermis, the dermis, and the subcutaneous tissues.

Hair shaft

Duct of sweat gland

Epidermis

Dermis

Subcutaneous tissue

The most superficial layer, the epidermis, is thin, devoid of blood vessels, and itself divided into two layers: an outer horny layer of dead keratinized cells and an inner cellular layer where both melanin and keratin are formed.

The epidermis depends on the underlying dermis for its nutrition. The dermis is well supplied with blood. It contains connective tissue, sebaceous glands, sweat glands, and hair follicles. It merges below with subcutaneous tissue, or adipose, also known as fat.

'air, nails, and sebaceous and sweat glands are considered appendages of the in. Adults have two types of hair: vellus hair, which is short, fine, incon-icuous, and relatively unpigmented; and terminal hair, which is coarser, icker, more conspicuous, and usually pigmented. Scalp hair and eyebrows are examples of terminal hair.

Hair shaft

Hair follicle

Duct of sweat gland

Epidermis

Dermis

Subcutaneous tissue

Horny layer Cellular layer Sebaceous gland

Muscle that erects hair shaft

Sweat gland

Hair follicle

Nails protect the distal ends of the fingers and toes. The firm, rectangular, and usually curving nail plate gets its pink color from the vascular nail bed to which the plate is firmly attached. Note the whitish moon (lunula) and the free edge of the nail plate. Roughly a fourth of the nail plate (the nail root) is covered by the proximal nail fold. The cuticle extends from this fold and, functioning as a seal, protects the space between the fold and the plate from external moisture. Lateral nail folds cover the sides of the nail plate. Note that the angle between the proximal nail fold and the nail plate is normally less than 180°.

Fingernails grow at about 0.1 mm daily; toenails grow more slowly.

Sebaceous glands produce a fatty substance that is secreted to the skin surface through the hair follicles. These glands are present on all skin surfaces except the palms and soles. Sweat glands are of two types: eccrine and apocrine. The eccrine glands are widely distributed, open directly onto the skin surface, and by their sweat production help to control body temperature. In contrast, the apocrine glands are found chiefly in the axillary and genital regions, usually open into hair follicles, and are stimulated by emotional stress. Bacterial decomposition of apocrine sweat is responsible for adult body odor.

The color of normal skin depends primarily on four pigments: melanin, carotene, oxyhemoglobin, and deoxyhemoglobin. The amount of melanin, the brownish pigment of the skin, is genetically determined and is increased by sunlight. Carotene is a golden yellow pigment that exists in subcutaneous fat and in heavily keratinized areas such as the palms and soles.

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