The integument (Fig. 11.1) consists ofthe skin and its associated appendages, the hair, nails, sweat glands, mammary glands, and sebaceous glands. The skin, the largest organ in the body, has an outer epithelial layer, the epidermis, adhering firmly to an underlying layer ofdense, irregular connective tissue, the dermis. Beneath this is a soft flexible layer offat and loose connective tissue, the hypodermis. The principal, and probably phylogenetically original, function of skin is to maintain a constant internal environment for the cells of the body, a culture medium, as it were, separated from the variable external environment. Loss or destruction of large areas of skin, as in burns, results in serious, often fatal fluid loss and changes in electrolyte balance and blood concentration. In addition, the uncovering of the underlying tissues exposes them to infections. Thus a major function of skin is protection from invaders as well as from gross physical insults. It also functions in excretion, temperature regulation, sensory reception, and photochemistry. It is flexible, elastic, self-sealing, and self-replacing. All of these properties are self-adjusting to environmental changes; instead of being worn away by friction, for example, skin thickens to form a callus.
KeywordsCholesterol Depression Tyrosine MolD Phenylalanine
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