The containment function relates specifically to the ability of the skin to confine underlying tissues and restrain their movements. The skin draws the strength it needs to perform this mechanical role from its tough, fibrous dermis (2). Ordinarily, the skin is taut even when under resting tension, yet it stretches easily and elastically when the body is in motion, quickly returning to normal contours when the stretching ceases. This extensibility of the skin is attributable to an alignment of collagen fibers under tension and in the direction of a load, which are otherwise nonaligned in the ground tension state. If the skin becomes stretched beyond its ability to elastically restore its initial condition, it folds over itself or wrinkles. Lost elasticity is advanced through extended exposure to ultraviolet radiation (sunlight) and thus wrinkling is often pronounced on dedicated sunbathers.
The behavior of the epidermis when distended is also of importance. It is the stratum corneum's role to fend against tearing (2). This tissue is actually stronger per unit mass than the dermal fabric and, as a rule, is sufficiently elastic to adjust to stretching. Its pliability, however, is conditional, and it fissures and cracks if stretched when excessively dry. Arid atmospheres alone can produce this condition (windburn). Detergents and solvents, which extract essential, water-sequestering lipids from the stratum corneum, and diseases such as psoriasis associated with a malformed horny structure render the stratum corneum brittle and prone to Assuring.
Although much is still to be learned about the factors that contribute to the pliability of the stratum corneum, it is generally accepted that its elasticity is dependent on a proper balance of lipids, hygroscopic, water-soluble substances, and water, all in conjunction with its keratin proteins. Water is its principal plasticizer, or softening agent, and it takes roughly 15% moisture to maintain adequate pliability. The capacity of the stratum corneum to bind and hold onto water is greatly reduced by extracting it with lipid solvents such as ether and chloroform. Amino acids, hydroxy acids, urea, and inorganic ions, cosmetically referred to as the skin's natural moistening factor, and the stratum corneum's lipids assist the stratum corneum in retaining moisture necessary to plasticize its mosaic, filamented matrix. In effect, the water makes the tissue less crystalline through its interposition between polymer strands.
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