Human skin
Human skin, in human anatomy, the covering, or integument, of the body’s surface that both provides protection and receives sensory stimuli from the external environment. The skin consists of three layers of tissue: the epidermis, an outermost layer that contains the primary protective structure, the stratum corneum; the dermis, a fibrous layer that supports and strengthens the epidermis; and the subcutis, a subcutaneous layer of fat beneath the dermis that supplies nutrients to the other two layers and that cushions and insulates the body.
The apparent lack of body hair immediately distinguishes human beings from all other large land mammals. Regardless of individual or racial differences, the human body seems to be more or less hairless, in the sense that the hair is so vestigial as to seem absent; yet in certain areas hair grows profusely. These relatively hairy places may be referred to as epigamic areas, and they are concerned with social and sexual communication, either visually or by scent from glands associated with the hair follicles.
The characteristic features of skin change from the time of birth to old age. In infants and children it is velvety, dry, soft, and largely free of wrinkles and blemishes. Children younger than two years sweat poorly and irregularly; their sebaceous glands function minimally. At adolescence hair becomes longer, thicker, and more pigmented, particularly in the scalp, axillae, pubic eminence, and the male face. General skin pigmentation increases, localized pigmented foci appear mysteriously, and acne lesions often develop. Hair growth, sweating, and sebaceous secretion begin to blossom. As a person ages, anatomical and physiological alterations, as well as exposure to sunlight and wind, leave skin, particularly that not protected by clothing, dry, wrinkled, and flaccid.
Human skin, more than that of any other mammal, exhibits striking topographic differences. An example is the dissimilarity between the palms and the backs of the hands and fingers. The skin of the eyebrows is thick, coarse, and hairy; that on the eyelids is thin, smooth, and covered with almost invisible hairs. The face is seldom visibly haired on the forehead and cheekbones. It is completely hairless in the vermilion border of the lips, yet coarsely hairy over the chin and jaws of males. The surfaces of the forehead, cheeks, and nose are normally oily, in contrast with the relatively greaseless lower surface of the chin and jaws. The skin of the chest, pubic region, scalp, axillae, abdomen, soles of the feet, and ends of the fingers varies as much structurally and functionally as it would if the skin in these different areas belonged to different animals.
skinA cross section of mammalian skin and its underlying structures.
The skin achieves strength and pliability by being composed of numbers of layers oriented so that each complements the others structurally and functionally. To allow communication with the environment, countless nerves—some modified as specialized receptor end organs and others more or less structureless—come as close as possible to the surface layer, and nearly every skin organ is enwrapped by skeins of fine sensory nerves.
The dermis makes up the bulk of the skin and provides physical protection. It is composed of an association of fibres, mainly collagen, with materials known as glycosaminoglycans, which are capable of holding a large amount of water, thus maintaining the turgidity of the skin. A network of extendable elastic fibres keeps the skin taut and restores it after it has been stretched.
The hair follicles and skin glands are derived from the epidermis but are deeply embedded in the dermis. The dermis is richly supplied with blood vessels, although none penetrates the living epidermis. The epidermis receives materials only by diffusion from below. The dermis also contains nerves and sense organs at various levels.
Human skin is enormously well supplied with blood vessels; it is pervaded with a tangled, though apparently orderly, mass of arteries, veins, and capillaries. Such a supply of blood, far in excess of the maximum biologic needs of the skin itself, is evidence that the skin is at the service of the blood vascular system, functioning as a cooling device. To aid in this function, sweat glands pour water upon its surface, the evaporation of which absorbs heat from the skin. If the environment is cold and body heat must be conserved, cutaneous blood vessels contract in quick, successive rhythms, allowing only a small amount of blood to flow through them. When the environment is warm, they contract at long intervals, providing a free flow of blood. During muscular exertion, when great quantities of generated heat must be dissipated, blood flow through the skin is maximal.
In addition to its control of body temperature, skin also plays a role in the regulation of blood pressure. Much of the flow of blood can be controlled by the opening and closing of certain sphincterlike vessels in the skin. These vessels allow the blood to circulate through the peripheral capillary beds or to bypass them by being shunted directly from small arteries to veins.
Human skin is permeated with an intricate mesh of lymph vessels. In the more superficial parts of the dermis, minute lymph vessels that appear to terminate in blind sacs function as affluents of a superficial lymphatic net that in turn opens into vessels that become progressively larger in the deeper portions of the dermis. The deeper, larger vessels are embedded in the loose connective tissue that surrounds the veins. The walls of lymph vessels are so flabby and collapsed that they often escape notice in specimens prepared for microscopic studies. Their abundance, however, has been demonstrated by injecting vital dyes inside the dermis and observing the clearance of the dye.
Because lymph vessels have minimal or no musculature in their walls, the circulation of lymph is sluggish and largely controlled by such extrinsic forces as pressure, skeletal muscle action, massaging, and heat. Any external pressure exerted, even from a fixed dressing, for example, interferes with its flow. Since skin plays a major role in immunologic responses of the body, its lymphatic drainage is as significant as its blood vascular system.
The intact surface of the skin is pitted by the orifices of sweat glands and hair follicles—the so-called pores—and is furrowed by intersecting lines that delineate characteristic patterns. All individuals have roughly similar markings on any one part of the body, but the details are unique. The lines are oriented in the general direction of elastic tension. Countless numbers of them, deep and shallow, together with the pores, give every region of the body a characteristic topography. Like the deeper furrows and ridges on the palms and soles, the skin lines are mostly established before birth. The fine details of each area of body surface are peculiar to each individual. Fingerprints are used as a means of personal identification because they have a high relief, more evident patternings, and can be easily obtained.
Some of the lines on the surface of the skin are acquired after birth as a result of use or damage. For example, furrows on the forehead are an accentuation of preexisting congenital lines that become strongly emphasized in old age. As the skin becomes less firm with aging, it also forms wrinkles. Certain occupations leave skin marks that, depending upon duration and severity, may be transient or permanent.
The palms of the hands and the soles of the feet are etched by distinct alternating ridges and grooves that together constitute dermatoglyphics. The ridges follow variable courses, but their arrangement in specific areas has a consistent structural plan. Though apparently continuous, the ridges have many interruptions and irregularities, branching and varying in length. Every small area of surface has ridge details not matched anywhere in the same individual or in any other individual, even in an identical twin. This infallible signature makes dermatoglyphics the best-known physical characteristic for personal identification.