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skin

skin
皮肤【名词】肌肤,皮,肤,护肤,表皮,肤色

Definitions

the outer covering of the body. The skin is the largest organ of the body, and it performs a number of vital functions. It serves as a protective barrier against microorganisms. It helps shield the delicate, sensitive tissues underneath from mechanical and other injuries. It acts as an insulator against heat and cold, and helps eliminate body wastes in the form of perspiration. It guards against excessive exposure to the ultraviolet rays of the sun by producing a protective pigmentation, and it helps produce the body's supply of vitamin D. Its sense receptors enable the body to feel pain, cold, heat, touch, and pressure. The skin consists of two main parts: an outer layer, the epidermis, and an inner layer, the corium (or dermis).Epidermis. The epidermis is thinner than the corium, and is made up of several layers of different kinds of cells. The number of cells varies in different parts of the body; the greatest number is in the palms of the hands and soles of the feet, where the skin is thickest.The cells in the outer or horny layer of the epidermis are constantly being shed and replaced by new cells from its bottom layers in the lower epidermis. The cells of the protective, horny layer are nonliving and require no supply of blood for nourishment. As long as the horny outer layer remains intact, microorganisms cannot enter.Corium. Underneath the epidermis is the thicker part of the skin, the corium, or dermis, which is made up of connective tissue that contains blood vessels and nerves. It projects into the epidermis in ridges called papillae of the corium. The nerves that extend through the corium end in the papillae. The various skin sensations, such as touch, pain, pressure, heat, and cold, are felt through these nerves. The reaction to heat and cold causes the expansion and contraction of the blood capillaries of the corium. This in turn causes more or less blood to flow through the skin, resulting in greater or smaller loss of body heat (see temperature).The sweat glands are situated deep in the corium. They collect fluid containing water, salt, and waste products from the blood and carry it away in canals that end in pores on the skin surface, where it is deposited as sweat. Perspiration helps regulate body temperature as well, because cooling of the skin occurs when sweat evaporates. The sebaceous glands are also in the corium. They secrete the oil that keeps the skin surface lubricated.Beneath the corium is a layer of subcutaneous tissue. This tissue helps insulate the body against heat and cold, and cushions it against shock.The hair and nails are outgrowths of the skin. The roots of the hair lie in follicles, or pockets of epidermal cells situated in the corium. Hair grows from the roots, but the hair cells die while still in the follicles, and the closely packed remains that are pushed upward form the hair shaft that is seen on the surface of the skin.The nails grow in much the same way as the hair. The nail bed, like the hair root, is situated in the corium. The pink color of the nails is due to their translucent quality which allows the blood capillaries of the corium to show through.Normal skin. From Frazier et al., 2000. Epidermis. The epidermis is thinner than the corium, and is made up of several layers of different kinds of cells. The number of cells varies in different parts of the body; the greatest number is in the palms of the hands and soles of the feet, where the skin is thickest.The cells in the outer or horny layer of the epidermis are constantly being shed and replaced by new cells from its bottom layers in the lower epidermis. The cells of the protective, horny layer are nonliving and require no supply of blood for nourishment. As long as the horny outer layer remains intact, microorganisms cannot enter. Corium. Underneath the epidermis is the thicker part of the skin, the corium, or dermis, which is made up of connective tissue that contains blood vessels and nerves. It projects into the epidermis in ridges called papillae of the corium. The nerves that extend through the corium end in the papillae. The various skin sensations, such as touch, pain, pressure, heat, and cold, are felt through these nerves. The reaction to heat and cold causes the expansion and contraction of the blood capillaries of the corium. This in turn causes more or less blood to flow through the skin, resulting in greater or smaller loss of body heat (see temperature).The sweat glands are situated deep in the corium. They collect fluid containing water, salt, and waste products from the blood and carry it away in canals that end in pores on the skin surface, where it is deposited as sweat. Perspiration helps regulate body temperature as well, because cooling of the skin occurs when sweat evaporates. The sebaceous glands are also in the corium. They secrete the oil that keeps the skin surface lubricated.Beneath the corium is a layer of subcutaneous tissue. This tissue helps insulate the body against heat and cold, and cushions it against shock.The hair and nails are outgrowths of the skin. The roots of the hair lie in follicles, or pockets of epidermal cells situated in the corium. Hair grows from the roots, but the hair cells die while still in the follicles, and the closely packed remains that are pushed upward form the hair shaft that is seen on the surface of the skin.The nails grow in much the same way as the hair. The nail bed, like the hair root, is situated in the corium. The pink color of the nails is due to their translucent quality which allows the blood capillaries of the corium to show through. Normal skin. From Frazier et al., 2000. Normal skin. From Frazier et al., 2000.





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