Skin is the largest organ in the body. In an average-sized adult, skin covers approximately 20 square feet and weighs from 6 to 8 pounds. The skin's thickness varies depending on the area it covers. For example, the eyelids are covered with 1/50 of an inch of skin, while the palms of the hands and feet are covered with approximately 1/3 of an inch. Moreover, the appearance and quality of skin depend on what body part it covers, the flow of blood to that part, and the amount that is covered with hair. The quality of skin can also tell something about a person's lifestyle. For example, a person with smooth, unblemished skin probably has avoided the sun, while a person with tanned and wrinkled skin probably has spent a fair amount of time in direct sunlight.
Skin may be likened to a four-ply tissue, because it has four layers (see Figure 8.1). The top, outermost layer, or epidermis, is the body's defense against infection. This layer constantly sheds through daily wear
and tear and is replaced with newer cells from a different part of the epidermis. This layer also contains the cells that determine the pigment or color of your skin.
The dermis, located directly beneath the epidermis, provides support and nutrition to the epidermis. The dermis contains blood vessels, nerves, hair follicles, and sweat glands. The supply of blood to the dermis originates in the underlying muscles. Although the skin usually does not require all of its blood supply to function, the muscle and fatty layers beneath it cannot function properly when the blood supply is compromised.
The dermis also contains sensory nerves, which help you determine changes in pressure, temperature, pain, and touch. Some people with MS have areas of demyelination over the nerves that should tell them when to change position, when to regulate the source of heat or cold, or if there is an irritation. People with this condition need to protect themselves by always being aware of what is touching their skin and potentially damaging it.
The hypodermis, or subcutaneous tissue, is also referred to as the fat layer. It provides insulation, absorbs shock, and cushions the layers above. It acts as a nutrient reserve when the body is exposed to illness or starvation. The distribution of fat is influenced by age, heredity, and gender.
The muscles, blood vessels, and nerves are covered by fascia. It is dense, firm tissue that varies in thickness and strength throughout the body. Fascia's elastic properties facilitate the movement of various parts of the body.
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