| Skin Forms a remarkable protective | | | | dead "horny" cells, and underlying |
| barrier against the outside world, | | | | "prickle" cells, which move upward to |
| helping to regulate temperature and | | | | replace them as they are shed (around 90 |
| fluid balance, keeping out harmful germs | | | | percent of household dust is dead skin |
| and chemicals, and offering natural | | | | cells). The epidermis also contains |
| protection against sunlight. It grows | | | | melanocytes, cells that secrete melanin, |
| hair, sweats, produces oil for | | | | the pigment responsible for varying skin |
| self-lubrication, and gives an accurate | | | | color. Melanin production is stimulated |
| perception of touch, heat, cold, and | | | | by sunlight: in white races this creates |
| pain, as well as the more complex | | | | a tan, which offers some protection |
| sensations such as tickle, itch, and | | | | against further exposure to the sun's |
| pressure. | | | | ultraviolet rays. |
| Skin caliber varies in different body | | | | Under the epidermis is the middle layer, |
| sites, from the thin, delicate tissue of | | | | or dermis, a bulky fibrous and protein |
| the eyelid, to the thick, hard padding | | | | layer containing hair follicles, sweat, |
| of the heel. Some sites have particular | | | | and sebaceous (oil-producing) glands, |
| characteristics - luxuriant hair growth | | | | and specialized sensory receptors for |
| on the scalp, sweating in the armpits, | | | | conveying information about touch, |
| highly-tuned touch discrimination in the | | | | temperature, and pain. Mast cells in the |
| fingertips - yet the basic structure of | | | | dermis respond to physical or chemical |
| the skin is the same everywhere, | | | | damage by releasing histamine, a |
| although its actual thickness varies | | | | substance leading to allergic responses. |
| between different physical types. | | | | Finally there is a layer of subcutaneous |
| Redheads, for example, have considerably | | | | fatty tissue that gives the skin its |
| thinner skin than brunettes, and, | | | | feeling of softness and plumpness and |
| consequently, their skin often reacts | | | | acts as insulation against the cold. The |
| with more sensitivity. | | | | blood vessels supplying nourishment to |
| The outer of the three skin layers, the | | | | the outer skin layers are carried in |
| epidermis, comprises the skin surface of | | | | this layer. |