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