Respuestas
Hair (and a coat of hairs, called fur or pelage) is uniquely mammalian. No other creature possesses true hair, and at least some hair is found on all mammals at some time during their lives.
Hairs grow out of pits in the skin called follicles. The base of the hair, sunk in the skin, is called the root, and the part that emerges to the exterior is the shaft. Follicles often lie next to a skin gland called a sebaceous gland. These glands secrete an oily substance, which lubricates the hair and conditions it. Next to the follicle may be a small, involuntary arrector pili muscle. Hairs normally lie nearly parallel to the skin or at an acute angle to it. Contraction of the arrector pili muscles causes the hair to erect (become more perpendicular to the skin), and at the same time, pulls down on the skin and causes the bumps and pits known as "gooseflesh."
The shaft of a hair is made up primarily of a protein called keratin and consists of three distinct and easily recognizable parts. The outer layer is made up of dead, transparent cells called cuticular scales. These are arranged in distinctive, often overlapping patterns. Beneath the cuticular scales is the cortex, an often-thick layer that appears featureless except that it may be pigmented. At the center of most hairs is the medulla, made up of large, cuboidal cells, often distinctively colored and interspersed with air pockets. The arrangement of scales, thickness and color of the cortex, and distribution and color of medulla cells are often diagnostic of species. Using a microscope, a researcher can easily examine the cortex and medulla of individual hairs. Cuticular scales are more difficult to visualize; to see them, we usually imbed a hair in substances such as ethyl acetate, then remove it and examine its impression. An excellent set of instructions for examining hairs can be found in Teerink (1991). Numerous keys for identifying individual hairs have been published (e.g., Miles, 1965; Mayer, 1952; Teerink, 1991).
Hairs also differ in cross-sectional shape. Round hairs tend to be straight; oval or flattened hairs are curly.
The color of hair is due to a group of proteins called melanins. Eumelanin is very dark, pheomelanin is paler. Most individual hairs contain at least a few alternating bands of eumelanin and pheomelanin (human hairs are exceptions); this pattern is called agouti. White hair results from the lack of pigment; black is due to dominance of eumelanin. The overall color of the pelage comes from (1) the color of the individual bands, (2) the relative size of individual bands, and (3) the distribution of hairs characterized by different banding patterns.