Arrectores pilorum: Tiny muscles that act as the hair erector muscles. The arrectores pilorum play a key role in goose bumps, a temporary local change in the skin The chain of events leading to this skin change starts with a stimulus such as cold or fear. That stimulus causes a nerve discharge from the sympathetic nervous system, a portion of the autonomic (involuntary) nervous system. The nerve discharge causes contraction of the arrectores pilorum (the hair erector muscles). Contraction of these muscles elevates the hair follicles above the rest of the skin. And it is these tiny elevations we perceive as goose bumps. The words used to describe this condition are curious and colorful. "Goose bumps" are listed in the Merriam-Webster Collegiate Dictionary (as two words in the plural). The term entered English in 1933. The word "gooseflesh" (written as one word or as two) is older than "goose bumps." Gooseflesh dates back to about 1810, according to the Oxford English Dictionary (OED), which defines it as "a rough pimply condition of the skin, produced by cold, fear, etc." A fancier term for this familiar phenomenon is "horripilation." Horripilation was compounded from the Latin "horrere", to stand on end + "pilus", hair = hair standing on end. (If you think "horripilation" sound horrible, you're right. (The word "horrible" also came from the Latin "horrere" and referred to something that was so awfully dreadfully frightful that it made your hair stand on end.) Medicine does not use a horrible term such as "horripilation" and rarely resorts to the commonplace words, goose bumps or gooseflesh. Medicine has a special term, "cutis anserina", that sounds like a scary dermatologic diagnosis. But it goes back to the goose again, since "cutis", skin + "anser", goose = goose skin. Some biologists believe that goosebumps evolved as part of the fight-or-flight reaction along with heart rate increases that send the heart racing while blood rushes to the muscles to give them additional oxygen. A similar phenomenon, bristling, in fur-covered animals may have made them look larger and more frightening and kept them warmer by increasing the amount of air between hairs which traps body heat. But in people there seems to be no practical purpose for goose bumps except, of course, to make our skin crawl.
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