arrector pili A small muscle in the dermis of the skin that is attached to the base of a hair follicle. Contraction of the arrector pili, in response to cold or fear, pulls the hair into a vertical position, thus trapping an insulating layer of air around the body and causing `goose flesh' in humans.

arrector pili (ă-rek-tor py-ly) n. (pl. arrectores pilorum) a small erector muscle attached to the hair follicle. Contraction of the arrectores pilorum causes goose flesh.