Arrector Pili
Historical document, translated for reference. It reflects medical knowledge of the 1920s–30s and is not medical advice.
Summary
The arrector pili is a smooth muscle bundle in the dermis that attaches to hair follicles, causing hairs to stand upright when contracted. This muscle is weakly developed in humans but more prominent in other animals, particularly on the back and tail.
Encyclopedia entry (1928–1936)
ARRECTOR PILI (from Latin arrigere-to raise and pilus-hair), the muscle that raises the hair. From the upper part of the skin's dermis, a bundle of smooth muscle fibers indirectly extends inward, weaving into the hair follicle on the side where the sebaceous glands empty into the follicle. In different animals, this bundle is developed to varying degrees: in humans it is weak, while in other animals it is strong (especially on the back, on the tail). When this muscle contracts, the hair, which is usually inclined and lying smoothly, becomes more vertical, stands on end. The muscle has its own nerve endings in the form of terminal bulbs. In birds, a similar muscle attaches to the base of the feather and, when contracting, changes its position.
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“Arrector Pili.” Soviet Medical Encyclopedia. English translation of Bolshaya Meditsinskaya Entsiklopediya, 1st ed. (Moscow, 1928–1936), ed. N. A. Semashko. https://sovietmedicalencyclopedia.pages.dev/article/arrector-pili/