These are little bundles of smooth muscle fibers attached to the hair follicles.
Contraction makes the hair stand erect and raises the skin around the hair, causing 'goose flesh'.
The muscles are stimulated by sympathetic nerve fibers in response to fear and cold.
Erect hairs trap air, which acts as an insulating layer.
This is an efficient warming mechanism especially when accompanied by shivering, i.e. involuntary contraction of the skeletal muscles.
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| The Arrector Pili |
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