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Hair follicles. Coloured scanning electron micrograph (SEM) of a section through hair follicles (oval, upper centre-left) in skin, with the hairs (dark, round) within. A follicle is the structure from which a hair grows from the skin. From centre outwards: the hair shaft is surrounded by an inner and outer root sheath, which in turn is surrounded by a sheath of connective tissue. The surrounding tissue contains the smooth muscle of the arrector pili muscle, which makes the hair shaft stand erect. Magnification: x133 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)