Illustration of a longitudinal section through a human hair follicle. Hair (black) is formed from the protein keratin. It is a dead tissue that is pushed up by growth from the bulb (bottom centre) at the base of the follicle, which lies in the dermis layer of the skin. Next to the hair are the inner (red) and outer (blue) root sheaths and a connective tissue sheath (yellow). From Traite d'Anatomie Humaine, Vol 3 (1905) by French anatomist Leo Testut (1849-1925).