Hair structure. Artwork showing the structure of a human hair. Hairs are thread-like structures embedded in the skin and composed of dead cells containing keratin. They have little function in humans. The outermost layer of overlapping cells (brown) is the cuticle. To the inside of the cuticle is the cortex (pink) composed of long, thin fibres (seen here in cross section). Protruding from the cortex is the semi-hollow core called the medulla. The ultrastructure of the medulla is indicated further (not to scale) to reveal macrofibrils, microfibrils and the alpha-helical molecular structure (spiral).