Cuticle - An external layer of light-sensitive, water resistant cells that surround and protect the hair. Designed to open and close allowing certain products to penetrate the hair.
Cortex - The main component of the hair fibre mass consisting of spindle shaped cells. Also contains natural colour pigments. This is where the chemical changes take place.
Medulla - An assembly of medullary cells, detectable under a light microscope and found in the centre of the hair fibre. Has no proven function and isn't always present (Particularly in fine hair).