A strand of hair is the visible part above the skin and the deeper segment under the scalp is known as the root. Composed of three layers, each strand has a diameter of 70-100 microns.
THE CUTICULE is the external layer of a strand of hair, which is formed of hardened protective cells. It is without pigment and arranged much like the shingles on a roof, intertwined and overlapping. These thick cells collectively give hair its texture.
THE CORTEX is under the scalp and directly related to the hair volume. The fibrous cells of the cortex contain keratin (the principle ingredient in hair), and melanin (the pigment that gives hair its color).
THE MEDULLA is the innermost center, composing 0 to 10% of the volume. In the case of very fine hair, the medulla is absent.
THE ROOT is the living part of the hair anchored in the scalp. At the tip of the root, a slightly larger area known as the bulb bears a thick, whitish appearance when one plucks a strand of hair.
THE PAPILLA is the curved and empty area at the base of the bulb. It connects the hair to the head. Here, the blood vessels and nerves meet and the hair receives its nourishment.