The hair shaft is made of a hard protein called keratin and is made in three layers. This protein is actually dead, so the hair that you see is not a living structure.
The inner layer is the medulla. The second layer is the cortex and the outer layer is the cuticle.
The cortex makes up the majority of the hair shaft.
The cuticle is a tightly formed structure made of shingle-like overlapping scales. It is both the cortex and the medulla that holds the hair's pigment, giving it its color.
Cycle of Growth
Hair on the scalp grows about .3 to .4 mm/day or about 6 inches per year. Unlike other mammals, human hair growth and shedding is random and not seasonal or cyclical. At any given time, a random number of hairs will be in one of three stages of growth and shedding: anagen, catagen, and telogen:
Anagen is the active phase of the hair. The cells in the root of the hair are dividing rapidly. A new hair is formed and pushes the club hair (a hair that has stopped growing or is no longer in the anagen phase) up the follicle and eventually out. During this phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for two to six years.
Catagen phase is a transitional stage and about 3% of all hairs are in this phase at any time. This
phase lasts for about two to three weeks. Growth stops and the outer root sheath shrinks and attaches to the root of the hair. This is the formation of what is known as a club hair.
Telogen is the resting phase and usually accounts for 6% to 8% of all hairs. This phase lasts for about 100 days for hairs on the scalp and longer for hairs on the eyebrow, eyelash, arm, and leg. During this phase, the hair follicle is completely at rest and the club hair is completely formed. Pulling out a hair in this phase will reveal a solid, hard, dry, white material at the root. About 25 to 100 telogen hairs are shed normally each day.