When a human being is cold or frightened, the muscles contract and produce little bumps around the hair. The color of hair is determined largely by the amount and distribution of a brown-black pigment called melanin. Hair also contains a yellow-red pigment that is most visible in people whose hair has little melanin. Most people's hair gradually becomes gray or white as they grow older, because pigment no longer forms. The texture of hair depends largely on the shape of the hair, which can be seen in cross section under a microscope.
Straight hairs have a round shape, and wavy and curly hairs are flat.
The flattest hairs are the waviest or curliest. Cycles of hair growth. The growth of hair in the follicles occurs in cycles. In each cycle, a follicle goes through a growth phase, called anagen, and a resting phase, called telogen. A hair stops growing during every telogen, when it is known as a club hair.