Extending into the indentation is the papilla, the center of hair growth, which contains the capillaries and nerves that supply the hair. Newly dividing cells at the base of the hair multiply, forcing the cells above them upward. As the cells move upward, they gradually die and harden into the hair shaft. The hair shaft has two layers, the cuticle and the cortex. Coarse hair, such as that of the scalp, contains an additional inner core called the medulla.
Hair is lubricated by sebaceous glands that are located in the hair follicle.
Illness or stress may lessen the secretion of pigment, which normally gives color to hair, and cause the hair shaft to whiten. However, the normal process of whitening that comes with age is determined by heredity. In humans, scalp hairs are generally shed every two to four years, while body hairs are shed more frequently. Straight-textured hair, round in cross section, is common among Native Americans, Eskimos, and Mongoloid peoples. Kinky or woolly hair, flat in cross section, prevails among the dark peoples of Africa, Australia, and elsewhere.