Hairs grow out of pits in the skin called follicles . The base of the hair, sunk in the skin, is called the root , and the part that emerges to the exterior is the shaft . Follicles often lie next to a skin gland called a sebaceous gland . These glands secrete an oily substance, which lubricates the hair and conditions it. Next to the follicle may be a small, involuntary arrector pili muscle .
Hairs normally lie nearly parallel to the skin or at an acute angle to it.
The shaft of a hair is made up primarily of a protein called keratin and consists of three distinct and easily recognizable parts. The outer layer is made up of dead, transparent cells called cuticular scales . These are arranged in distinctive, often overlapping patterns. Beneath the cuticular scales is the cortex , an often-thick layer that appears featureless except that it may be pigmented. At the center of most hairs is the medulla , made up of large, cuboidal cells, often distinctively colored and interspersed with air pockets.