Hair grows out of hair follicles embedded in the dermis. The color and texture of the hair is determined by its structure. In the center of the shaft of hair is the medulla. The cortex of the hair contains melanin granules, which determines the hair color, along with rodlike bundles of keratin fibers. The shape of the fibers in the cortex determines whether the hair is curly, wavy, or straight. Finally, the outer layer of the hair, or the cuticle, contains overlapping dead cells that line the shaft.
There are three different types of hair. The first hair type, called lanugo, is produced by fetal hair follicles of around 16 weeks of gestation. It usually disappears around birth and is replaced by the second hair type, called vellus hair. Vellus hair is sometimes also called peach fuzz because it is so fine. It is present on both children and adults. The third hair type is called androgenic hair (terminal hair) because it grows in response to androgen hormones during puberty. This is the full, thick hair found on the heads of children and adults and on the bodies of adults after puberty.
Hair serves a variety of different functions in mammals, depending on the species, and the location on the body. The functions of hair include protection, regulation of body temperature, sensory perception, and as a form of social and sexual communication. Hair protects the skin from unwanted parasites and debris that may fall on it. Additionally, the eyebrows keep matter from falling into the eyes.
Hair is important in thermoregulation, providing an insulating layer that keeps the body warm. In response to cold, an arrector pili muscle, which is a muscle attached to hair follicles that supports each hair that exits the dermis, contracts to release thermal energy. This causes the hairs attached to the follicles to stand up, creating an insulating layer of air trapped beneath the hair. In humans, this phenomenon persists despite the lack of insulating body hair on most of the body and is called horripilation, commonly known as the appearance of goose bumps. When the body is too warm, the piloerector muscles flatten the hairs. This releases trapped heat and aids cooling.
Hair also serves an important sensory function. Nerve receptors at the hair follicle sense movements of the hair shaft caused by the movement of air or contact with physical objects. In humans, eyelashes are particularly sensitive to touch, which helps keep dirt and other foreign particles out of the eyes. Eyebrows and eyelashes keep dirt, sweat, and rain from running into the eyes.