Hair is a thin filamentous (fingernail-like) outgrowth of the skin found only in mammals, which can be seen on the head, hands, feet, arms, and buttocks. It is composed of non-living cells primarily of the protein keratin and is colored by melanin. A hair follicle produces the cells that eventually become a strand of hair. These cells are then hardened by a chemical process called keratinization, which occurs as the follicle grows out of the skin. Glands in the follicle, such as the sebaceous glands, produce the natural oil that helps keep hair moist.
Hair has an important social significance for humans and is a major indicator of biological sex, age, and ethnic ancestry. The way a person wears their hair is also a strong indicator of their mood and attitude. Hair can be damaged by the environment, poor grooming habits, and some types of chemical treatments. It can also be affected by underlying medical issues, such as thyroid disease and autoimmune diseases like alopecia areata.
There are two main functions of human hair: insulation and aesthetics. The vellus (ve-LUS) hairs, which are soft and fuzzy and are generally found everywhere on the body, provide insulation against cold temperatures. The follicles of the vellus hairs are small, and their hair shafts contain little melanin. The follicles of the terminal (te-me-LAH) hairs on the head, however, are larger and more melanin-rich. The follicles of the hair on the head can be either regressive or progressive, and they produce a thicker and darker shaft of hair than their vellus counterparts.
The color of the hair is primarily determined by the genes of the individual, but environmental factors can influence the pigmentation of the hair. The outer layer of the hair, known as the cuticle, is composed of a single layer of cells that are arranged in scale-like fashion and protect the inner cortex and medulla of the hair fiber. The cortex is a fibrous and helically-arranged protein that provides the strength and structure of the hair. The medulla, in turn, determines the color of the hair.
The hair follicles are fed by a network of dermal nerves, which include sensory afferents and autonomic nervous innervation. Sensory information from the hair can be interpreted by the brain and is used to enhance tactile ability, while autonomic nervous innervation of the arrector pili muscle enables the hair to stand on end. This is likely a vestigial function inherited from scaly skin: it may serve to trap air and conserve heat, or to cut a larger silhouette to intimidate rivals or would-be predators. The muscles that make the hairs stand on end are elastin-rich and can also be used to flexibly bind the skin to the skull.