xxiii Introduction Throughout history, people have cared about their appearance and sought ways to personalize their looks or gain social approval or other rewards through groom- ing and self-adornment. Anthropologists believe that this desire to groom and beau- tify the body is an inherent part of being human. Ancient carvings dating back more than twenty thousand years show hair in deliberately arranged styles. Different types of combs and hairpins dating back thou- sands of years attest to the human desire to groom and adorn the hair. Hair is a key aspect of appearance, one that is always on view unless it is purposely concealed, removed, or lost due to aging or disease. The act of grooming hair is com- mon to certain fur-bearing animals, but, unlike other animals, human beings often cut, remove, or otherwise alter their hair, in addition to cleansing it for hygienic pur- poses. Also, unlike fur-bearing animals, human hair is most obvious only on certain parts of the body: head, face, armpits, legs, and the pubic region. Composition and Growth of Hair Hair itself is a natural polymer, made up mostly of proteins, called keratins. The fibrous types of protein in hair are held together by covalent bonds. These chemical components and chemical bonds make hair a durable material. Hair that is sealed in a tomb or other container can last for thou- sands of years. A hair consists of three layers: the medulla (innermost layer), the cortex, and a protective covering called the cuticle, which is made up of overlapping scale-shaped pro- tein layers. Human hair cuticles typically have six to ten thin layers of cells. The larg- est portion, the cortex, gives hair its color and texture. Each flexible shaft of hair grows from a follicle, and healthy hairs end with a root nourished by tiny blood vessels called capillaries. The root of the hair is surrounded by a white-colored bulb, and when hair is pulled out by its root, this bulb remains attached. Hair follicles are pocket-like holes located in the outer layer of skin. According to the American Academy of Dermatology, the average number of follicles on the human scalp is around 100,000. Human hair folli- cles operate independently rather than as a group. As a result, large numbers of hairs are not shed at once, as they are in some animals. Unless a disease process occurs, at any one time about 90 percent of a human’s hair follicles will be in the growing phase. During that growing phase, cells in and around the matrix, located at the base of the follicle, produce keratin, as well as melanin, the substance that gives hair its color.