Hair is a thin, fibrous structure that covers most of the body surface of some mammals. It is one of the most distinctive characteristics of mammals and it helps to identify them from other non-mammalian animals such as reptiles, birds, and fish. Hair is made of a protein called keratin and it also contains some fatty acids, both of which give hair its luster and strength. Hair grows from hair follicles which are tiny pores or saclike holes in the skin of the scalp or other parts of the body. A hair follicle contains a bulb at its base and it is this bulb that produces the new hair cells that form a new hair strand. Hair has a very high tensile strength which means that it can stretch quite far before it breaks. Hair also has glands that produce natural oils to help moisturize the strands and keep them healthy.
Hair can be divided into three major categories: fine, medium, and coarse. Fine hair has a silky, transparent quality and sheds strands that are hard to see even against a contrasting background. Medium hair is thicker and feels like a cotton thread. Coarse hair is the most common and resembles a string of barbed wire and sheds strands that are very easy to see.
The hair follicles which grow our hair are buried deep in the skin beneath the dermis. Beneath the hair follicles are layers of fat and connective tissue, and large sensory nerve branches and blood vessels run through this layer. Hair follicles are located within this layer and they produce the new hair cells that eventually form a strand of hair.
A hair strand is actually two structures, the shaft and the root. The shaft is the part that sticks out above the skin’s surface and it is made of multi-layered keratinized (dead) flat cells whose rope-like filaments make up the hair. Each strand of hair is protected by an outer layer of cuticle which gives the hair its sheen and lustre. The tip of a hair is also protected by the cuticle but if it is damaged then this protective layer can wear away. This is what causes the appearance of split ends.
In most fur-bearing animals, hair serves as a means of defense. When an animal is confronted by a potential predator, the hairs stand on end and this makes the animal appear larger and more threatening to the enemy. Porcupines have even adapted this defense mechanism by converting their hair into sharp quills that they use to attack their enemies.
In humans, the majority of our hair is on our head but we also have some on our arms and legs. Hair has a huge cultural significance and many societies have rules and laws regarding hairstyles and color, for example Muslim women wear headscarves while Sikh men cover their hair with turbans. Hair has also been used as a marker of group membership, for example in the English Civil War the followers of Oliver Cromwell cropped their hair close to their heads which gave them the name of Roundheads.