Hair

Hair (sometimes referred to in Somali as timo when discussing multiple strands) is a thin, cylindrical filament made of a protein called keratin that grows from the skin of mammals. Hair emerges from a small cavity in the skin called a hair follicle. The part of hair above the skin consists of dead, hardened cells, which means that cutting or shaving hair causes no pain.

The presence of hair is one of the defining characteristics of mammals, distinguishing them from other groups of vertebrates. Many animals have a complete coat of hair covering the body; in humans, hair is found as short, fine strands covering most of the body, and as longer strands on the head, eyebrows, eyelashes, and other areas.

Structure

Hair has two main parts: the root, which lies within the follicle, and the shaft, which extends above the skin. The root sits inside the follicle, where cells rapidly divide, fill with keratin, and then harden. The shaft of hair is divided into three layers: the outer cuticle, the cortex in the middle which gives hair its color, and the medulla at the center, which is not present in all hairs.

Color

Hair color depends on the type and amount of melanin present in the cortex. Eumelanin produces dark brown tones, while pheomelanin produces red and yellow tones. When melanin-producing cells in the hair follicles reduce their activity—usually with age—hair becomes gray or white.

Growth cycle

Each hair passes through three phases of growth: the anagen phase (active growth), the catagen phase (transition), and the telogen phase (resting), after which the hair falls out. The follicle then begins to produce new hair. The length of the anagen phase determines the maximum length a hair can reach, and this duration varies across different parts of the body.

Functions

Hair serves to regulate body temperature by trapping a layer of air near the skin. It also provides protection from the sun and physical injury, and is connected to the sense of touch, since sensory nerves surround the hair follicle. Some animals use hair as a communication signal, for example with body hair standing up when the animal is frightened.