Each hair on the human scalp is divided into a hair shaft that protrudes from the skin and a hair root, which is embedded in the skin. At the end of the root is a thickened bulb that works in conjunction with the underlying papillae to facilitate the nourishment, development, and growth of the hair. Each hair is further organized into three distinct layers that are tubular in shape. The outermost layer consists of dead, transparent cells known as cuticular scales, while the innermost section, called the medulla, is composed of large cells that are interspersed with air pockets. The cortex that lies between these two layers is composed of several strata of cells that may contain air bubbles and pigments.