Hair is organized in three distinct tubular layers, the outermost of which is composed of dead, transparent cells known as cuticular scales. The innermost component is the medulla and consists of large, often distinctively colored cells that are interspersed with air pockets. Between the sections is the cortex, several layers of cells that may contain pigments or air bubbles. Specific characteristics, such as arrangement of scales, thickness of the cortex, and the color of medulla cells, vary greatly from species to species.