Hair is made up of several layers, including the cuticle, the cortex, and the medulla. The cuticle is the outermost layer, made up of overlapping scales that protect the inner layers of the hair. The cortex is the middle layer, which provides strength and elasticity to the hair. The medulla is the innermost layer, which is not always present in every hair strand. It is the core of the hair shaft and is composed of cells and air spaces. Together, these layers make up the structure of the hair strand.