The three main layers of the hair shaft are the cuticle, cortex, and:

The three main layers of the hair shaft are the cuticle (the tough, outermost protective layer of overlapping scales), the cortex (the middle layer that gives hair its strength, elasticity, and natural color through melanin), and the medulla (the innermost core, which may be absent in fine or very light hair). Most chemical services act on the cuticle and cortex. Knowing these three layers and their roles — especially that the cortex is where color and most chemical changes occur — is essential hair science.