During chemical processing, swelling raises which hair structure to allow liquids to penetrate?

• A. Medulla
• B. Cortex
• C. Hair shaft
• D. Cuticle

Answer: (D)

Swelling during chemical processing involves the outer protective layer of the hair, the cuticle. When the hair swells, the cuticle scales lift away from the cortex, creating gaps that let processing liquids penetrate. This access is crucial because the cortex contains the keratin fibers that chemical changes target, so lifting the cuticle opens a path for the chemicals to reach where they can act. The central medulla is an internal core and doesn’t govern how liquids penetrate, and the term hair shaft refers to the whole strand rather than a specific lifting layer.