The cortex is a crucial component of hair structure, primarily responsible for providing strength and determining color. It lies between the outer protective layer called the cuticle and the innermost layer known as the medulla. The cortex is composed of long, coiled protein chains, primarily keratin, which give hair its tensile strength and resilience. Additionally, it contains pigment granules that contribute to the hair's color, making the cortex essential for both the durability and aesthetic appearance of the hair.
While other layers and elements in hair contribute to its growth, protection, and moisture absorption, the cortex specifically plays a central role in enhancing strength and defining coloration, which is why this choice is the most accurate description of its function.