West Virginia Cosmetology Board Practice Exam

The cortex is the part of the hair that is primarily responsible for its strength. This layer is located beneath the cuticle and makes up the bulk of the hair strand. It contains fibrous proteins and a large amount of melanin, which contributes to both the structure and color of the hair. The cortex is made up of long chains of amino acids, providing tensile strength and flexibility to the hair.

The cuticle serves as a protective outer layer and can affect the hair's shine and smoothness but doesn't contribute to the structural strength as significantly as the cortex does. The medulla, which can be absent in some types of hair, is the innermost layer and generally does not impact strength. Finally, the scalp is not part of the hair shaft at all; it's the skin on the head where the hair follicles reside. Thus, strength in hair comes predominantly from the cortex.