What forms the inner structure of a hair strand?

The inner structure of a hair strand is formed by the medulla. The medulla is the innermost layer of the hair shaft and plays a crucial role in the overall composition and properties of hair. It is a soft, spongy, and sometimes hollow core that can vary in its presence depending on the type of hair. Not all hair types have a medulla; for example, fine hair often lacks this structure. However, in hair strands where the medulla is present, it can contribute to the weight and flexibility of the hair.

The cortex, which is another option, constitutes the middle layer of the hair and is primarily responsible for the hair's strength and color because it contains melanin, keratin, and other proteins. The cuticle is the outermost layer that protects the inner layers and helps in moisture retention, while the follicle is the skin structure that houses the hair root. Understanding these structures helps in comprehending the overall functionality and characteristics of hair.