Which part of the hair is primarily responsible for its strength and structure?

The cortex is primarily responsible for the strength and structure of the hair. It is the thick, fibrous middle layer of the hair shaft and contains the majority of the hair's mass. The cortex is made up of long chains of keratin, which provide tensile strength and elasticity, allowing hair to withstand various forces without breaking. This layer also contains melanin, which gives hair its color.

In contrast, the cuticle is the outermost layer that serves primarily as a protective barrier. While it plays a crucial role in protecting the inner layers, its main function is not to contribute to the strength of the hair. The medulla, which is often absent in fine or thin hair, is a soft, spongy core that does not significantly influence the overall structure or strength of the hair. The follicle, on the other hand, is the part of the skin that surrounds the hair root but also does not contribute to the structural integrity of the hair strand itself. Therefore, the cortex stands out as the key component responsible for the hair's strength and structure.