Which part of the hair structure is primarily responsible for the protective outer layer?

The cuticle is the part of the hair structure that serves as the protective outer layer. It is made up of overlapping scales that lie flat against the hair shaft, which helps to protect the inner layers of the hair, including the cortex and medulla, from environmental damage, moisture loss, and chemical exposure. The cuticle's structure provides both physical protection and contributes to the overall appearance of the hair, influencing its shine and smoothness.

In contrast, the cortex is the main body of the hair that contains proteins and determines its strength and elasticity, while the medulla is the innermost layer that is often absent in finer hair types and does not play a significant role in hair protection. The bulb is the part of the hair located beneath the scalp that contains cells that divide and grow to produce hair, playing a crucial role in hair growth but not in the protection of the hair shaft itself. This distinction highlights the specific function of the cuticle as the outermost barrier.