What is the role of the medulla in hair?

The medulla is the innermost layer of the hair shaft, and its primary role is related to determining hair thickness. The presence and structure of the medulla can influence the overall diameter of the hair strand, as not all hair types possess a medulla; for instance, finer hair often has a less developed or absent medulla. This characteristic helps categorize hair types and can impact styling choices since it contributes to the hair's overall bulk and appearance.

In contrast, the other choices pertain to different aspects of hair structure; for instance, shine is typically provided by the outer layer, the cuticle, which reflects light. Strength comes primarily from the cortex, which contains the fibrous proteins that give hair its resilience. Lastly, while the cuticle acts as a protective layer against environmental damage, the medulla does not serve this protective function. Understanding the role of the medulla helps in recognizing how different hair types are characterized and managed in hair design and care.