What is the innermost layer of the hair called?

The innermost layer of the hair is known as the medulla. This central layer is often referred to as a more primitive feature of hair structure, found in some types of hair, especially in coarser hair types. The medulla is made up of a soft, spongy tissue, which may or may not be present in finer hair types. It has little to no impact on the overall appearance of the hair, but it can contribute to the strength and flexibility of the hair strand.

In contrast, the cortex is the middle layer of the hair shaft and is responsible for the hair's strength, color, and texture. The cuticle is the outermost layer that protects the hair from damage and maintains moisture but does not play a role in the inner structure of the hair itself. The follicle, while related to hair growth, is not a layer of the hair strand but rather the structure from which the hair grows. Understanding these layers is crucial for comprehending how hair functions and how different treatments can affect it.