The medulla of a hair is the innermost (so the central) layer of the hair.
To appreciate this in the overall context of the structure of hair, note that, structurally, hairs are threads of fused (i.e. attached together), dead, keratinized cells. They consist of two main parts:
- The hair shaft is the visible part of the hair that protrudes through the skin. It is composed of three layers. The innermost layer is called the medulla and is only present in large thick hairs. The middle layer is called the cortex and outermost layer is called the cuticle.
- The hair root is the part of the hair below the surface of the skin that includes and/or interacts with many other associated structures within the dermis and hypodermis layers of skin.
When present, the medulla of a hair extends all the way along its length. However, a medulla is not present in all hairs, only in large thick ones e.g. protruding from the scalp rather than from the abdomen or upper-arms (where hairs tend to be lighter and less dense).