The cortex of a hair is the middle layer of many hairs. That is, it is the middle layer of many thick hairs (i.e. those that include a medulla at their centre, forming their innermost layer). However, as not all hairs have a medulla, the cortex forms the innermost layer of some thinner, finer, hairs.
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.
More about the cortex of hairs:
The cortex of a hair extends along its length, i.e. through the hair root as well as the hair shaft. The cortex of a hair provides its strength and determines both the colour and the texture of the hair.
The topic of the structure of a hair follicle is included in some courses in health sciences, including subjects such as nursing, skin care, hair care, beauty therapy and various holistic therapies.