What is the central structure of the hair shaft known as?

The central structure of the hair shaft is known as the medulla. This portion of the hair is located at the innermost layer and is not present in all hair types, but when it is, it can influence the overall characteristics of the hair. The medulla is often described as being composed of a soft, spongy material and is surrounded by the cortex, which provides strength and color to the hair.

The medulla's role may be more prominent in certain types of hair, such as thicker or coarser hair. This feature can be crucial in understanding the properties of hair for coloring and treatment processes, as it can affect how products interact with each strand. The other components of the hair shaft, such as the cuticle and cortex, have their own distinct functions, but the medulla specifically refers to the central part.