Each strand of hair contains three layers: the cuticle, medulla, and cortex.
The next layer, in the middle of the hair shaft, is the cortex, which makes up most of the actual hair. Melanins, which are color pigments, are located here in the cortex. They determine the color of the fiber of the hair, based on how many melanins there are and what types they are.
The shape of the hair follicle determines the shape of the cortex, which therefore determines if hair is straight, wavy, or curly. The cortex also holds water and is packed with keratin protein. The process of coloring, perm/straighteners or other styling all takes place in the cortex.
The innermost layer of hair is called the “medulla,” although some people (with fine hair) don’t have a medulla.