Cortex
The middle layer of a hair, which gives it strength and colour and holds a perm or relaxer.
Also: hair cortex
The cortex sits under the cuticle and makes up most of a hair strand. It is built of long keratin fibres and holds melanin, the pigment that gives hair its natural colour. Chemical bonds inside the cortex fix the hair's shape. Perms and relaxers work by breaking and resetting these bonds, and bleach works here by breaking down the pigment. Heat styling changes weaker bonds only for a while, which is why a blow-dry is undone by water.