Each hair strand is made up of three main components, the cuticle, cortex and medulla.. 

Cuticle:  This is the outermost layer.  It consists of a single, overlapping layer of scale-like cells that overlap like the shingles on a roof.   It acts as a protective shield to the hair shaft and is coated with sebum, a natural lubricant.  Sebum is what gives hair its natural sheen.

When hair appears dull or dry, the sebum has been stripped from the hair shaft.  When you color, perm or chemically process your hair, you are swelling the hair and raising the cuticle layer.  This allows the liquids to penetrate.  If there were more than one layer of cuticle, nothing would be able to penetrate the hair.

Cortex:  The cortex determines the strength, elasticity and texture of the hair.  It accounts for 90% of the hair's weight. The changes involved in hair coloring, wet setting, thermal styling permanent waving and chemical hair relaxing all take place within the cortex.

 If the cortex is damaged it will appear fragile and weak and will break easily.  Deep conditioning and regular treatments are necessary to strengthen the cortex.

Medulla:  The medulla is the innermost layer that is sometimes called the marrow of the hair shaft.  The exact function of the medulla is mere speculation.  Much more research needs to be done.  The medulla is frequently broken or missing from the hair shaft in fine or very fine hair.  Many suspect that poor health and use of drugs have major effects on the medulla.