Hair is a fibrous protein known as alpha keratin. It is elastic in texture, insoluble in water and resistant to chemical change. Each hair shaft grows from a single follicle in the scalp. The follicle secretes sebum [natural skin oil] from a sebaceous gland which provides lubrication for scalp and hair. Sebum gives hair its pH rating.
Keratin is composed of amino acids joined into long interwoven molecules called polychains. Adjacent polypeptide chains contain several links and bonds [Cystine or Disulphide bonds; and weaker Hydrogen bonds]. Cystine bonds are the foundation of hair and support the structure of the shaft.
The Cuticle is the outer protective layer composed of overlapping scales from the scalp up, as in fingernails. It is the framing structure that determines the shape and direction of the hair shaft. Curly, wavy and kinky hair appear to have twice as much cuticle as straight hair; however, both have the same amount.
The Cortex of the hair shaft is the layer that contains natural colour and helps protect the inner structure of the hair from potential harm such as the sun’s UV rays. Artificial colour when using peroxides, reside here [by literally punching microscopic holes in the cuticle and allowing colour to reach the cortex]. The transparent outer cuticle enables the color in the cortex to be seen. All affected bonds during relaxing reside in the cortex, making it the most important part of the hair in relaxing.