A fibrous protein made up of long linear molecules arranged parallel to the axis of the fibre. It is the basic substance of the hair, and is elastic and insoluble in ordinary aqueous media. It contains 50 to 60% carbon, 8 to 12% nitrogen, 4 to 5% hydrogen, 25 to 30% oxygen and 2 to 4% sulphur. The cohesion of keratin is provided by cystine and saline links and hydrogen bonds. The keratin chains are very long and are parallel to the hair shaft. In normal, unstretched hair, the chains are not straight but twisted in a kind of spiral: alpha-keratin. When the hair is stretched by setting (perming, blow-drying, etc.), the chains are pulled
straight, and become beta-keratin.