Chapter 1 5 In broad respect, hair consists of a central core of longitudinally oriented and closely packed spindle-shaped cortical cells containing the ‘keratin composite’, sheathed by thin sheet-like and tough layers of cuticle that overlap each other from root-to-tip along the fiber. What follows is a detailed description of the composition and internal structure of these two major components. The nomenclature for many of the hair’s substructures was established in 1959 by Professor George Rogers from Adelaide9 arising from his early pioneering transmission electron microscope work on sheep’s wool and human hair. Cuticle: Figure 3 illustrates schematically the overall geometry of the hair cuticle. In the context of toiletry treatments this component of the fiber is arguably the most important. Its influence upon tactile perceptions of the coarseness/fineness of a fiber goes far beyond the small proportion (at most 25%) it occupies of the total cross-sectional area.10, 11 This is the component onto which materials such as hair sprays and conditioning materials are deposited and, in undamaged hair, it provides a significant barrier to the penetration of cosmetic agents such as colorants into the central core of the fiber (cortex). It helps protect the fiber as a whole from degradation and helps sustain it for at least the duration of approximately three years on the head. That hair compressed between stainless steel platens indents the steel to the pattern of the overlapping scales attests to the considerable Figure 2. Scanning electron micrograph of a human head hair close to the scalp surface.