The Structure and Chemistry of Human Hair 6 hardness of the outer cuticular surface, a property that helps the fiber withstand external mechanical insult. Only at our peril do we engage upon toiletry treatments of hair that will diminish the chemistry, structure and physical behavior of the cuticle. The cuticle consists of a series of flattened cells wrapped around the cortical core of the hair (c.f. Figure 3). Where the hair emerges from the scalp, approximately 10 cell layers are seen in the transverse cross-section (Figure 3), but this number diminishes along the hair’s length by frictional attrition at the exposed edges of the scales. The cells overlap each other from root-to-tip along the hair, providing an imbricate surface. The coefficient of friction of undamaged hair for rubbing contacts in a root-wise direction (against scales) is significantly greater than in a tip-wise direction. This differential frictional behavior helps the hairs to be aligned parallel to each other, to be relatively free from entanglement, and provides for the migration of irritant scalp detritus to the surface of the pelage.12 Figure 3. Schematic diagram showing cuticle geometry.