The Structure and Chemistry of Human Hair 4 Hair Structure Human hair is mainly composed of a hardened protein known as keratin or trichokeratin, and its structure is best described as a tough hierarchical composite at all levels of scale from the macroscopic fiber down to molecular dimensions. Figure 1 is a schematic diagram illustrating the overall structure of a human hair, and Figure 2 is a low-power scanning electron micrograph showing the surface of a hair close to where it emerges from the scalp. Here the scales are in pristine condition as evidenced by the smooth contour of the scale edges in the picture, but further along the hair these edges normally adopt a serrated appearance due to the effects of mechanical wear8. The diagram of Figure 1, drawn by this author in 1968, incorporates information about the orthocortex and paracortex, the distribution of which in the hair cross-section even in 1968 had been shown by the author to be related to the extent of natural curliness of human head hair from a range of ethnic subjects. Figure 1. Schematic diagram of human hair structure.