The Structure and Chemistry of Human Hair 20 the presence of two major classes of protein, one sulphur-rich and the other sulphur-poor, found in polyacrylamide gel electrophoretic separations of human hair proteins.47 According to earlier nomenclature, the rods of the keratin composite were known as microfibrils, but on account of similarities in internal structure and chemistry of these fibrils with those found in a wide range of other biological tissues, they are now known as intermediate filaments (IFs) or more specifically keratin intermediate filaments (KIFs). In a similar vein the matrix used to be known as the intermicrofibrillar matrix but is now referred to as containing keratin associated protein (KAPs). For comprehensive information on all the human hair KIF proteins and KAPs the reader is referred to the very excellent papers from the Cancer Research Center in Heidelberg, Germany.48, 49 The KIFs are of approximately 70 Å diameter and are aligned roughly parallel to each other along the length of each cortical macrofibril. They are packed in a pseudo-hexagonal fashion and embedded in the KAP matrix with a center-to-center spacing of roughly 100 Å. In images of transversely sectioned macrofibrils examined under the electron microscope one often sees individual KIFs as unstained dots at the center of each macrofibril surrounded by what seems like a thumbprint (c.f. Figures 9 and 10). The dots are end-on projections of KIFs aligned parallel with the electron beam of the microscope. The surrounding whorl arises because, within the thickness of the section being imaged, the KIFs are tilted with respect to the electron beam direction as to present a series of overlapping oblique projections giving the characteristic striated form. Kajiura et al.45 investigated hair by microbeam small-angle X-ray scattering methods (SAXS). They found the angular dispersion of the equatorial reflection associated with the KIF-KIF spacing increased from naturally straight hairs to very curly ones. Furthermore this trend was apparent irrespective of the racial origin of the hairs. Given there is a general increase in the amount of orthocortex in the fiber section with increasing curliness, this indicates the KIFs are less well aligned relative to each