The Structure and Chemistry of Human Hair 16 during passage through the neck of the hair follicle to occupy the intercellular space now recognized as the δ-layer. In passing it is noteworthy Kreplak et al.36 used X-ray microdiffraction to accurately measure the thickness of the δ-layer in dry hair (175 Å) and found its thickness increased by only 10% in wetted hair. To place this in context it is worth recalling the radial increase of an undamaged hair taken from the dry to the wet state is 15%, whilst radial swelling of the endocuticle is as high as 100%.34 Given the thinness of the δ-layer, its relative lack of swelling by water and its being bounded on both sides by a layer of lipid, it is hard to believe, as some have claimed,37 that this is the primary route for the diffusion of dyes into the bulk of mammalian keratin fibers. Far more likely access is gained via the much thicker, significantly swellable endocuticle.38 Cortex: Cortical cells are the major component of the mature human hair shaft. Each cell is spindle shaped 50 to 100 µm long and 3 to 6 µm in diameter with its long axis parallel to the fiber axis (c.f. Figure 1). The outer surfaces of the cells are longitudinally fluted and there are finger-like projections at the two ends. The cells closely interdigitate both longitudinally and transversely at these surfaces with adjacent cortical cells from which they are separated by a thin cell membrane complex of somewhat different character to that found between the cells of the cuticle (c.f. Figure 8). The outer surface of the cortex presents a relatively smooth interface with the fiber cuticle from which it is also separated by a cell membrane complex. The overall form of the cells has arisen as a consequence of the extrusion of the original cells as they have passed through the neck of the hair follicle. Coupled with this, hard longitudinally arranged keratin macrofibrils deposited inside the cells have distorted the longitudinal cell surfaces to a close fit with adjacent cells. In a similar fashion the keratin macrofibrils have caused the ends of each cell to be finger-like with the fingers interdigitating between cells in a longitudinal direction. The internal components of the cortical cells are now described in detail. Keratin macrofibrils: These tough fibrils of between 400 and 2000 Å in diameter and up to 50 µm in length occupy most of the