EXTENSION OF HUMAN HAIR 189 presence of the methyl group in 18-MEA is likely to lead not only to a greater mobility of the fatty acid monolayer but also to a possible reduction of the adhesion between cells. The reduced amount of cystine crosslinks in the endocuticle certainly would result in a higher degree of swelling in water. Furthermore, the mechanical opposition to distortion would be reduced. However, the fact that the endocuticle distorts considerably more than the exocuticle when stressed does not of itself mean that the endocuticle will fracture under a lower stress than the remainder of the cuticle. The endocuticle is more extensible, as a lightly cross-linked elastomeric structure within which the molecular chains are able to flow past each other with greater ease because of the lower presence of cross-links. The 18-MEA surface in contact with the underside of a cuticle above it (see Figure 2a) can, because of its surface mobility, slide past the upper cuticle. From the above, the proposal that the surface of the upper-[•-layer is the region of weakness allowing flow of cuticle past cuticle (see Figure 2a) during fiber extension must be considered as an important alternative mechanism to the breakup of the endocuticle under stress that might result in the "decementation" between cells. Near the edge of the upper endocuticle and the upper-[•-layer of the lower cuticle (see "X" in Figure 2b), the distorted endocuticle will not only tend to relieve the distortion stress by allowing the lower cuticle to slide, exposing more upper-[3-1ayer, but would also tend to lift. The lifting action would progress along the junction with the upper-[•-layer, allowing a film of air to penetrate along the junction. The results of Guiolet eta/. (2) suggest, from Figure 1 and their own data on loss of transparency, that at around 8% strain on the hair fibers under the experimental conditions of relative humidity and rate of straining, failure at the surface of fracture of the upper-[•-layer commences. This failure is pro- gressive up to the maximum extension applied. DISCUSSION WITH REFERENCE TO PERMING AND CONDITIONING The failure of the adhesion of cuticle cells to the hair cortex has been observed to lead to the complete breakdown of the hair at the fiber ends. This observation was obtained in experimental comparison of the action of combing of hair with and without condi- tioner (10). The use of hair conditioner resulted in the cuticle being present on much longer lengths of hair prior to exposure of the cortex and breakdown of the hair structure. The progressive loss of cuticle can be clearly seen in SEM pictures. Once the cortical structure is exposed, the main hair shaft fibrillates into split ends and begins to break up. As mentioned earlier in this paper, 18-MEA in the upper-[•-layer not only allows for sliding of the lower cuticle relative to the upper cuticle, but plays an important role in the adhesion between cuticle cells. Failure of this adhesion would be expected to make the cuticle fragments much easier to remove, leading to exposure of the cortex. In experimental perming procedures at elevated temperatures, as described elsewhere (11), no neutralizer was used. The resultant permed hair felt smooth and was not left harsh, as is the case in the normal perming procedure. The permed hair required a minimum of conditioning, and on successive brushing and combing there was little evidence of broken pieces of hair ends, as can occur after a normal perming procedure. It appears that this perming process preserves the lubrication and adhesion of the cuticle cells better than the conventional perming process. It should be noted that hair fibers when they first protrude from the follicles have up to ten overlapping cuticle cells. The result is that in the life of a hair fiber the action of

190 JOURNAL OF COSMETIC SCIENCE combing and brushing will not remove the last layer of cuticle cells for some consid- erable time and thus for the length of hair. However, the amount of 18-MEA per unit length of hair would progressively decrease until the last cuticle layer is removed, exposing the hair cortex, leading to subsequent breakup of the fiber end. The action of cationic conditioners serves to replace the lubricating surface where 18-MEA has been depleted, helping to preserve the cuticle layer for substantially greater lengths of hair. Unlike the 18-MEA layer, conditioners are not covalently bonded to the cuticle surface and need to be replaced after the hair is washed to retain the lubricating action and protective effect. APPENDIX The overlapping scales, which form the cuticular outer layer of a human hair fibers, are attached by their inner edge to the hair cortex, the main shaft of the fiber. The over- lapping scales form a serrated system of scale edges on the surface of the cuticle. For the purposes of this analysis the scales are considered as platelets rectangular in cross section and capable of rotation about a line fulcrum (A and B in Figure 3a,b). With the fiber unextended, the angle of the scales to the direction of the fiber is 0. If the fiber is extended by strain e, the attachment of scales to the main shaft (A and B in Figure 3a, b) will move apart, and the scale angle, 0, will reduce to (0-80). The dotted scale in Figure 3b represents the changed position of Scale 2 after extension, and AB' = (1 + e)AB (1) In Figure 3b, BC is perpendicular to the Scale 2 surface with the fiber unextended and B'C' with the fiber extended. Similarly, BD is perpendicular to AB, and B'D' is perpendicular to AB'. The thickness of the scale, t, is for the unextended fiber. With low adherence between overlapping scales, extension of the fiber will result in slippage between scales, as indicated in Figure 3a. The thickness, t, of the scales will remain unchanged because of the negligible lateral forces transferred between the scales. In the unextended state, the scale thickness, t, is given by t=BC = AB sin 0 (2) If slippage occurs between the scales on extension of the fiber and the thickness remains unchanged, then BC = B'C' = t It follows that because then That is, from equation 1 B'C' -- AB' sin(0 - 80) AB' sin(0 - 80) = AB sin 0 (1 + e)sin(0 - 80) = sin 0 (3)