HAIR CUTICLE 847 cant variation in the reactivity of hair as a function of its diameter. Ex- perimental evidence supporting this view has been obtained by us in the course of au investigation of the supercontraction properties of hair. Fibers of varying diameter were boiled slack in 5% NaHSO.• for 30 min and their contraction was determined after rinsing in water and drying. The results are given in Fig. 5. The level of supercontraction decreases with increasing relative cuticle content (decreasing diameter) of the fiber. The direct reason for this negative effect of the cuticle may be more structural than chemical. This is apparent from the scanning electron microscopic examination of supercontracted fibers. Figure 6 shows an intact mohair fiber, and Fig. 7 the same fiber after supercontraction in hot, aqueous bisulfite. The cuticle cells show little change in their di- mensions but are tightly folded over to conform to the length changes _Figure 7. Scanning electron micrograph of supercontracted mohair. Magnification, 2250

848 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS sustained by the contractile cortex. In wool fibers, which contain only a single layer of cuticle cells, the restraining effect of the cuticle is marginal. In human hair, it becomes a dominant factor. The most likely explanation for the resistance of the cuticle material to contraction is its isotropic character. The breakdown of disulfide bonds (or for that matter secondary bonds) may cause an intensification of swelling, but there is no vectorial force which could develop and bring about significant change in the axial direction of the fiber. The combination of the thickness of the cuticle and its heavy cross- linking is also responsible for the stress birefringence phenomenon which can be observed on cross-sections of hair immersed in formic acid (Fig. 8). Here again, the cuticle acts as a restraining element on the expanding cortex interior. In the course of this, some preferential arrangement of this isotropic material takes place, and this can be readily perceived un- der polarized light. MECHANICAL PROPERTIES The contribution of the cuticle to the mechanical performance of hair has been usually regarded as marginal, the similarity of the stress-strain curves of wool and hair being used as an argument in support of such a view. However, while the cortical elements may be dominant in the overall mechanical characteristics, it is difficult to see how a heavily cross-