2006 TRI/PRINCETON CONFERENCE 363 Figure 3. TEM micrograph of microfibrils in medulla transversally oriented to he section. Figure 4. TEM micrograph of microfibrils in medulla parallel oriented to he section.

364 JOURNAL OF COSMETIC SCIENCE 550 500 unmedullated hair 450 400 350 300 en en 250 200 en 150 100 50 0 0 10 20 30 40 50 strain(%) Figure 5. Stress-strain curves of 40 fibers of unmedullated hair. 550 500 medullated hair 450 400 350 300 - en en 250 200 en 150 100 50 0 0 10 20 30 40 50 60 strain (%) Figure 6. Stress-strain curves of 40 fibers of medullated hair. A series zone model has been proposed (25) to explain the behavior of hair in the yield and post-yield region. According to the series zone model, hair can be assumed to consist of two alternating zones along the microfibrils, namely the X zone and the Y zone. 30% of the a-helices in the keratin are capable of exhibiting a reversible folding-unfolding nature without breaking the disulfide bonds and these form the X zone, which charac-