98 JOURNAL OF COSMETIC SCIENCE 100 80 60 40 20 0 '[__ ! [•j •"1 80 Temper •:v 70 oc 5 4 3 2 1 0 60 of hair / Cycle times of heating / times Figure 12. Increase of porous medulla as function of both numbers of cycle times of heating and heating temperature. and the low-angle region (around 10 degrees), while the contrast is higher in the case with fewer medulla pores to show a distinct peak around 55 degrees. This peak in the chroma curve leads to a brilliant impression. Futhermore, the hue angle of hair with fewer medulla pores shows variation according to the receiving angle (Figure 8c), but it is almost constant with more medulla pores. These results mean that apparent hair color with less porous medulla is highly dependent on the viewing angle, while the color with more medulla pores is relatively independent on the angle and is monotonous From the above results, it is understood that the appearance of hair with more porous medulla is lusterless and dull because of low contrasts of lightness and monotonous apparent color, while the appearance of hair with less porous medulla is shiny and brilliant because of that high contrast. PORE GENERATION AT THE MEDULLA BY DAILY HAIR CARE PROCESSES Figure 9 shows a distribution histogram of panelists where the abscissa denotes the scores of the total amount of light scattering from the medulla. The scores of scattering light must roughly correspond to the lengthwise ratio of the total whitish portion against the total fibers in microscopic views. It should be noted that the score does not strictly correspond to the amount of porous medulla, because it is sometimes difficult to evaluate scattering at the fiber center in the case of considerably dark hair, even by using an intense light source. It can be said that, more or less, 80% of the panelists have porous medulla. Through this survey, it was also noticed that the amount of light scattering from the medulla in the panelists may have some relationship with the hair care behaviors of the individual panelists. Figure 10 shows the histogram of the same panelists as those in

MEDULLA STRUCTURE AND HAIR APPEARANCE 99 Figure 9, which was reviewed again by classification according to the way a hair dryer is used. The peaks of the distribution are located at the higher score of light scattering for the panelists drying their hair fully with the heat of a hair dryer, suggesting that the pores in the medulla can be generated by heat. Figure 11 shows the effect of dryer heat upon enhancement of the light scattering at the medulla (optical microscopic views 11 a, 1 lb, 1 l c) and the effect upon the pore generation at the medulla (SEM cross-sectional views 1 ld, 1 le). It is clearly shown that heat of the dryer can cause pore generation in the medulla and lead to enhancement of light scattering. Further experiments were carried out by simulating a general hair care cycle in order to confirm that pore gen- eration actually occurs in daily life. Figure 12 shows the increase of light scattering from the medulla as functions of both cycle time of a model hair care process including heat drying and also the temperature of the hair surface. One cycle is composed of sham- pooing, conditioner treatment, and heat drying processes. The heat drying was carried out for five minutes. Medullae of hair used in this test were originally non-porous. The higher the hair temperature, the more pores are generated. Furthermore, the pores thus generated are accumulated through the hair care cycles, as shown in Figure 12. It is known that the existence of the medulla is correlated with the hair cycle and the thickness of the hair shaft (4,16). Our results clearly show that the apparent amount of medulla is also strongly affected by hair care behavior. SUMMARY 1. Medulla pores cause light scattering inside the hair fiber and make the hair look whitish and chalky (less shiny). 2. Medulla pores can be generated using a hair dryer in daily hair care processes. ACKNOWLEDGMENTS The authors express their gratitude for helpful discussions and guidance to Mr. Itomi Homma, Director of Hair Care Laboratories, and Dr. Koichi Nakamura of the Institute of Beauty Science in the Hair Care Laboratories of Kao Corporation. Thanks are also due to the late Professor Toyoichi Tanaka of the Massachusetts Institute of Technology for many helpful discussions on the structural changes of the medulla. REFERENCES (1) P. K. Chattopadhyay, K. Gonmori, and N. Yoshioka, Medulla types of hair among the Japanese, Act. Crim. Japon, 60, 142-148 (1994). (2) R. Iwamoto and E. Munakata, The evaluation of the validity of morphological characteristics in forensic hair comparison, Kagakukeisatsu Kenkyuhoukoku, 45, 75-83 (1992). (3) G. Mahrle and C. E. Orfanos, The spongious keratin and the medulla of the human scalp hair, Arch. Derre. Forsch. 241, 305-316 (1971). (4) P.E. Hutchinson and J. R. Thompson, The size and form of the medulla of human scalp hair is regulated by the hair cycle and cross-section size of the hair shaft, Br. J. Dermatol., 140, 438-445 (1999). (5) G. E. Rogers, in The Epidermis, W. Montagna and W. Lobitz, Eds. (Academic Press, New York, 1964), p. 202. (6) J.-L. Bantignies, G. Fuchs, G.L. Carr, G.P. Williams, D. Lutz, and S. Marull, Organic reagent