376 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ß _e ' .,.? ,• . •.- • . • . . . Figure 12. Polarized light photomicrograph of beard hair with an •termittent medulla. Magnification 150x. b. Solubility in Urea-Bisulfite Solution The solubility of wool in urea-bisulfite solution, under standard conditions (18), is a test used in the wool industry that provides an index of the extent of change in the fiber's chemical properties, resulting from certain processing steps. The formation of new cross-links leads to decrease in solubility, whereas degradative treatments result in an increase. Table V Tensile Properties of Caucasian Beard and Scalp Hair in Water at 21øC Property Beard Hair" Scalp Hair" Initial Modulus, dynes/cm 2 Stress at Yield Point, dynes/cm 2 Stress at 20% Extension, dynes/cm 2 Post Yield Modulus, dynes/cm 2 Extension to Post Yield, (%) (1.18 + 0.22) x 10 •ø (3.57 -+ 0.51) X (4.47 _+ 0.53) x 10' (3.60 + o.8o) x 10' 26.2 _ 1.4 (1.55 + 0.13) X 10 •ø (4.3• + o.21) x (5.18 + 0.42) X 10 (4.92 + 0.42) X 109 25.1 + 0.7 "For each fiber type twenty fibers, ten fibers each from two subjects, were tested. Average values with standard deviations are reported.

COMPARISON OF BEARD AND SCALP HAIR 577 %,:, Figure 15. Transmission electron micrograph of Caucasian scalp hair cross-section showing melanin in cuticle and cortex. Magnification 91,600x. The much faster rate of solubilization of Caucasian beard hair, as compared to scalp hair, in urea-bisulfite (Figure 16) is in good agreement with the lower disulfide content of facial hair. c. Swelling Studies in Formic Acid Caldwell and Milligan (19) developed a gravimetric method for measuring the extent of swelling of wool in concentrated formic acid and correlating the swelling data with the