426 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS X10 • I J I , =% RH VS FORCE/AREA ••_""• .... % RH VS NUMERICAL ........• •-"•,.•_.,, _-"..•,• BIREFRINGENCE - ........,, _• • .-.•_ _• __-• , II 8 7 6.5 o' zs' 50' •5' /4 /o 5 /oo' gm/ /.,,m' X10' % RELATIVE HUMIDITY Figure 13. Linear regression with 95 per cent confidence interval of force/area and numerical birefringence versus per cent relative humidity In the context of this paper, the term condition is used synonymously with the parameter descriptions body and manageability. These, in turn, are more specifically defined as elasticity or the resistance to and recovery from defor- mation induced by external force. It can be seen by the nature of this defini- tion, that hair condition is traditionally thought of in a mechanical light (1, 9). Obviously, there is a molecular basis to this mechanical behavior (7, 9, 16). The a-helix molecular chain arrangement, basis of the keratin fibril system, is a symmetrically ordered configuration mainly responsible for this longi- tudinal stability in hair. In addition, both surrounding and infiltrating the fiber system is an amorphous cement-like matrix, which is high in cystine cross-linking, and has both a and/3 keratin chains (7, 16, 17, 18). It is this relationship between the fiber system and matrix which •becomes important as the contributing factor to both optical and mechanical properties of the hair (3, 7, 9, 16, 17, 18) (Fig. 14). The mechanical parameters of this relationship can be explained in te]xns of a two-phase model, which becomes apparent through experimentation with the aqueous swelling of hair (9, 16). A hair fiber undergoes various changes due to absorbed moisture between the relative humidity levels of 0 to 100 per cent. Approximately 2.7 times less force is required to stress a hair to yield point in a saturated versus dry state. At the same time, swelling occurs at a rate of 16 per cent radially but only

POLARIZATION MICROSCOPY 427 •..., w.• ...... ..•.•4!•71 • ?•:•' .-.-•.• •.•...• '•.•-.•......• ?.:.." '...%: •??":'": •: 4:' '• .•" •..• •g" . .-::. .... ß '•:..7:.:.":-'.'-' : • .'•.': '-L .-.'. •: •:..•-..'•' . ., •.:,. { "..,.? ...•.'.,. :-' :,. ,, '-. .... .•, •:? .? .i•:,::•'" ..•. •.....:•:'•'•....• .,• ß •97'-:•: - ..':*" '. ' -'Z•-?'•..** :½ .--, /' .'.•i• :•:S::::.•:'..-:-•::.t _C&.:,•::'...:• * ,. .•? .. ... il '-,'-... •. ß .• •. :•.? ':::' ..•.: ? :.•..• ,' ..• ß ½!i?' 'i•:•:. •' : • : .":2 : :•-...:)' •:•.. •:". -....2' •: .'½7' ß .. :.. ß :. i ..: % ..• ..• .. . .-- ....-:' . • .... '-• • .. •..•.• :' •::.' .:..•.. .... • . ..:'-:.,.: .. if! ::. ' .' . ' ..... .,.. ..:'•': . .... :. ....% •' "........ •.?.: '(•. :.. •- •.. *.• . '. • • . :.. •-•.. •. -•...-...----. . ½ • ...- . . •. '•. .-- •:• •. , .•: .:.. •. •... ,. ...... . ': • •. .......• .: %.. •..- .:• -•-.•-. . ..::. . .-:.• ...... .. ..... :• ..... -. ,' -.-:'.'.--, ,' "-"• -•- - "..-.•' •:•:.. ? •: ...•. ' :'" d ' . • . -.:• t, .", :',:½ •'. •:..•,:..• .•- ......... 4:":-½-:: ....... . :::.:. :.• ..,•..... •.. •.•. :.:... ':.'.•' - ...... .. •:....,,.::. .•.,. '..:: ': ....... •-• ......... .. • Figure ]4. (A) Scanning •d (B) transmission electron micrographs of fibril and matrix system • human hak cortex. •ote fiber bundles i.n (A) made up of small fibers, and in (B) the 80 • protofibd• (small white oiroles), su•ounded by an amo•hous matrix (stained •rk)