174 KERATIN AND KERATINIZATION 



water or in steam. It is supposed that at these higher extensions a further 

 non-crystalline fraction must extend. 



An important feature of these extensions and the X-ray changes that 

 accompany them is that (excluding the phenomena of set, p. 249) they are 

 reversible and that the type of X-ray pattern is closely connected with the 

 degree of extension. 



Astbury and Woods suggested that the step-like nature of the curve 

 OABCD was due to the presence in the fibre of three " phases " of keratin 

 which differed in their ease of extensibility and which were effectively in 

 series. On stretching a fibre each extended in turn: at A the " transfor- 

 mation tension " of phase I was reached, at B phase II, etc. Since no 

 marked change in X-ray pattern occurs in the range AB or beyond C 

 these portions of the curve must correspond to the extension of non- 

 crystalline material (phases I and III); the range B-C where the a — /3 

 transformation occurs must be where the crystalline phase extends 

 (phase II). 



Since the steps in the curve are not sharp it was assumed that the 

 separate phases were not simply in series but that, as a result of restrictions 

 exerted by one phase on another, they were also partly in parallel. With 

 the assumption of three extensible elastic elements, with different re- 

 sistance to extension, this model has sufficient variables to provide a fit 

 even for the infinitely varied responses of the wool fibre. 



Woods (1938) met the possible criticism that, since hairs were histo- 

 logically complex, certain features of the step-curve might be due to the 

 extension of various histologically-recognizable components joined partly 

 in series and that the extension of these parts (including the crystalline 

 region) might not be the same as that of the whole fibre. He stretched 

 wool fibres to various percentage extensions, " set " them at these lengths 

 by steaming and, by means of tryptic digestion, isolated the stretched and 

 set, keratinized contents of the cortical cells. By plotting the percentage 

 increase in length of the whole fibre against that of the cell residues, he 

 showed that the cell contents parallelled very closely the extension of the 

 fibie as a whole. A slight lag was interpreted as due to either a loss of set 

 during the isolation of the cells or, as is most likely the case, to the extension 

 of the component which links the cells together, i.e. in current histological 

 terms, to the cell membranes and intercellular cement. It is thus possible 

 to say that the elastic properties of the fibre are mainly those of the 

 keratinized contents of the cortical cells. 



In water at ordinary temperatures extensions of the order of 70% 

 are possible; at elevated temperatures (steam) or in solutions of dilute 

 caustic soda a limiting extension of the order of 100% was found by 

 Astbury and Woods (Fig. 71). They assumed from this that at 100% 

 extension all the polypeptide chains were fully extended. By making a 



