THE KERATINIZATION PROCESS 273 



soft keratin containing transformed keratohyalin may be " mixed " in 

 various proportions with a fibrous keratin of the hard type. These 

 differences in degree of keratinization produce differences in physical and 

 chemical properties having obvious functional value and this has not been 

 overlooked in the evolution of the various epidermal appendages. It takes 

 perhaps its most interesting form when the function of a particular organ 

 is found to depend on differences in keratinization in its several histological 

 parts. The case of the hair follicle has been discussed above. There we find 

 the soft cells of the root sheaths desquamating and freeing the hair ; the 

 tough impact-resisting cuticle protecting the fibrous cortex; the light open 

 framework of the medulla providing a rigid girder-like internal skeleton. 



The epidermis itself varies in thickness and toughness from site to site 

 and the distribution is clearly related to the special demands made on each 

 site. The thickened epidermis with its marked pattern of papillary ridges 

 found on the palmar surface of the hands and feet is genetically determined 

 as is also the ability to respond to mechanical friction and pressure 

 by further thickening. On the histological level, hard and softer regions in 

 the papillary ridges are said to have the effect of enhancing mechanically 

 the sensitivity to touch (Cauna, 1954). 



In the terminal appendages (claws, hoofs, etc.) two parts, the unguis 

 and the softer subunguis, can normally be distinguished (Fig. 29, p. 68) 

 and the different rates of wear of these two parts has much to do with the 

 maintenance of the shape and the functional efficiency of the parts. For 

 example, in the ungulate hoof the hard cylinder of the unguis largely 

 surrounds the softer subunguis which wears the more rapidly and thus 

 maintains a more or less flattened, load-bearing surface. The sharp cutting 

 or piercing tip of a claw is maintained by a difference between the hardness 

 and orientation of the two layers of which the claw is composed. These 

 two layers, the superficial stratum and the deep stratum (unguis and 

 subunguis) may be seen in Figs. 29 d, e and f, and it is obvious how the 

 sharp cutting edge of the claw is maintained automatically by the more 

 rapid wearing of the deeper layer. 



Hair also contains parts varying in keratinization in the cortex. An 

 elaborate relationship is found between morphology, physical behaviour 

 and variations in keratinization in fine crimpy or curly hair or wool 

 (Mercer, 1954). This type of hair, already discussed in Chapter IV, p. 156, 

 forms waves which are very nearly uniplanar and it is found that, in terms 

 of stabilization and of chemical reactivity, these fibres, like many of the 

 larger appendages, are also bilateral (Horio and Kondo, 1953). They con- 

 sist in fact of two hemi-cylinders differing in stability which are twisted 

 together in such a way that the helix of the two hemi-cylinders is always 

 in phase with the crimp wave and that the outside face of the fibre is 

 always less keratinized than the inside face (Horio and Kondo, 1953; 



