68 



KERATIN AND KERATINIZATION 



(a) 



(b) 



(e) 



(f) 



Fig. 



29. The various epidermal appendages containing hard keratin 

 shown black and softer varieties shown stippled. 



(a) and (b) are two kinds of horns : (a) is the pronghorn consisting of a 

 " thimble " of true horn capping a " bony horn " covered by hairy 

 skin. The cap is shed annually; (b) is the true horn covered entirely 

 with horn keratin which is not shed. The antlers of deer are not true 

 keratinous horns but consist of bony growths at first covered by a hairy 

 skin, (c) The principal parts of a hoof which consists of an outer covering 

 of hard keratin (the unguis) and an inner subunguis of softer keratin. 

 This combination of a harder and softer keratin recurs in claws 

 (d, e and f). Claws of a carnivore, a bird and a rat. The wearing away 

 of the softer subunguis helps to maintain the sharp cutting edge of the 

 carnivore claw (Le Gros Clark, 1936). (g) The human nail in which 

 only the hard keratin layer remains. 



