THE KERATINIZED TISSUES 



69 



The Digital Tips: Claws, Nails, Hoofs 



The fingers and toes of all vertebrates above the amphibians are re- 

 inforced by horny appendages which are adapted to the way of life of their 

 owners. The homology of all these structures will be apparent from 

 Fig. 29. Claws are structurally similar wherever found and consist of two 

 unequally-developed scale-like surfaces which meet over the end of the 

 digit (Fig. 29 (d), (e) and (f)). The dorsal surfaces are formed of a tougher 

 and more compact keratin than the lower or sole, and this contrivance 

 ensures that where the two meet, a sharp projecting cutting edge will be 

 formed. The orientation of the component cells may be different in the 

 two layers (Fig. 29 (f) ). The nails of primates are developed from claws 

 by flattening and losing the underlayer leaving only the compact layer 

 (Clark, 1936 and Horstmann, 1955). The sole is perhaps represented 

 by a small area beneath the projecting nail (Fig. 29 (g)). In hoofs, the sole 

 although softer forms a more important part of the weight-bearing surface, 

 and is encased in a sheath of more compact and harder horn (Fig. 29 (c)). 

 This use of keratins of different degrees of toughness to effect special 

 functional properties is an important aspect of keratinization which will 

 be returned to in Chapter 6. 



Feathers 



The structure of a typical feather is shown in Fig. 30. The simple 

 down-feather or plumule consists of a cylindrical quill opening into a tuft 

 of barbs and barbules; the filoplume is a fine hair-like feather; contour 

 or flattened flight feathers are more complex, consisting of a quill or 

 calamus and a shaft or rhachis which bears the barbs and barbules. 



Feathers, the uniquely-distinguishing mark of birds, are believed on 

 embryological and paleontological grounds, to be homologous with 

 reptilian scales. As in the case of scales the first indication of the site of 

 a presumptive feather is in the gathering of dermal cells beneath the 

 epidermis which then projects to form a papilla containing dermal elements. 

 At this stage, feather and scale " germs " are much alike. Later the 

 whole formation sinks into the skin to form the follicle. Regarded in its 

 simplest ideal form, a feather is a hollow type of cornified epidermis 

 growing from a ring of germinal cells at the bottom of the follicle (Fig. 45, 

 Chapter 3). The development of a plumule (down feather) which consists 

 of a short cylindrical quill opening into a circle of soft barbs and smaller 

 barbules can be understood from Figs. 46 and 47, pp. 103-105. The 

 germinal layers at the base of the papilla first form a number of longitudinal 

 columns (seen in cross-section in Fig. 46) ; these separate and keratinize 

 each to form a barb. The basal part of the growing feather does not 

 separate into columns, remaining as a continuous cylinder to form the 



