io6 HISTORY OF THE HUMAN BODY 



crests and other cornified structures, many of which are un- 

 doubtedly scale modifications ; and in birds there are occa- 

 sionally horny structures, often with a core of bone, like the 

 spurs of the game cock, of doubtful morphological value. The 

 lower legs and feet of birds are encased by a horny epidermis, 

 a part of which 'is covered by definite scales, while other parts 

 of it are divided by grooves into square or polygonal areas. 

 The skin of crocodiles is marked in much the same way and 

 does not form overlapping scales, yet it is highly probable that 

 in both cases the areas separated are the equivalent of scales, 

 since overlapping is not a necessary characteristic of these 

 organs. 



In mammals there are many special organs composed of 

 keratin. The " whalebone " of whales is derived from the 

 epidermis of the hard palate and forms a thick fringe which 

 hangs from the upper jaw and is employed as a strainer. 

 There are three types of horns: that of the rhinoceros, formed 

 by a coalescence of numerous keratin fibers, probably the 

 morphological equivalent of hairs; the hollow type found in 

 some ruminants, in which a hollow keratin structure is fitted 

 over a core of bone ; and, thirdly, the solid horn of deer and 

 antelopes, where the final structure is composed of the bony 

 core alone, the epidermis being represented by the " velvet," 

 an external covering which atrophies after the horn is com- 

 pleted and is rubbed off by the animal. Thus this last, in its 

 final condition, cannot be counted among epidermic structures. 



In reptiles, birds and mammals the ends of the digits are 

 armed by horny structures, strictly homologous throughout, 

 although variously denominated as claws, nails or hoofs, in 

 accordance with their shape. In the typical claw (Fig. 28, a) 

 the parts to be noted are the convex dorsal plate (Krallen- 

 platte), the concave 'ventral plate (Sohlenhorn) and the apical 

 pads of the digit (Zehenballen). In the sauropsidan claw (a) 

 the two plates are of about equal importance and the terminal 

 pad is represented by an unmodified scale or by several 

 scales. In the typical mammalian claw (b) the ventral plate 

 is somewhat reduced and the terminal pad is well developed - 



