THE INTEGUMENTARY SYSTEM 



171 



bits of the old skin at the tip of the tail. These become the rattle, which 

 therefore grows two rings a year. 



Most reptiles have substituted horny scales for the bony scales charac- 

 teristic of fishes. But crocodiles have both sorts on the same individual. 

 On the ventral side of some snakes, large scales are attached to muscles 

 and become organs of locomotion. 



The largest reptilian scales are those of chelonia, in which horny scales 

 fuse with the bony carapace and plastron. In birds horny scales cover 

 the feet. 



Among mammals, the East Indian manis, and the tails of rats and 

 mice are scaled. 



Fig. 129. — Section of developing scales of lizard, Sceloporiis. c, papilla of corium; 

 e, outer layer of epidermis which later becomes cornified; /, fibrous layer of skin; m, 

 Malpighian (stratum germinativum) layer; p, periderm; ts, tela subjunctiva. (From 

 Kingsley's "Comparative Anatomy of Vertebrates.") 



It is a curious fact that while horny scales are purely epidermal 

 structures, their development is initiated, like that of bony scales, by the 

 corium. 



HORNS 



To produce such diverse structures as hairs, feathers, scales, nails, 

 and hoofs, demands most exceptional evolutionary potentialities on the 

 part of the horny layer of the skin. Among the surprising developments 

 of keratin-forming tissues are the horns of ruminants and rhinoceros. 

 Those of rhinoceros are formed wholly of keratin produced by the stratum 

 corneum on the snout. The hollow horns of cattle have, in addition to 

 external keratin, a bony base and core, which grows from the frontal bone 

 into the cavity of the horn. The antlers of the deer tribe are bony out- 

 growths with no covering of horn, but only the skin or "velvet" which 

 is soon lost. 



Horns are best interpreted as weapons of defense and offense. 



NAILS, CLAWS, AND HOOFS 



Nails are scale-like thickenings of the stratum lucidum at the ends 

 of the fingers and toes, formed of homogeneous keratin identical with that 

 of the stratum lucidum from which they develop. 



Nails and claws are strikingly alike except in form. Both develop 

 from a matrix at the base, which in man appears as the whitish "lunula." 



