Fig. 312. — Section through 

 a cow's horn : c, cuticle ; 

 he, horny cuticle ; ml, Mal- 

 pighian layer ; her, horn 

 core ; d, dermis. 



456 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



In hollow horns (Fig. 312) we have first a conical 

 projection of the frontal bone, covered as usual with 

 dermis, -and this in its turn with active epithelium hard- 

 ening into horn. About the base 

 there is, as usual, an infolding in 

 which the growth of horn is espe- 

 cially rapid. Successive layers 

 are formed one within another 

 precisely as in claws. The hol- 

 low horns are permanent. 



Solid horns, as in the deer, elk, 

 etc., are bosses which grow out 

 from the two frontal bones with 

 great rapidity, and are at first 

 covered with skin and fine hair 

 (so-called velvet). In this condition it is very vascular. 

 In a little while — a month or so — the blood gradually 

 withdraws, the skin dies and is rubbed off, and the antler 

 is really dead though firmly attached to the skull. At 

 the end of the year a separation takes place at the 

 skull, the antler drops, and the skin of the skull grows 

 over and covers the wound. Soon growth begins again 

 at the same place and a new pair of antlers is formed, to 

 pass again through the same annual cycle of changes. 



In comparing the antler of a deer with the horn of 

 an ox or sheep it is evident that the mature antler of 

 the former corresponds with the horn core of the latter, 

 while the horny exterior of the latter corresponds to the 

 skin and velvet of the former. 



Thus, then, the ruminants are divided into two groups, 

 the hollow-horned and the solid-horned. The horns of 

 the former are permanent, those of the latter are decidu- 

 ous. The two grade into one another in the antelopes. 



Feathers. — This most wonderful of all skin struc- 

 tures is wholly characteristic of birds. 



