240 



ZOOLOGY. 



are sometimes mere appendages of the skin, and seem to be 

 formed of the hair matted together ; this is the case as 

 regards the horn or horns of the rhinoceros ; but generally it 

 is otherwise, and it is a prolongation of the frontal bones, 

 which forms the axis of these horns. All mammals provided 

 with such horns are ruminants. Sometimes the osseous pro- 

 tuberance remains covered with the common integument, and 

 continues so; this happens in the giraife (Fig. 271). In some, 

 the skin which covers the osseous core or axis disappears, 

 and leaves the bone exposed ; which, after being so for a time, 

 falls or is thrown off, to be replaced annually by another 

 growth : such are called antlers, and are found only in the 

 deer kind (Fig. 203) ; finally, in others the osseous axis is 

 never shed, but continues to grow during the life of the 



Fig. 198. lihmoceros of India ; a single-horned Khinoceros. 



animal, covered with a kind of sheath composed of an elastic 

 substance, especially called horn, which grows by layers, and 

 is analogous to nail and hair. The term hollow horns is 

 given to horns thus covered with a horny sheath, formed 

 seemingly of agglomerated hairs, and found in different 

 species of the goat (Fig. 202), ox, sheep, and antelope. It 

 ought further to be remarked, that in all these animals, with 

 the exception of the antelope, the osseous axis of the horn is 

 hollowed out into cells communicating with the frontal 



