OF THE CLASS MAMMALIA. *2'2i) 



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 giraffe (Fig. 217). 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. 177) ; finally, in others the osseous axis is 

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

 covered with a kind of sheath composed of an elastic sub- 

 stance, 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 



Fig. 175. Rhinoceros of India. 



goat (Fig. 176), 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 sinuses, and thus re- 

 ceiving into their interior the external air. 



The mode of formation of the horns called antlers, is as 

 follows : At a certain age there appears a growth of bone on 



