RUMINANT QUADRUPEDS. 513 



in the construction of the horns of the ox and 

 of the sheep : for in these the bony processes 

 arising from the frontal bones are invested with 

 a covering composed of horn, the nature of 

 which is totally different from bone. Two tu- 

 bercles may be seen in the young calf, proceed- 

 ing from the bones of the forehead : the skin 

 covering these tubercles, unlike that which pre- 

 cedes the antlers of the deer, is unusually thick 

 and hard. As the skull expands, this portion 

 of integument becomes more and more callous, 

 till it is converted, by the action of the subjacent 

 vessels, into a solid, hard, elastic, and insensible 

 fibrous substance, fitted to give effectual protec- 

 tion to the subjacent bony layers which are 

 forming underneath it. The highly vascular 

 membrane, from which these new structures 

 chiefly arise, appears to have different powers of 

 production at its two surfaces : for while the inner 

 surface is forming the osseous portion of the horn, 

 and supplying the phosphate of lime required for 

 the construction of its plates and fibres, the ex- 

 terior surface is adding successive layers of horny 

 substance to the inner side of those portions which 

 had been before deposited. These two opera- 

 tions, which offer a remarkable contrast, both as 

 to the mode of their performance, and as to the 

 nature of the resulting products, are carried on 

 at the same time, and by the same organ, but 

 on different sides. The bony basis of the horn 



VOL. I. L L 



