354 THE MECHANICAL FUNCTIONS. 



dant nourishment, but continue to maintain an exact equili- 

 brium between the expenditure and the supply ; so that the 

 horns of the cameleopard are never shed, and remain perma- 

 nent bony structures. 



A farther modification of this process occurs in the con- 

 struction 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 tubercles may 

 be seen in the young calf, proceeding from the bones of the 

 forehead: the skin covering these tubercles, unlike that 

 which precedes the antlers of the deer, is unusually thick 

 and hard. As the skull expands, this portion of integu- 

 ment 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 eflfect- 

 ual protection to the subjacent bony layers which are form- 

 ing 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 con- 

 struction of its plates and fibres, the exterior surface is add- 

 ing successive layers of horny substance to the inner side 

 of those portions which had been before deposited. These 

 two operations, w^hich oflfer 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 is an organic structure, which continues to be 

 nourished by vessels forming part of the general system: 

 the horn is a mere excretion, which appears to be destitute 

 of vessels, and is, consequently, removed from the influence 

 of the living powers. Thus the growth of horn is somewhat 

 analagous to that of shell; for the layers which compose it 

 are deposited in succession; each new^ layer is agglutinated 

 to the inner surface of the preceding; and each has the shape 



