454 THE ;mj:chamcal functions. 



the foieljcad : 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 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 

 protection to the subjacent bony layers which are 

 forming underneath it. The highly vascular mem- 

 brane, from which these new structures chiefly 

 arise, appears to have different powers of produc- 

 tion at its two surfaces ; for while the inner sur- 

 face is forming the osseous portion of the horn, and 

 supplying the phosphate of lime required for the 

 construction of its plates and fibres, the exterior 

 surface is adding successive layers of horny sub- 

 stance to the inner side of those portions which 

 had been before deposited. These two operations, 

 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 is an organic 

 structure, which continues to be nourished by ves- 

 sels 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 succes- 

 sion ; each new layer is agglutinated to the inner 

 surface of the preceding ; and each has the shape 

 of a hollow cone, occupying the part towards the 

 apex of the former cone, and extending farther to- 



