514 THE MECHANICAL FUNCTIONS. 



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, conse- 

 quently, removed from the influence of the 

 living powers. Thus the growth of horn is 

 somewhat analogous to that of shell ; for the 

 layers w^hich 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 ex- 

 tending farther towards the base. Hence a 

 longitudinal section of the whole presents the 

 appearance represented in the annexed figures 

 (218*), where a is the section of the horn of an 

 Ox, and b, a similar section of the horn of an 

 Antelope. C is a magnified view of the extre- 

 mity of the latter, together with a portion of the 

 bone D, which occupies the axis of the horn. 



In this process of the formation of horn, as 

 happens in that of shells, there sometimes occur 

 irregularities, or periodical intermissions and 

 increase of action in the secreting organs, giving 

 rise to transverse grooves, or ridges. These 

 may be seen in the horns of the goat, in which 

 the fibres are short, and laid one over another 

 with the same regularity as the tiles of a house. 

 The tendency in these horns to assume a spiral 

 form is explicable on the same principles as 



