NO. 17. 



THE FARMERS' CABINET. 



259 



that its vessels secrete horn and not bone — the 

 latter, being nourished by its own proper ves- 

 sels. On a closer examination of tiie struc- 



FiL'. 711. 



Fig. 71. 



Fig. 72. 



section be made of the sflme part, it will bd 

 soen that the layers run into rach other, like 

 so many hollow cones, — see Fig. 74. 



I'iff. TX 



ure and growth of the shell, we shall find 

 that the periosteum not only secretes horn to 

 repair injuries, but that it constantly secretes 

 new layers of horny fibres over the whole in- 

 terior surface of the shell. In a transverse 

 section of the horn, above the pith, we ob- 

 i«rve a number of concentric circles or lay- 

 MTS — often varying in color, and sometimes, \ 

 eparating^, — see Fig. 73. If a longitudinal | 



Fig 74. 



These layers are successively formed on 



the interior of the shell, extending from the 



root to the tip of the core. But while the 



layers are thus forming, they also grow out 



from the root, so as to push the external ones 



