382 THE MECHANICAL FUNCTIONS. 



and not extending to the adjacent bones. These 

 pieces unite together, as if by a natural affinity ; 

 and they refuse to unite with the bony fibres 

 proceeding from neighbouring centres, and be- 

 longing to other groups. The groups themselves 

 are not arbitrary, but are pre-established parts of 

 the original design. Circumstances occasionally, 

 indeed, arise, which may overrule this inherent 

 tendency to preserve the line of separation be- 

 tween two bones ; and we then find them coales- 

 cing to form a single piece. Such unions are 

 technically called anchyloses. 



Were this the whole of what takes place in 

 the formation of a bone, the process would not, 

 perhaps, differ very materially from that by 

 which a shell is produced ; for a shell, as we 

 have seen, is the result of successive depositions 

 of calcareous matter, forming one layer after 

 another, in union with a corresponding deposit 

 of animal membrane. But the subsequent 

 changes which occur, show that the constitution 

 of bone is totally dissimilar to that of shell : for 

 no portion of the shell that is once formed, and 

 has not been removed, is subject to any farther 

 alteration. It is a dead, though perhaps not 

 wholly inorganic mass; appended, indeed, to 

 the living system, but placed beyond the sphere 

 of its influence. But a bone continues, during 

 the whole of life, to be an integrant part of the 

 system, partaking of its changes, modified by 

 its powers, and undergoing continual altera- 



