OSSIFICATION. 267 



of a saw, presenting externally the zig-zag line of junction 

 which is called a sulicre. This is seen in Figures 175 and 

 176, the former of which represents the upper side of the 

 skull of an infant; and the latter, the same bones when com- 

 pletely ossified. 



The union of bony fibres proceeding from different cen- 

 tres of ossification is not indiscriminate, but is found to be 

 regulated by definite laws, and to have certain relations to 

 the general plan of conformation originally established. 

 Each distinct bone is formed from a certain number of ossi- 

 fic centres, which altogether constitute a system appertain- 

 ing to that bone only, and not extending to the adjacent 

 bones. These pieces unite together, as if by a natural affi- 

 nity; and they refuse to unite with the bony fibres proceed-- 

 ing from neighbouring centres, and belonging 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 inhe- 

 rent tendency to preserve the line of separation between 

 two bones; and we then fiind them coalescing to form a sin- 

 gle 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, perha])s, differ very mate- 

 rially 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 deposite 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, thougli per- 

 haps 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 in- 

 tegrant part of the system, partaking of its changes, modi- 

 fied by its powers, and undergoing continual alterations of 

 shape, and even renewals of its substance, by the actions of 

 the living vessels. 



