340 THE MECHANICAL FUNCTIONS. 



proceeding in a contrary direction. Yet the arte- 

 ries still continuing to deposit ossific matter, each 

 set of fibres insinuate themselves between those of 

 the opposite set, for some little distance, and until 

 their further progress is stopped by the increasing 

 resistance they encounter. The consequence is 

 that the edges of the bones, which have thus met, 

 are irregularly jagged, like the teeth of a saw, pre- 

 senting externally the zig-zag line of junction which 

 is called a suture. 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 completely ossified. 



The union of bony fibres proceeding from dif- 

 ferent centres of ossification is not indiscriminate, 

 but is found to be regulated by definite laws, and 

 to have certain relations to the general plan of con- 

 formation originally established. Each distinct 

 bone is formed from a certain number of ossific 

 centres, which altogether constitute a system apper- 

 taining to that bone only, 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 belonging to other groups. .The groups 

 themselves are not arbitrary, but are pre-established 

 parts of the original design. Circumstances occa- 

 sionally, indeed, arise, which may overrule this 

 inherent tendency to preserve the line of separation 

 between two bones ; and we then find them coa- 

 lescing to form a single piece. Such unions are 

 technically called anchi/loses. 



Were this the whole of what takes place in the 

 formation of a bone, the process would not, perhaps, 



