OSSIFICATION. 381 



exposed to dangers from the contact of external 

 bodies. The divergent fibres shoot out rapidly, 

 coalescing with those in their immediate neigh- 

 bourhood, which co-operate to form an extensive 

 bony plate. When they have reached the pre- 

 scribed line, they have become so much ex- 

 panded as to have lost the power of coalescing 

 with the fibres which have originated from other 

 centres, and are proceeding in a contrary direc- 

 tion. Yet the arteries still continuing to deposit 

 ossific matter, each set of fibres insinuate them- 

 selves between those of the opposite set, for some 

 little distance, and until their further progress is 

 stopped by the increasing resistance they en- 

 counter. The consequence is that the edges of 

 the bones, which have thus met, are irregularly 

 jagged, like the teeth of a saw, presenting exter- 

 nally 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 imion of bony fibres proceeding from 

 different centres of ossification is not indiscri- 

 minate, but is found to be regulated by definite 

 laws, and to have certain relations to the gene- 

 ral plan of conformation originally established. 

 Each distinct bone is formed from a certain 

 number of ossific centres, which altogether con- 

 stitute a system appertaining to that bone only, 



