OSSIFICATION. 339 



room for the deposits of bony substance- When 

 the bone is long, separate points of ossification 

 appear in the extremities, before the central por- 

 tions are ossified ; and the ends, thus formed into 

 bone, are afterwards united to the shaft, so that the 

 whole shall form a continuous bony mass. In the 

 fiat bones, also, if the surface be extensive, an addi- 

 tional number of arteries are engaged to perform 

 the work, which is begun from several auxiliary 

 centres of ossification, and the completion of which 

 is materially accelerated by their co-operation. 



This mode of increase often gives rise to a curious 

 result, of which a striking example is presented in 

 the bones of the skull. The brain, which these 

 bones are designed to protect, requires this protec- 

 tion at a very early period of life. The growth of 

 so large a surface of bone, as would be required for 

 covering the brain, could not have proceeded with 

 sufficient quickness for the exigencies of the occa- 

 sion, if it had originated from a single point. There- 

 fore it is that, besides being commenced at a very 

 early age, the process goes on from a great number 

 of separate points at the same time. The ossifica- 

 tion is evidently expedited in order to complete 

 the roofing in of the edifice by the time at which 

 the animal is to be ushered into the world, and ex- 

 posed 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 prescribed 

 line, they have become so much expanded as to 

 have lost the power of coalescing with the fibres 

 which have originated from other centres, and are 



