266 THE MECHANICAL FUNCTIONS. 



bone is regulated. The process of ossification has, however, 

 this peculiarity, that the cartilage is progressively absorbed 

 to make room for the deposites of bony substance. When, 

 the bone is long, separate points of ossification appear in the 

 extremities, before the central portions 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 flat 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 pro- 

 tect, requires their protection at a very early period of life. 

 The growth of so large a surface of bone, as would be re- 

 quired for covering the brain, could not have proceeded 

 with sufficient quickness for the exigencies of the occasion, 

 if it had originated from a single point. Therefore it is 

 that, besides being commenced at a very early age, the pro- 

 cess goes on from a great number of separate points at the 

 same time. The ossification is evidently hurried on 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 exposed to 

 dangers from the contact of external bodies. The divergent 

 fibres shoot out rapidly, coalescing with those in their im- 

 mediate neighbourhood, which co-operate to form an exten- 

 sive 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 origi- 

 nated from other centres, and are proceeding in a contrary 

 direction. Yet the arteries still continuing to deposite ossific 

 matter, each set of fibres insinuate themselves between those 

 of the opposite set, for some little distance, and until their 

 farther 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 



