380 THE MECHANICAL FUNCTIONS. 



ever, this peculiarity, that the cartilage is pro- 

 gressively absorbed to make room for the depo- 

 sits of bony substance. When the bone is long, 

 separate points of ossification appear in the ex- 

 tremities, before the central portions are ossified ; 

 and the ends, thus formed into bone, are after- 

 wards 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-ope- 

 ration. 



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, re- 

 quires their protection 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 quick- 

 ness 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 process goes on from a great number of 

 separate points at the same time. The ossifica- 

 tion 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 



