OSSIFICATION. 



265 



arranged longitudinally, and parallel to one another, and to 

 the axis of the bone. They first constitute a ring in the 

 middle of its length: this ring enlarges in all its dimensions, 

 but principally in its length; the spicula becoming larger, 

 not by the stretching of their parts, in consequence of the 

 insinuation of fresh materials between those already depo- 

 sited, but by the addition of new particles at both their ex- 

 tremities. In like manner, the ring increases in thickness, 

 not by the deposition of phosphate of lime between the ori- 

 ginal layers, but by the application of fresh layers on the 

 outside of those already existing. 



In the flat bones, the process of ossification is very simi- 

 lar to what I have just described; only the fibres have a ra- 

 diated arrangement, shooting out from the spot where the 

 first deposite took place, as from a common centre. This is 

 seen in Fig. 174, which represents the parietal bone of the 



175 iT/i 175 



human skull, in an early stage of its ossification, and shows 

 the radiating fibres very distinctly. In the cubical, and 

 more irregularly shaped bones, the process is, doubtless, 

 conducted with the same order and regularity, although it 

 cannot so readily be followed by the eye. 



The same process is repeated in different parts of the bone, 

 wherever nature has, in conformity with determinate laws 

 of development, appointed particular centres of ossification. 

 The bone continues to extend from each of these centres, 

 proceeding gradually towards the circumference, or the re- 

 moter parts of the cartilage, on which the ossific materials 

 are moulded, and by the form of which that of the future 



Vol. I. 34 



