DEVELOPMENT OF BONE. 



273 



perhaps also by the calcification of the intervening cells. As the process ad- 

 vances, and the plate of bone thickens, a series of grooves or furrows, radiating 



Fig. 64. 



Fig. 65. 



Process of ossification in parietal bone of an embryo 

 sheep of 2^ inches in length. The small upper figure re- 

 presents the bone of the natural size, the larger figure is 

 magnified about 12 diameters. The curved line, a, 6, 

 marks the height to which the subjacent cartilaginous 

 lamella extended. A few insulated particles of bone are 

 seen near the circumference, an appearance which is 

 quite common at this stage. 



The growing ends of two bony spicula from 

 the frontal bone of an embryo dog, highly mag- 

 nified. The surrounding membrane has been 

 removed, and most of the corpuscles are washed 

 away, to show more evidently the transparent 

 soft fibres prolonged from the bone, with the 

 dark earthy deposit advancing into them. 



from the ossifying centre, are found upon its surface; and these, by a further 

 increase in thickness, occasioned by a deposit of ossific matter all around them, 

 are gradually converted into closed canals (the Haversian), which contain blood- 

 vessels, supported by processes of the investing membrane. Further deposits 

 subsequently take place in the interior of these canals ; which thus gradually 

 produce a diminution of their caliber, and a consolidation of the bone ; and in 

 this manner its two surfaces acquire their peculiar density, whilst the interven- 

 ing layer, or "diploe," retains a character more resembling that of the original 

 osseous reticulation. The mode in which the peculiar lacunae and canaliculi are 

 formed, in the concentric layers around the Haversian canals, probably corre- 

 sponds with that in which they are generated in the intra-cartilaginous form of 

 ossification, to which we shall next proceed. 



264. In a very large proportion of the skeleton, the appearance of the Bones 

 is preceded by that of Cartilages; which serve (so to speak) as the moulds for 

 their formation, and which also seem destined to afford a certain degree of sup- 

 port to the surrounding soft parts, until the production of bone has taken place. 

 The temporary Cartilages differ in no essential particular of structure or compo- 

 18 



