CH. V.] 



OSSIFICATION 



G:3 



Some, it is true, get walled in in the process, and become bone- 

 corpuscles, but the system of intercommunicating lacunae and 

 canaliculi maintains their nutrition. 



These two races are working side by side, and at first do not 1 

 interfere with each other. But soon comes a declaration of war, and 

 we enter upon the second stage of ossification, which is very appro- 

 priately called the stage of irruption (fig. 90). Breaches occur in the 

 bony wall which the osteoblaets have 



built like a girdle round the calcifying ~- ~j f ^ ^ M ^ 

 cartilage, and through these the peri- 

 chondrial tissue pours an invading army 

 into the calcified cartilage. This con- 

 sists of osteoblasts, the bone formers; 

 osteoclasts, or the bone destroyers; the 

 latter are large cells, similar to the mye- 

 loplaxes found in marrow (fig. 82). There 

 are also a few fibres, and a store of 

 nutrient supply in the shape of blood- 

 vessels. 



Having got inside, the osteoclasts set 

 to work to demolish the homes of the 

 cartilage-cells, the walls of the primary 

 areolae, and thus large spaces are formed, 

 which are called the secondary areolce, or 

 the medullary spaces. On the ruins of 

 the calcified cartilage, the osteoblasts pro- 

 ceed to deposit true bone in layers, just 

 as they were wont to do in their own 

 country, under the periosteum. 



The third stage of ossification is a 

 repetition of these two stages towards the 

 extremities of the cartilage. The carti- 

 lage-cells get flattened and arranged in 

 rows ; calcareous deposit occurs around 

 these, and primary areolao result; then 

 follows the advance of the subperiosteal 

 tissue, the demolition of the primary areolae, the formation of 

 secondary areolae, and the deposit of true bone. At the same time, 

 layer upon layer is still being deposited beneath the periosteum, 

 and these, from being at first a mere girdle round the waist of the 

 bone, now extend towards its extremities. 



The next figure (fig. 93) is a magnified view of the line of advance. 



The bone which is first formed is less regularly lamellar than that 

 of the adult. .The lamellae are not deposited till after birth, and 

 their formation is preceded by a considerable amount of absorption. 



FIG. 91. Longitudinal section of ossi- 

 fying cartilage. Calcified trabeculse 

 are seen extending between the 

 columns of cartilage-cells, c, Car- 

 tilage-cells ; a, b, secondary areolsc. 

 x 140. (Sharpey.) 



