68 THE CONNECTIVE TISSUES. [CH. v. 



and disappear, leaving only the walls of their home enclosing the 

 spaces called primary areolse. The osteoblasts, the other race of 

 cells under the perichondrium, are forming layers of true bone in 

 that situation. 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 

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

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

 appropriately called the stage of irruption (fig. 90). Breaches 

 occur in the bony wall which the osteoblasts have built like a 

 girdle round the calcifying cartilage, and through these the peri- 

 chondrial tissue pours an invading army into the calcified cartilage. 

 This consists of osteoblasts, the bone formers ; osteoclasts, or the 

 bone destroyers ; the latter are large cells, similar to the myelo- 

 plaxes 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 areolse, 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 proceed 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 cartilage-cells get 

 flattened and arranged in rows ; calcareous deposit occurs around 

 these, and primary areolse result ; then follows the advance of the 

 subperiosteal tissue, the demolition of the primary areolse, the 

 formation of secondary areolse, 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. 9 1 ) 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. To carry our simile further, the osteoblasts are not 

 satisfied with the rough constructions that they were first able to 

 make, but having exterminated the cartilage, they destroy (again 

 through the agency of the regiment of giant osteoclasts) their 

 first work, and build regular lamellse, leaving lacunse for the 

 accommodation of those who desire to retire from active warfare. 



About this time, too, the marrow cavity is formed by the 

 absorption of the bony tissue that originally occupied the centre 



