THE SUPPORTING TISSUES. 95 



This is generally understood to mean the turning of the 

 cartilage into bone, but such a change in no sense takes 

 place. Ossification really consists in the gradual removal 

 of the cartilage and in the formation of entirely new bone. 

 The changes which convert this cartilage into the bony 

 humerus, as we know it, are as follows: * 



A little before the process of bone formation begins, it 

 may be noticed that the cartilage at the point of ossification 

 becomes somewhat gritty, probably due to the deposit of 

 extra mineral matter in the cartilage. In the case of the 

 humerus this point is near the middle of the shaft. Soon 

 after this, peculiar large cells burrow and absorb a pas- 

 sageway for themselves from under the periosteum through 

 the cartilage until they reach the middle of the shaft. Here 

 these large cells continue their process of dissolving and 

 absorbing the cartilage, and so tunneling it in every direc- 

 tion. These large cells are called osteoclasts, or possibly 

 more generally myelo-plaques. By the eating away of the 

 cartilage in this way the central portion of the cartilagin- 

 ous humerus is soon converted into an intricate system of 

 tunnels, which gradually extend further and further towards 

 the ends of the bone. The absorption of the cartilage is 

 probably explained as due to the digestive action of the 

 liquid which these myelo-plaques secrete. What happens to 

 the cartilage cells themselves when their matrix is removed 

 is not yet definitely known. According to some observers the 

 cartilage cells, too, are absorbed; while according to other 

 observers these cells when liberated change into bone cells, 

 and figure in a way to be described later. In the tunnels 

 so made, blood-vessels from the periosteum grow, and from 

 the same place large numbers of osteoblasts follow up these 

 channels. These osteoblasts secrete bits of bone against 

 the walls of these tunnels, possibly not unlike the stone 

 masonry that lines many railroad tunnels. When finally all 



* As the changes in the case of this bone are, except for local differences the same 

 for every other, this one instance may suffice to explain the phenomena of ossification 

 wherever occurring. 



