BONE 85 



cells deposits the internal circumferential lamella. The Haversian canals 

 are actually continuations of the marrow cavity, and the larger are even 

 lined by endosteum. 



With the formation of the perichondrial bone the lateral expansion 

 of the organ by endochondral bone formation necessarily ceases. Hence- 

 forth increase in diameter of the bone is only produced by continued 

 absorption internally of the compact bony wall and the formation of new 

 bone beneath the periosteum by frequent repetitions of the processes of 

 periosteal (perichondrial) ossification as already described. The rem- 

 nants of those Haversian and circumferential lamellae which are only 

 partially absorbed in this process form the interstitial lamella of the ma- 

 ture bone. In the long bones and in flat cartilage bones ossification at 

 first proceeds in the perichondrium, endochondral ossification appearing 

 only later ; in the short bones ossification is endochondral until the carti- 

 lage is entirely replaced by bone. 



Epiphysial Ossification. During the processes of endochondral and 

 perichondrial ossification within the shaft of the bone, the epiphysial 

 cartilages continue to grow. Finally, however, ossification begins in the 

 epiphysis, osteogenic tissue having pushed in from the primary center of 

 the diaphysis, and proceeding in the same manner as in the shaft, 

 results in the formation of primary spongy bone, some of which is ab- 

 sorbed and replaced by more compact bony tissue, as occurs in the wall 

 of the diaphysis. In its central portions the tissue retains its spongy 

 arrangement and but few Haversian systems are formed. It is thus 

 that the cancellous bone of this part, as also of the ends of the diaphysis, 

 is formed. 



At the point where the expanding centers of ossification of the shaft 

 and epiphysis are about to meet, a line of unossified cartilage, the 

 epiphysial line, persists until growth of the bone is complete. It is by 

 growth of this cartilaginous disk, with continued formation of cartilage 

 mainly on its inner surface, and its concomitant replacement by bone, 

 that the bone increases its length. After ossification of this epiphysial 

 synchondrosis at about the twenty-first year, growth in length must 

 cease. Meanwhile the perichondrium has become periosteum. 



The following is a resume of the various stages of endochondral ossifi- 

 cation : 



1. Formation of the fetal hyaline cartilages from precartilage mesen- 



chyme blastema. 



2. Enlargement of the cartilage cells with a rearrangement into 



radiating cell rows at the center of ossification. 

 7 



