98 MANUAL OF HISTOLOGY. 



way inward, develop communications between the capsules, so 

 that in this way a cartilaginous network is formed that is filled 

 with the arborescent tissue. This change in the cartilage, 

 which is characterized by absorption and rarefaction, is called 

 chondro-porosis. 



At a more advanced stage the cartilage around the oldest 

 channels has become transparent in places, while the walls are 

 irregular, because portions- of calcified trabeculse project into 

 them. These irregular spaces are called primary marrow 

 cavities. Now upon the walls may be seen, not the cartilage- 

 corpuscles, but the osteoblasts, which are proceeding to develop 

 concentric layers of osseous tissue. 



When this process has been completed, the osseous tissue 

 will be found to have replaced the calcified cartilage, and true 

 bone has been formed. But this action may be no sooner 

 completed than absorption will again commence, and at first in 

 the last or most internal layer of the Haversian system. This 

 process is essential for the development of the central marrow 

 cavity. After an Haversian system has been removed, the 

 matrix will also disappear. 



Now, while this cavity is filling up with marrow a gradual 

 development of bone is taking place from the periosteum, 

 which slowly encroaches upon the bone whose formation we 

 have just described. 



This last stage results in the formation of 'adult bone. 

 When it has been completed all the first formed bone has 

 been absorbed before it. This periostea! or metaplastic bone 

 is at first spongy, as is all new bone ; in the fulfilment of its 

 task it next appears to form compact bone, and then part of 

 this latter is rarefied, as, for example, along the wall of the 

 central cavity. Thus, as we have already seen, compact bone 

 is formed from spongy, and spongy from compact. The peri- 

 pheric or interstitial lamellae are either the remains of calcified 

 and unabsorbed trabeculse, or perhaps the walls of other Haver- 

 sian systems forming sides of the bony network. 



Formation of bone from membrane. This second method 

 of bone-formation is seen in the bones of the skull and face. 

 The steps are precisely similar to those already described. The 

 inner layer of the periosteum, which is lined with osteoblasts, 

 produces both matrix and bone corpuscles by a process of bud- 

 ding. The change first begins at the points of ossification. 



