lOO 



HUMAN ANATOMY. 



These cells cover the exterior of the trabeculae as they lie surrounded by the young 

 marrow-tissue which extends from the osteogenetic layer of the periosteum into the 

 intertrabecular spaces. The union of the young trabeculae results in the production 

 of a subperiosteal net-work of osseous tissue, the peripheral spoyigy bone. The latter 

 forms a shell surrounding the central endochondral bone, or, where the latter has 

 already disappeared, the central marrow-cavity of the shaft. The two processes, 

 central and peripheral bone-formation, progress simultaneously, so that the produc- 

 tions of both lie side by side, often in the same microscopical field (Fig. 131). 



;^- 



•W "^-v 



-^^O^: 





c 

 o 





Fibrous layer of 

 periosteum 



Osteogenetic 

 layer of peri- 

 osteum 



formed 

 rabecula 



(S- 



^i^^^>'^ 

















rv5 





Bone trabecula 



Osteoblasts 



Primary mar- 

 row 



Bone trabecula 



-Remains of car- 

 tilage 





Portion of developing humerus of foetal sheep, showing periosteal and central spongy bone. X i6o. 



The development of compact bone involves the partial absorption of the 

 subperiosteal net-work of osseous trabeculae and the secondary deposition of new 

 bone-tissue. The initial phase in the conversion of the peripheral spongy bone into 

 compact substance is the partial absorption of the trabeculae by the osteoclasts of the 

 primary marrow-tissue ; in consequence of this process the close reticulum of perios- 

 teal bone is reduced to a delicate framework, in which the comparatively thin remains 

 of the trabeculae separate the greatly enlarged primary marrow-cavities, which, now 

 known as the Haversimi spaces, are of round or oval form. 



After the destructive work of the osteoclasts has progressed to the required 

 extent, the osteoblastic elements of the young marrow contained within the Haver- 



