NORMAL HISTOLOGY. 



new matrix at the periosteum, and in thickness by the deposition of new 

 layers of bone-matrix on the surface of the trabeculae by the osteoblasts, 

 some of these cells being surrounded by the matrix and thus converted into 

 bone-cells. The spaces between the trabeculae are occupied by the primary 

 marrow, the direct prolongation from the periosteal tissue. During their 

 further growth, the trabeculae unite to form a subperiosteal bony network, 

 the peripheral spongy bone, which surrounds the central spongy bone, or, 

 where that has already disappeared, the central marrow cavity. Towards 



Periosteum 



Osteogenetic 

 layer 



FIG. 61. Portion of developing humerus of foetal sheep, showing subperiosteal and central spongy 



bone. X 135 



the ends of the shaft, where the cartilage still intervenes between the central 

 spongy bone and the surface, the subperiosteal bone forms a thin perichon- 

 dral shell. The two processes, central and peripheral bone-formation, 

 progress simultaneously, so that their products are often seen in the same 

 microscopical field lying side by side, separated, however, by a thin and 

 incomplete layer of calcified cartilage-matrix, known as the boundary line. 

 From their relations to cartilage, it is evident that the subperiosteal bone 

 never contains the remains of the calcified cartilage, while such enclosures 

 are very common within the trabeculas of the central spongy bone (Fig. 61). 

 The conversion of the peripheral spongy bone into the typical compact 

 bone of the shaft involves the partial absorption of the subperiosteal network 

 and the secondary deposit of new osseous tissue. The initial phase of this 



