76 NORMAL HISTOLOGY. 



which the bone is formed under the influence of the 

 osteoblasts, we find that just beneath these cells, as 

 they lie along the walls of the new-formed medul- 

 lary spaces, the basement substance of true bone 

 begins to be deposited, at first in the form of a nar- 

 row shell beneath each osteoblast. These deposits, 

 which on cross-sections have a crescentic shape, be- 

 come thicker and thicker, rising up around the cell, 

 which they finally enclose the enclosed osteoblast 

 becoming, as it would seem, a bone-cell. This 

 process going on around each osteoblast, the walls 

 of the medullary cavities soon become covered with 

 a layer of bone containing bone-cells. New osteo- 

 blasts appear on the walls, and in turn become en- 

 closed in a layer of bone, and thus the lamellar 

 arrangement of bone-tissue is produced. The re- 

 mains of cartilage basement substance between the 

 medullary space thus covered by bone finally dis- 

 appear in a manner unknown to us. 



2. Hand in hand with the formation of bone 

 within the cartilage, new bone is formed on its sur- 

 face beneath the perichondrium, which thus becomes 

 periosteum. The process of sub-periosteal ossifica- 

 tion, by which the bone increases in thickness, is 

 dependent also upon the presence of osteoblasts. 

 We find these arranging themselves along the blood- 

 vessels which enter the bone, and along the inner 

 layer of connective-tissue fibres of the periosteum, 

 and bone is formed around them in the manner 



