74 NORMAL HISTOLOGY. 



about the middle of the diaphysis the cartilage-cells 

 begin to enlarge, the basement substance between 

 them becoming partially absorbed, and what re- 

 mains of the latter becoming infiltrated with fine 

 granules of lime salts. Around this calcified por- 

 tion we find the blood-vessels of the perichondrium 

 accompanied by marrow-tissue and the above- 

 mentioned osteoblasts, growing into the calcified 

 cartilage, absorbing the latter as they go, and form- 

 ing irregular channels or cavities called medullary 

 spaces. These channels are first separated from one 

 another by narrow, irregular septa of the cartilage 

 basement substance which remains unabsorbed, and 

 are lined by layers of osteoblasts, by whose agency 

 the septa become covered with new bone, in a 

 manner presently to be described. 



The region in which this new bone is first depos- 

 ited is called the ossification zone. As the blood- 

 vessels, accompanied by the osteoblasts and marrow- 

 tissue, proceed further and further into the cartilage, 

 channelling out the medullary spaces as they go, we 

 always find just in advance of the ends of the 

 blood-vessels and the extremity of the spaces in 

 which they lie a zone of calcified cartilage ; and 

 beyond this, cartilage-tissue which apparently pre- 

 pares the way for the advancing marrow-spaces and 

 newly forming bone, by very characteristic modifi- 

 cations, chiefly in the form and arrangement of its 

 cells. 



