INTRODUCTORY. DEVELOPMENT OF BONE. 11 



osteoblasts. These osteoblasts gradually get surrounded by 

 the matter which they secrete and become converted into bone 

 cells, arid in this way a mass of spongy bone is gradually pro- 

 duced. Meanwhile a definite periosteum has been formed round 

 the developing bone, and on its inner side fresh osteoblasts 

 are produced, and these with the others gradually render the 

 bone larger and more and more compact. Finally, the middle 

 layer of the bone becomes again hollowed out and rendered 

 spongy by the absorption of part of the bony matter. 



Endochondral ossification 1 . This is best studied in the 

 case of a long bone like the femur or humerus. Such a long 

 bone consists of a shaft, which forms the main part, and two 

 terminal portions, which form the epiphyses, or portions 

 ossifying from centres distinct from that forming the shaft or 

 main part of the bone. 



In the earliest stage the future bone consists of hyaline 

 cartilage surrounded by a vascular sheath, the perichondrium. 



Then, starting from the centre, the cartilage becomes per- 

 meated by a number of channels into which pass vessels from 

 the perichondrium and osteoblasts. In this way the centre 

 of the developing shaft becomes converted into a mass of 

 cavities separated by bands or trabeculae of cartilage. This 

 cartilage next becomes calcified, but as yet is not converted 

 into true bone. The osteoblasts in connection with the 

 cavities now begin to deposit true endochondral spongy bone, 

 and then after a time this becomes absorbed by certain 

 large cells, the osteoclasts, and resolved into marrow or vas- 

 cular tissue loaded with fat. So that the centre of the shaft 

 passes from the condition of hyaline cartilage to that of 

 calcined cartilage, thence to the condition of spongy bone, and 

 finally to that of marrow. At the same time beneath the 



1 In compiling these paragraphs on Histology, free use has been 

 made of Klein and Noble Smith's Atlas of Histology, the small Histo- 

 logies of Klein and Schafer, Huxley's Elementary Physiology, and Lloyd 

 Morgan's Animal Biology. 



