BONE TISSUE. 



n 



of its owner, or whether it be transplanted into that of another 

 animal; again produces new bone tissue, only the deepest 

 layer must be uninjured. 



If we examine this deepest layer with the help of the 

 microscope, we discover our old friends, the osteoblasts. This 

 cell layer then grows downwards in a conical form into a re- 

 trogressing, indifferent cell substance. 



With the bone-producing force of the osteoblasts we are 

 already familiar. Therefore the osteoblast-cones {sit venia 

 verba) produce the Haversian lamellae, while the general 

 lamellae are produced by the flat osteoblast layer, which is 

 immediately beneath the periosteum. In this manner is also 

 explained the regular structure of the diaphysis and its increase 

 in thickness. Concerning the latter, further remarks are 

 scarcely necessary. 



We may, therefore, say : The endochondral bone dis- 

 appears as an embryonic structure, the periosteal remains 

 during the subsequent life. 



As we have already learned above, a number of cranial and 

 face bones never were cartilage. 



They arise from a soft, foetal connective substance, and have 

 been badly named the "secondary" bones. Here, also, 

 when there is to be a production of osteoid tissue, we 

 meet with osteoblasts and the same process of origin of the 

 bone tissue as when formed from the periosteum. The 

 development of the bone substance commences centrically 

 in certain places, and advances from these peripherically. 

 These are, therefore, true points of ossification, in contra- 

 distinction to the false, or the calcifying centres of endo- 

 chondral bone. 



That connective-tissue fragments are frequently hardened 

 with the periosteal and secondary bones, we readily under- 

 stand. These things — they sometimes appear like a board 

 with nails driven in — have received the name of Sharpey's 

 fibres. 



Many modern investigations also favor an immediate trans- 

 formation of one or another cartilage into osteoid substance, 



