8 PRACTICAL ANATOMY. 



It must be remembered that the formation of 

 bone is not a simple process of calcification, but the 

 result of the functional activity of the osteoblasts. The 

 formation of all the spaces and canals in cartilage and 

 bone is due to absorption. The canals formed by the 

 process described are known as medullary sinuses, and 

 are lined by osteoblasts and medullary cells. Through 

 the agency of the osteoblast, layers of bone begin to 

 encroach concentrically upon the medullary sinuses, 

 forming the primary lamellae surrounding the Haver- 

 sian canals, which these medullary sinuses ultimately 

 become. 



The formation of the lacunae and canaliculi is not 

 definitely settled. Briefly, the process is probably as 

 follows : The medullary sinuses or canals, as previously 

 stated, are lined by layers of osteoblasts, from three to 

 seven deep. The peripheral layer undergoes bony trans- 

 formation, and then the next, and thus layer after layer 

 ossifies, forming the concentric lamellae of the Haversian 

 system. The nucleus of the osteoblast resists the change 

 described, although the cell-wall and contents in part 

 undergo ossiric transformation. The nucleus remains as 

 a space or cavity, which is gradually fashioned into a 

 lacuna. The formation of the canaliculi is not under- 

 stood, although it is probable that the contents of the 

 osteoblasts are not entirely absorbed, but that delicate 

 fibres of protoplasm remain, radiating from the nucleus 

 and communicating with the adjoining cell, and that 

 these fibres ultimately become the canaliculi. In the 

 inter-membranous form of ossification, the first evidence 

 is the formation of a vascular centre, from which rays 

 of fibre-cartilaginous material issue. This material is 

 calcified, and then passes through the process of ossifica- 

 tion already described. 



