OSTEOLOGY 



so build up a bony reticulum. As the osteogenetic fibres grow and shoot 

 out, they carry with them coatings of osteoblasts, and the process of cal- 

 careous incrustation goes on, so that the bony spicula increase in length, 

 and gradually approach the periphery. During this process of spicular bony 

 formation many of the osteoblasts are left behind, and become imprisoned 

 in the lacunar spaces of the forming bone, where they represent the future 

 bone cells. The ossification of a membrane bone is thus effected by means 

 of osteogenetic fibres plus osteoblasts, the fibres acting as outrunners and 

 becoming calcified. 



Ossifieation in Cartilage. — The cartilage is covered by a membrane, called 

 the perichondrium, which corresponds to the periosteum, and the process of 

 ossification takes place in three stages. 



First Stage. — In this stage the ossification is 'partly endochondral, and partly 

 ectocliotuiral or on the surface beneath the perichondrium. In the endo- 

 chondral lormthe cartilage cells at the centre become 

 enlarged, and the intervening matrix becomes cal- 

 cified. Above and below the centre the cartilage 

 cells are arranged in long columns, directed towards 

 each extremity. The matrix between these columns 

 becomes calcified by an extension of the calcareous 

 matter at the centre, which now surrounds the cell- 

 columns. The spaces in the calcified matrix, which 

 contain these columns, are known as the primary 

 areolcB. At the same time, ectochondral cr sabperi- 

 ehondral ossification is proceeding in a manner 

 similar to what takes place in membranous ossifica- 

 tion, that is to say, by osteogenetic fibres, osteo- 

 blasts, and calcareous impregnation. In this way 

 several layers cf bone are laid down at the surface 

 beneath the perichondrium, and these constitute the 

 circumferential lamellae. During this process some 

 of the osteoblasts are detained in lacunar spaces, and 

 form the bone cells. 



Second Stage. — This is known as the stage of irrup- 

 tion. The inner or osteogenetic layer of the peri- 

 chondrium bursts in through openings in the cir- 

 cumferential lamellae, in the form of osteogenetic 

 fibres, osteoblasts, and osteoclasts or bone destroyers. 

 These incursions reach the calcified matrix, and the 

 osteoclasts now commence their destructive work. 

 The cartilage cells of the primary areolae, as well as 

 the walls of these areolae, are absorbed, and larger 

 spaces, called secondary areolx or medullary spaces, 

 are formed in the original calcified matri,x. These 

 spaces contain osteogenetic fibres and osteoblasts, 

 and the latter now build up lamellae of bone. 



Third Stage. — ^This is a repetition of the preceding 

 two stages. The cartilage cells arrange them- 

 selves in rows ; the intervening matrix becomes 



calcified, and invests them; the enclosed cartilage cells atrophy and give 

 rise to primary areolae; the osteoclasts produce partial absorption of the 

 calcified cartilage, giving rise to medullary spaces; and the osteogenetic 

 fibres and osteoblasts build up lamellae of bone. Simultaneously with these 

 processes, subperiosteal ossification is going on. The medullary canal is 

 due to absorption by the osteoclasts of the osseous tissue in the centre of the 

 shaft. 



The ossification of the epiphyses of a bone is endochondral. 



Periosteum. — ^The periosteum is a fibrous, vascular membrane, 

 v.hich closely invests bones, except where there is articular cartilage. 



Fig. 5. — Ossification 

 IN Cartilage. 



