OSSIFICATION IN CARTILAGE. 



273 



the shaft when ossification is about to commence. As it grows, the cartilage 

 acquires firmer consistence ; it represents in figure the future bone, though of 

 course much smaller in size, and it is surrounded with a fibrous membrane, the 

 future periosteum. Vessels ramify in this membrane, but none are seen in the 

 cartilage until ossification is about to begin. In a long bone the ossification 



Fig. 312. SECTION OP PART OF ONE OF THE 



LIMB BONES OF A FffiTAL CAT, AT A 

 MORE ADVANCED STAGE OF OSSIFICATION 

 THAN IS REPRESENTED IN FIG. 311, AND 

 SOMEWHAT MORE HIGHLY MAGNIFIED. 



(Drawn by Mr. J. Lawrence.) (E.A.S.) 



The calcification of the cartilage matrix 

 has advanced from the centre, and is extend- 

 ing between the groups of cartilage-cells 

 which are now arranged in characteristic 

 rows. The subperiosteal bony deposit (im) 

 has extended pari passu with the calcifica- 

 tion of the cartilage matrix. The cartilage 

 cells in the primary areolas are mostly 

 shrunken and stellate, in some cases they 

 have dropped out of the space. At ir and in 

 two other places an irruption of the sub- 

 periosteal tissue, composed of ramified cells 

 with osteoblasts and growing blood-vessels, 

 has penetrated the subperiosteal bony crust, 

 and has begun to excavate the secondary 

 areolse or medullary spaces ; p, fibrous layer 

 of the periosteum ; o, layer of osteoblasts, 

 some of them are embedded in the osseous 

 layer as bone-corpuscles in lacunae ; bl,' blood- 

 vessels occupied by blood-corpuscles. Be- 

 yond the line of ossific advance the perios- 

 teum may be noticed to be distinctly in- 

 curved. This incurvation is gradually 

 moved on, the cartilage expanding behind 

 it until the head of the bone is reached, 

 when it forms the periosteal notch or groove 

 represented in fig. 313, p. 



commences in the middle and pro- 

 ceeds towards the ends, which 

 remain long cartilaginous, as re- 

 presented in fig. 310. Much 

 later, separate points of ossification 

 appear in them, and form epiphyses, 

 which at last are joined to the body 

 of the bone. 



The manner in which the process of ossification of a cartilage bone takes place 

 is as follows : 



In the middle of the cartilage the cells are enlarged, and are separated from one 

 another by a relatively larger amount of matrix than elsewhere (fig. 311). This 

 matrix becomes hardened by calcareous deposit, assumes a granular opaque appear- 

 ance, and has a gritty feel to the knife. Meanwhile the cartilage-cells above and 

 below the centre of ossification become enlarged and flattened, and piled up in 

 elongated groups or columns which radiate from the centre for a certain distance 

 towards either end. The columns taper towards their ends, where the cartilage-cells 

 which compose them are smaller. Into the matrix between these oblong groups the 

 calcareous deposit extends between and around the groups of cells, so that the 

 calcified substance encloses the columns ; the cell-spaces in the calcified matrix 



bl 



