THE CONNECTIVE TISSUES. 



53 



columns of the cells suffer much less reduction, and, as a result, 

 remain and project into the marrow-cavity as irregular trabeculse of 

 calcified cartilage. The marrow-cells rapidly multiply and arrange 



<^vw .' 



themselves as a layer upon 

 the surface of the cartilage- 

 trabeculae ; now called 

 osteoblasts, they busy 

 themselves in enveloping 

 these with a covering of 

 true osseous tissue. Si- 

 multaneously with the 

 deposition of the bone the 

 calcified cartilage within 

 the trabeculae undergoes 

 absorption, so that the 

 amount of cartilage en- 

 cased by the new bone 

 gradually diminishes and 

 finally disappears, the 

 entire net-work of anas- 

 tomosing trabeculae being 

 now composed of true os- 

 seous tissue. This newly- 

 formed net-work consti- 

 tutes the central 

 primary spongy bone, 

 a structure which, in the 

 shafts of the long bones, 

 is but temporary, after- 

 wards entirely disappear- 

 ing, except at the ends of 

 the bones, where it per- 

 sists as the cancellous 

 tissue of the extremities. 



It will be noticed that 

 in the changes above described the cartilage is not directly converted 

 into bone, ossification being a process of substitution, the new bone 

 replacing the primary cartilage. 



Starting near the middle of the long bones, the process of calci- 

 fication and absorption of the cartilage and the formation of the 

 primary spongy bone proceed towards the extremities, the original 

 cartilage gradually disappearing, the growth of the young bone being 

 maintained by increments of new cartilage deposited at the ends be- 

 neath the perichondrium. 



Developing bone from the end of a long bone : a, area 

 of rearranging cartilage-cells ; e, area of enlarged lacunae ; c, 

 zone of calcified matrix ; nt, primary marrow-spaces contain- 

 ing the osteogenetic tissue ; b, trabeculae of new bone cover- 

 ing the remains (r) of the calcified cartilage-matrix. 



