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 trabeculae of 

 calcified cartilage. The marrow-cells rapidly multiply and arrange 

 themselves as a layer upon 



the surface of the cartilage- FlG - 6l - 



trabeculae ; now called 

 osteoblasts, they busy 

 themselves in enveloping 

 these with a covering of 

 trtre~0ssep,us 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- 



P formed net- work consti- 

 tutes the central 



I primary spongy J^flne, 

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



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 loss being made up by incre- 

 ments of new cartilage deposited on the surface beneath the peri- 

 chondrium. 



n. 



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 ; m, primary marrow-spaces contain- 

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



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