BONE 



297 



Cartilage 



Endochondral Bone Formation. The cartilage cells enlarge, become ar- 

 ranged in characteristic rows and resorb the cartilage matrix (Fig. 287). The 

 perichondrium becomes the periosteum. From its inner or osteogenic layer, 

 which is densely cellular, ingrowths invade the cartilage as it is resorbed and fill 

 the primary cavities. The invading osteogenic tissue gives rise to osteoblasts 

 and bone marrow. By the osteoblasts bone is differentiated directly upon 

 persisting portions of the cartilage. As new bone is developed peripherally, it is 

 resorbed centrally to form large marrow spaces. Eventually, all of the cartilage 

 matrix is destroyed. The fate 

 of the cartilage cells is un- C 



known. 



Perichondral Ossification. 

 Compact bone is developed 

 after birth by the osteogenic 

 layer of the periosteum and thus 

 are produced the periosteal la- 

 mella. In the ribs this is said 

 to be the only method of ossi- 

 fication. The bone lamellae de- 

 posited about a blood-vessel are 

 concentrically arranged and 

 form the concentric lamella of a 

 Haversian system. The Haver- 

 sian canal of adult bone is 

 merely the space occupied by 

 a blood-vessel. 



Growth of Cartilage Bones. 

 In cartilage bones there is no 



interstitial growth as in cartilage. Most of the cartilage bones have more than one 

 center of ossification and growth is due to the expansion of the intervening cartil- 

 age. Flat bones grow at the periphery, ring-like bones, such as the vertebrae, have 

 three primary centers of ossification, between which the cartilage continues to grow 

 (Fig. 288 A). In the case of the numerous long bones of the skeleton, the primi- 

 tive ossification center forms the shaft or diaphysis (Fig. 288 C-F) . The cartilage 

 at either end of the diaphysis grows rapidly and thus the bone increases in length. 

 Eventually, osteogenic tissue invades these cartilages and new ossification centers, 

 the epiphyses, are formed, one at either end. When the growth of the bone in 



Paraphysis 



FIG. 288. Diagrams to show the method of growth 

 of A, a vertebra; B, of sacrum; C-F, of a long bone 

 (the tibia). 



