BONE 289 



coalesce, form a network of bony plates, and constitute the bone matrix upon the 

 surfaces of which osteoblasts are arranged in a single layer like the cells of an 

 epithehum (Fig. 294 B) . These cells may be cuboidal, columnar, or may flatten 

 out as bone formation ceases. As the matrix of the bone is laid down, osteoblasts 

 become engulfed and form bone cells. The bone cells are lodged in spaces termed 

 lacuncB. These are connected by microscopic canals, the canaliculi, in which 

 delicate cell processes course and anastomose with those of neighboring cells. 



The plates of the spongy membrane bone are formed about blood vessels 

 as centers. As the bone grows at the periphery, the bone matrix is resorbed 

 centrally. At this time large multinucleated cells (43 to 91 ;u long) appear upon 

 the surfaces of the bone matrix. These cells are known as osteoclasts (bone 

 destroyers). There is, however, no positive evidence that the osteoclasts are 

 active in dissolving the bone. They may be interpreted also as degenerating, fused 

 osteoblasts (Arey, Anat. Rec, vol. 11, 1917). The cavities in which they are 

 frequently lodged are known as Hows hip's lacunce. The bone lamellae of the cen- 

 tral portion of the membrane bone are gradually resorbed and this portion of the 

 bone is of a spongy texture. Some time after birth, compact bone lamellae are 

 laid down by the inner osteoblast cells of the periosteum. In the case of flat 

 bones, compact inner and outer plates or tables are thus developed with spongy 

 bone between them. The spaces in the spongy bone are filled by derivatives of 

 the mesenchyme: reticular tissue, blood vessels, fat cells, and developing blood 

 cells. These together constitute the red bone marrow. The ossification of mem- 

 brane bone begins at the middle of the bone and proceeds in all directions from 

 this primary center. 



Cartilage Bone. — The form of the cartilage bone is determined by the pre- 

 formed cartilage and its surrounding membrane, the perichondrium (Fig. 296). 

 Bone tissue is developed as in membrane bones, save that the cartilage is first 

 destroyed and the new bone tissue develops (1) in, and (2) about it. In the first 

 case, the process is known as endochondral bone formation. In the second case, 

 it is known as perichondral or periosteal bone formation. 



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

 ranged in characteristic rows, and lime is deposited in the matrix (Fig. 295). The 

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

 which is densely cellular, ingrowths invade and resorb the cartilage and fill the 

 primary marrow 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 

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