CHAPTEE XI 

 BONE AND BONE MARROW 



BONE is a firm calcareous tissue which is found only in the 

 skeletal system. In the flat bones it forms a double layer of dense 

 bony tissue between which is a narrow space, bridged across at 

 frequent intervals and thus subdivided into a number of com- 

 partments, the marrow cavities. This central stratum presents a 

 spongy appearance as compared with the denser periphery ; it is 

 therefore said to contain spongy or cancellous tone, while the more 

 superficial lamellae contain compact bone. 



In the long bones a similar condition exists i-n the epiphyses, 

 which consist of a wall of compact bone within which the marrow 

 cavity is subdivided by bony partitions into numerous compart- 

 ments ; the epiphysis consists, therefore, of spongy bone. The 

 shaft or diaphysis of the bone, however, contains a single large 

 marrow cavity whose walls, except for a thin layer at either end, 

 consist entirely of compact bone. A little spongy structure is 

 present at either end of the shaft, in that portion which adjoins 

 the marrow cavity. 



The ends and facets of the bones are covered by a disk of hya- 

 line cartilage, which forms the articulating surfaces of those bones 

 which enter into the formation of the movable joints. These 

 articular cartilages are peculiar in that they are not covered by a 

 perichondrium, and their deeper cells, which adjoin the bone, are 

 so arranged that their long axes are perpendicular to the free sur- 

 face, as is the case in the central portion of free cartilaginous 

 plates. Toward the free surface of the cartilage the long axis of 

 the cell lies more nearly parallel to the surface, as is likewise the 

 case at the surface of cartilaginous plates elsewhere. In the long 

 bones of younger individuals a plate of hyaline cartilage is also 

 found at the epiphyseal lines between the epiphyses and the dia- 

 physis. This plate, which extends through the entire axis of the 

 bone, becomes ossified later in life. It represents the line <rf 



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