HISTOLOGY 



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cartilage. A variety in which elastic fibrils have been developed in 

 the matrix is known as elastic cartilage ; and still another, in which white 

 ........ ,^ y .. tendon fibrils have pene- 



trated the hyaline matrix, 

 is known as fibrous carti- 

 lage. 



The bone of the verte- 

 brates is perhaps the most 

 highly developed and effi- 

 cient of the rigid connective 

 tissues. It is formed from 

 the mesodermic tissues and 

 is always an internal struc- 

 ture. It has many varieties 

 in the groups of chordate 

 animals, and we shall select 

 the best-known form as an 

 example, the bone of the 

 mammals and of man. 



This structure is formed 

 in two ways, producing two 

 slightly different structures. 

 The most primitive, per- 

 haps, is the endochondral 

 bone, formed by the trans- 

 formation of hyaline carti- 

 lage (Fig. 72). The second 

 is formed directly from the 

 connective tissue, especially 

 that which surrounds a car- 

 tilage or a bone. This is 

 known as perichondral bone 

 formation. It can be 

 formed in the connective 

 tissue quite apart from any 

 cartilage. Endochondral 

 bone is formed as follows 

 in an embryonic finger car- 

 tilage. 



The bone transformation begins in the middle of the joint and ad- 

 vances as a curved line in both directions, distad and proximad. This 

 line is the point at which the cartilage may be said to change into bone 

 and, as it moves, the cartilage cells in front of it go through a series 



-gi. c. 



FIG. 72. Reconstruction of cartilage into bone, car.c., 

 cartilage cells in successive stages of degeneration; 

 ost., osteoblasts; gi.c., giant cells or osteoclasts; b., 

 young bone ; bl.c., blood cells. 



