6 4 



HISTOLOGY. 



cartilage may be found throughout life. Fig. 68 shows a part of the 

 humerus of a fetus in which the calcified cartilage remains, forming in one 

 place a boundary between endochondral and perichondral bone. The 

 vascular tissue within the shaft becomes marrow, a reticular tissue 

 associated with fat cells, and having developing blood corpuscles in its 

 meshes, to be described later. 



In brief review it may be said that cartilage bones are formed by the 

 deposition of perichondral bone on the outside of a hyaline cartilage, 

 and of endochondral bone upon the lining of excavations within the carti- 

 lage. The cartilage is not transformed into bone, although the matrix 

 in part becomes calcified and encased in bone. In the long bones this 

 process of ossification produces a shaft of bone tipped with a mass of 



cartilage at either end, Fig. 69, A, B, C. 

 The shaft is the diaphysis; the cartilage 

 ends are epiphyses. At various times after 

 birth, or in the tibia shortly before birth, 

 osteogenic tissue invades the epiphysis and 

 gradually replaces its cartilage by bone. 

 A layer of epiphyseal cartilage between 

 the epiphysis and diaphysis, and a layer 

 of articular cartilage covering the joint 

 surface persist longest. Until adult life 

 the epiphyseal cartilage grows, chiefly 

 toward the diaphysis, arid the addition as 

 fast as it forms is replaced by bone. Thus 

 the epiphyseal cartilage is an essential 

 provision for the lengthwise growth of 

 bones. The epiphyseal cartilages be- 

 come entirely calcified at different ages in the various bones, generally 

 from 18 to 22 years, at which time the epiphysis is said to unite with the 

 diaphysis. After that the articular cartilages are all that remain of the 

 original cartilaginous structure wiiich preceded the corresponding bone. 



THE JOINTS. 



Bones may be joined in two ways, either by a synarthrosis which 

 allows little or no motion between them, or by a diarthrosis which permits 

 them to move freely upon one another. 



In a synarthrosis the mesenchymal tissue between the adjacent bones 

 may become a dense connective tissue, either like a fibrous tendon or an 

 elastic ligament, thus forming a syndesmosis; or it may become cartilage, 

 usually of the fibrous type, making a synchondrosis. The sutures are forms 



FIG. 69. PLAN OF OSSIFICATION IN A 



LONG BONE, BASED UPON THE TIBIA. 

 Cartilage is drawn in black, and bone is 



stippled. Art., Articular cartilage; 



ep., epiphysis; diaph., diaphysis. 



