9 



HISTOLOGY 



as elsewhere. Thus the spicules of calcified matrix, staining blue, become 

 encased in the matrix of bone which stains red (Figs. 75 and 76). 



From what has been said, it is clear that bone is formed both around 

 the cartilage (perichondrial bone) and within the cartilage (endochon- 

 drial bone). In long bones and flat bones, ossification is at first perichon- 

 drial and later endochondrial; in short bones it is endochondrial until 

 the cartilage has been entirely replaced. Thus the part taken by endo- 

 chondrial and perichondrial ossification varies greatly in different bones. 

 As the bone grows, the older parts which have formed in relation with 



Periosteum. 



Haversian 



Endochondrial Perichondrial 

 bor 



Haversian canal. 



Calcified matrix 

 between endo- 

 chondrial and 

 perichondrial 

 bone. 



Blood vessel. 



Marrow. 



Remains of calci- 

 ned matrix of 

 cartilage. 



FIG. 77. FROM A CROSS SECTION OF THE SHAFT OF THE HUMERUS, FROM A HUMAN EMBRYO OF THE FOURTH 



MONTH. X 80. 



the cartilage are resorbed. In the shaft of the humerus from a human 

 embryo of the fourth month (Fig. 77), only a thin and interrupted layer 

 of calcified cartilage remains to mark the boundary between perichondrial 

 and endochondrial bone, and in the adult all traces of this layer have dis- 

 appeared. This is true of most bones, but in the auditory ossicles cal- 

 cified cartilage is found throughout life. 



The final stages in the replacement of the cartilages by bone take 

 place long after birth, when the bones have increased greatly in diameter 

 and length. The growth in diameter is accomplished by the deposition 

 of new layers externally, and the enlargement of the marrow cavity. 



