STRUCTURE OF THE ELEMENTARY TISSUES. 



53 



Bone. The absorption of the endochondral spongy bone is now complete, 

 and the medullary cavity is bounded by periosteal bone: the inner layers 

 of this periosteal bone are next absorbed, and the medullary cavity is 

 thereby enlarged, while the deposition of bone beneath the periosteum 

 continues as before. The first-formed periosteal bone is spongy in char- 

 acter. 



Stage 6. Formation of Compact Bone. The transformation 

 of spongy periosteal bone into compact bone is effected in a manner 

 exactly similar to that which has been described in connection with ossi- 

 fication in membrane (p. 47). The irregularities in the walls of the 

 areolae in the spongy bone are 

 absorbed, while the osteoblasts 

 which line them are developed 

 in concentric layers, each layer 

 in turn becoming ossified till the 

 comparatively large space in the 

 centre is reduced to a well- 

 iormed Haversian canal (Fig. 

 57). When once formed, bony 

 tissue grows to some extent in- 

 terstitially, as is evidenced by 

 the fact that the lacunae are 

 rather further apart in fully- 

 formed than in young bone. 



From the foregoing descrip- 

 tion of the development of bone, 

 it will be seen that the common 

 terms "ossification in cartilage" 

 and "ossification in membrane" 

 are apt to mislead, since they 

 seem to imply two processes radi- 

 cally distinct. The process of 

 ossification, however, is in all 

 cases one and the same, all true bony tissue being formed from membrane 

 (perichondrium or periosteum); but in the development of such a bone 

 as the femur, which may be taken as- the type of so-called "ossification in 

 cartilage," lime-salts are deposited in the cartilage, and this calcified car- 

 tilage is gradually and entirely re-absorbed, being ultimately replaced by 

 bone formed from the periosteum, till in the adult structure nothing but 

 true bone is left. Thus, in the process of "ossification in cartilage," cal- 

 cification of the cartilaginous matrix precedes the real formation of bone. 

 We must, therefore, clearly distinguish between calcification and ossifica- 

 tion. The former is simply the infiltration of an animal tissue with 

 lime-salts, and is, therefore, a change of chemical composition rather 



FIG. 57. Transverse section of femur of a human 

 embryo about eleven weeks old. a, rudimentary Ha- 

 versian canal in cross section ; 6, in longitudinal section ; 

 c, osteoblasts ; ei, newly formed osseous substance of a 

 lighter color ; e, that of greater age ; /, lacunae with their 

 cells; g, a cell still united to an osteoblast. (Frey.) 



