THE STRUCTURE OF THE ELEMENTARY TISSUES. 55 



finally coalescing into one great cavity the medullary cavity of the 

 shaft. 



Stage 5. Absorption of the Inner Layers of the Periosteal 

 Bone The absorption of the endochondral spongy bone is now com- 

 plete, 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 ossification 

 in membrane (p. 49). 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-formed 

 Haversian canal (Fig. 64). When once formed, bony tissue grows to 

 some extent interstitially, as is evidenced by the fact that the lacunae are 

 rather further apart in fully-formed than in young bone. 



From the foregoing description of the development of bone, it will 

 be seen that the common terms "ossification in cartilage" and "ossifi- 

 cation in membrane " are apt to mislead, since they seem to imply two 

 processes radically distinct. The process of ossification, however, is in 

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

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

 bone as the femur, which may be taken as the type of so-called ' ' ossifi- 

 cation in cartilage/' lime-salts are first of all deposited in the cartilage; 

 this calcified cartilage, however, 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/' calcification of the cartilaginous matrix 

 precedes the real formation of bone. We must, therefore, clearly dis- 

 tinguish between calcification and ossification. The former is simply 

 the infiltration of an animal tissue with lime-salts, and is, therefore, a 

 change of chemical composition rather than of structure; while ossifica- 

 tion is the formation of true bone a tissue more complex and more 

 highly organized than that from which it is derived. 



Centres of Ossification. In all bones ossification commences at one 

 or more points, termed " centres of ossification." The long bones, e.g., 

 femur, humerus, etc., have at least three such points one for the ossifi- 

 cation of the shaft or diaphysis, and one for each articular extremity or 

 epipliysis. Besides these three primary centres which are always present 

 in long bones, various secondary centres may be superadded for the ossi- 

 fication of different processes. 



