56 NORMAL HISTOLOGY. 



matrix ; this gradually thickens and encloses the osteoblasts, which 

 now lie within minute bays or recesses, the entrances to which become 

 gradually contracted, until the opposed 

 edges join and the cells lie within lacunae 

 completely surrounded by the bone-matrix : 

 the osteoblasts have now become the bone- 

 cells. The matrix is deposited as lamellae, 

 especially marked in the bone formed in the 

 later stages of fcetal life ; between these are 

 included the lacunae. The matrix is at first 

 soft and exhibits an indistinct fibrillated 

 structure in which the subsequent deposit 

 Developing bone portion of of lime salts principally the phosphate and 



trabecula undergoing absorp- carbonate takes place. 



tion : b, bone-cells; c, osteo- -\-ITI i i 



blasts ;m, bone-matrix ; *, muiti- When, on the contrary, bone or carti- 

 nucieated osteociast lying within lage is absorbed, it is through the agency 



the^bsorption-pit, or lowship's Qf ^ g^^ the QSteOClaStS, Or choU- 



droclasts (Klein); these large multinucleated 



elements usually lie upon the surface of the bone-trabeculae within 

 larger or smaller pits which have been excavated by them ; these 

 are Howship's lacunae. 



In recapitulation, the following summary of the phases of de- 

 velopment during the growth of a tubular long bone may be 

 noted : 



1. Solid embryonal cartilage. 



2. Enlargement and rearrangement of cartilage-cells and lacunae 

 and calcification of matrix at centre of ossification. 



3. Penetration of periosteal tissue to the focus of calcification ; 

 vascularization of the cartilage. 



4. Formation of medullary spaces by the breaking down of lacunae 

 surrounded by the zone of calcifying cartilage. 



5. Covering of the surface of calcified cartilage trabeculae by the 

 layer of osteoblasts and the production of an enveloping sheath of 

 true bone. 



6. Resulting central net-work of endochondral bone, with gradual 

 absorption of encased cartilage trabeculae. 



7. Absorption of central spongy bone in shaft and formation of 

 central marrow-cavity. 



8. Formation, meanwhile, of peripheral periosteal net-work of 

 spongy bone. 



9. Conversion into compact bone by partial absorption of tra- 

 beculse to form Haversian spaces ; secondary deposit of concentric 

 lamellae within these spaces forming Haversian systems of compact 

 bone. 



