120 LOCOMOTION. [CHAP. v. 



cells corresponding in size with those of the temporary cartilage, 

 and having their nuclei disposed vertically, and of the same shape 

 and dimensions as the lacunae of bone. They are still granular, 

 however, and no pores can be seen emerging from them. The cells 

 are united together, and the lines of their junction have for the most 

 part disappeared. The curvilinear border of each can be still seen, 

 however, at its union with the central lamina, /. In the cancelli, 

 i. i. i., the granular blastema exists in great abundance. 



It hence appears, that, after the ossification of the intercellular 

 substance the rows of cartilage-cells arrange themselves on the 

 inner surface of the newly formed cancelli, and become ossified, 

 with the exception of their nuclei, which remain granular, and sub- 

 sequently form the lacunae of bone ; and that a new substance, or 

 blastema, appears within the cancelli, from which, probably, vessels 

 are developed, and the future steps in the growth of the bone 

 proceed. 



The cancelli when first formed are closed cavities. At a subse- 

 quent period they appear to communicate, and thus to form the 

 cancelli and Haversian canals of perfect bone; a complete net- 

 work of blood-vessels becoming developed within them at the same 

 time. 



The subsequent progress of ossification seems to consist essen- 

 tially of a slow repetition, on the entire vascular surface of the 

 bone, of that process which has been now briefly described. It is 

 probable that new cartilage-cells are developed on that surface, 

 and become ossified in successive layers, their nuclei remaining to 

 form the lacunae, the uniform dispersion of which through bone 

 is thus explained. The cause of the lamination of bone, parallel to 

 its vascular surface, is also thus illustrated. 



The first appearance of pores is in the form of irregularities in the 

 margin of the lacunae. These increase with the consolidation of the 

 tissue, and are converted into branching tubules which communicate 

 with those adjacent. These pores must consequently be formed in 

 the ossified substance of the cartilage-cells. In our account of the 

 lacunae of perfect bone it was mentioned, that, for the most part, 

 those of contiguous Haversian systems do not communicate across 

 the narrow interval that separates the Haversian rods ; this interval 

 having, in fact, no pores. It results from what has just been said of 

 the mode of deposition of new layers, that the primary osseous net- 

 work, formed in the intercellular substance of the temporary carti- 

 lage, must come to constitute the substance intervening between 

 the Haversian rods, the non -porosity of which is thus satisfactorily 



