DEVELOPMENT OF BONE. 141 



surface also, especially where they are in contact with the peri- 

 osteum. The tissue enclosed by the newly formed bony arches 

 soon becomes more transparent, highly vascular, and then con- 

 tains small roundish cells which appear granular, together with 

 others that are more transparent, elongated, and fusiform, 

 resembling those which are met with in young connective tissue. 

 In the mode illustrated by the example we have adduced, the 

 process of formation may be followed in the later stages of deve- 

 lopment, during the period of increase of the bone in thickness. 



Since the great medullary cavity occurs as a secondary 

 formation in the tubular bones, in consequence of a great part 

 of the bone developed by intra-cartilaginous ossification having 

 undergone re-absorption, we find in adult bones that the 

 medullary cavity in the middle portion of the shaft is bounded 

 for the most part only by the intercalated or Haversian sys- 

 tems of bone which have been formed by periosteal osteogenesis. 



As in the intra-cartilaginous mode of ossification, so also in 

 the periosteal form, a continuous layer of osteoblasts may be 

 observed to constitute the immediate precursor of the formation 

 of the osseous tissue. 



It still remains to be noticed, that, in transverse sections 

 made as before, strong fibrous bands may be followed, pursuing 

 a radiating course through all the layers of the periosteum, 

 from the exterior to the bony trabeculse ; these are continuous 

 with the fasciculi of the outer layers of the periosteum, appear 

 to be quite independent of the rings and arches of osteoblasts, 

 and are to be regarded as the earliest condition of the per- 

 forating fibres of Sharpey. 



If we examine the longitudinal section of one of the long 

 bones of an aborted embryo or very young child, in the same 

 way that we have already examined the transverse section, we 

 shall see stretching over spaces of considerable extent in the 

 periosteum, near the surface of the bone and imbedded in the 

 small-celled layer of the periosteum, a layer of well-defined 

 osteoblasts, arranged longitudinally in a striated manner, in 

 which the processes that take place are less distinctly visible 

 than in transverse sections, and, indeed, become intelligible 

 only by comparison with the latter. The appearances pre- 

 sented by such longitudinal sections are, however, very similar 



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