COMPACT BONE-SUBSTANCE DEVELOPMENT. 



357 



until the bone attains to nearly its full development, when the 

 general (fundamental) laminae (p. 328) are formed externally to the 

 Haversian rods from a plasma afforded by the vessels of the peri- 



Fig. 227. 



Fig. 228. 



Fig. 227. Vertical section from the surface of the shaft of the metatarsus of the calf; magnified 

 45 diameters. A. Periosteum. B. Ossifying blastema, c. Young layer of bone with -wide cavities 

 (a) in which are lodged remains of the ossifying blastema, and reticular spicula (Z>), which towards 

 the blastema present a tolerably abrupt border. D. More developed layer of bone, with Haversian 

 canals (c) surrounded by their lamellae. (KiMiker.) 



Fig. 228. Sub-periosteal layer from the extremity of the shaft of the ossifying tibia. The cartilage 

 and more open bony tissue have been scraped off from the inside of the crust, except at (a), where a 

 dark shade indicates a few vertical osseous areolae, out of focus and indistinctly seen. The part 

 (a, b) of the crust is ossified ; between (6 and c) are the clear reticular fibres, into which the earthy 

 deposit is advancing. (Magnified 150 diameters.) 



osteum, to constitute the structure represented by Fig. 217. And by 

 these the thickness of the bone is increased. But, while this change 

 is going on in the outer portions of the shaft of the long bones, 

 another is occurring in its interior; viz., the whole original shaft of 

 cancellated bone-substance, formed from the cartilage, becomes by 

 degrees absorbed, and thus the medullary canal is formed. The 

 extremities, however, of the long bones being formed entirely from 

 the original cartilages the latter constantly growing and becoming 

 ossified, while the bone is increasing in size are not absorbed in- 

 ternally; but, like the short bones (which also are not formed from 

 a collagenous matrix derived from the periosteal vessels), continue 

 to retain the cancellated structure through life. It is, however, suf- 

 ficiently obvious that the medullary canal is formed, not at the ex- 

 pense merely of the cancellated substance in the shaft of the foetal 



