HISTOLOGY. 



retained in the adult. The processes, moreover, are so fine as to be 

 invisible ordinarily, and formerly their existence was denied. 



The strands of bone containing bone cells, and beset with osteoblasts, 

 increase in size and unite so as to enclose areas of embryonic connective 

 tissue containing blood vessels, as shown in the upper part of Fig. 60, 

 and in the diagram, Fig. 61. The connective tissue surrounding the 

 entire network of bone becomes differentiated into a distinct layer, the 

 periosteum. This includes an outer stratum of ordinary connective tissue 

 (not drawn in the figures), a middle layer of dense fibrous tissue, and an 

 inner cellular layer including the osteoblasts in contact with the outer 

 surface of the bone. Fig. 61 shows the way in which a portion of this 

 inner stratum may be enclosed in the bone matrix. It is about to occur 



around the blood vessel. 

 B. Y., and has taken place 

 in the space H. O. Within 

 such an enclosure the osteo- 

 blasts continue to form 

 bone in concentric layers or 

 lamellae, thus gradually re- 

 ducing the central space 

 until it contains only a few 

 cells and the blood vessels 

 as in H. C 2 . Such spaces 

 occur abundantly in adult 

 bone, and are called Hai-cr- 

 sian canals (in recognition 

 of the anatomist Havers). 

 They are always surrounded 

 by concentric lamellae, or 

 layers of bone, of which the innermost is the youngest. Between these 

 Haversian systems there are irregular lamellae, called interstitial lamellae, 

 and sometimes a blood vessel runs through them, not surrounded by con- 

 centric layers. It is said to occupy a Volkmann's canal. Transitions from 

 a Volkmann's to an Haversian canal are gradual, and are made not by a 

 change in the canal but by a rearrangement of the surrounding lamellae. 

 Coarse fibers may extend from the periosteum into the interstitial lamellae, 

 known as Sharpens fibers. They consist of more or less calcified bundles 

 of connective tissue fibers, including both white and elastic elements, though 

 chiefly the former. If abundant, the periosteum is most closely adherent 

 to the bone. They are absent from the Haversian systems. Besides the 

 interstitial and concentric lamellae, another set is deposited under the pen- 



FIG. 61. DIAGRAM OF THE DEVELOPMENT OF BONE. 



(In part, after Duval.) 



f., Fibrous layer of periosteum; o., osteogenic layer of perios- 

 teum; os., ostepblast; b.c., bone cell; B. V., blood ves- 

 sel; H. C 1 ., beginning Haversian canal; H. C 2 ., complete 

 Haversian canal; i. 1., interstitial lamellae, c. 1., concen- 

 tric lamellae; Sh., Sharpey's fibers. 



