STRUCTURE OF BONE. 



61 



of areolar tissue, by thin bony partitions, which cross each 

 other in every direction, forming what is called the " cancel- 

 lated" structure. These chambers or cancelli are filled with 

 marrow, like the central cavity, with which they communi- 

 cate. In the flat bones, moreover, such as those of the 

 head we find that the two surfaces are composed of dense 

 plates of bone, like that which forms the shaft of the long 

 bones ; but that between them there is a layer of cancellated 

 structure, filled in like manner with marrow. But when we 

 examine with the microscope a thin section of even the 

 densest bony matter, we find it traversed by a network of 

 minute canals, continuous with the central cavity. These 

 canals usually run, in the shafts 

 of long bones, in the direction 

 of their length; and are con- 

 nected, every here and there, by 

 cross branches (fig. 16). They 

 are termed the Haversian canals, 

 after the name of their disco- 

 verer, Havers. The lining mem- 

 brane of the large central cavity 

 is copiously supplied with blood- 

 vessels; and this sends off pro- 

 longations into the cancelli at 



the extremities of the bone, and Fif? - ".-DIAGRAM REPRESENTING 



' THE STRUCTURE OP A PORTION OS 



into the Haversian Canals. Thus THE SHAFT OF A LONG BONE. 



hlonrl is rrmvpvprl into fhp in a b d> the surface as seen in 

 Lb COnvtyeu IE in- transverse section; b e f c, surface 



terior of the bone ; but no vessels 



can be traced absolutely into its 



texture, so that all the spaces 



which lie between the Haversian 



canals are as destitute of vessels as 



is healthy cartilage. These spaces are provided with nutriment 



by the following very remarkable arrangement. 



50. When we cut across the shaft of a long bone, and 

 examine a thin section with a microscope, we of course see 

 the open extremities of the Haversian canals (fig. 17, a) ; 

 just as we see the cut ends of the ducts and vessels of 

 wood, when we make a transverse section of a stem. 

 Around each of these apertures, the bony matter is arranged 

 in concentric rings, which are marked out and divided 



seen in longitudinal section ; i, Ha- 

 versian systems cut across, each 

 having an Haversian canal in its 

 centre ; g v, Haversian systems cut 

 longitudinally { /, lamellae near the 

 surface of bone, destitute of Haver- 

 sian systems. 



