BONE 



49 



compact substance except along the centre, which is hollow and filled 

 with marrow. The interstices of cancellated bone are also occupied 

 by marrow. Externally bones are covered except at the joints by a 

 vascular fibrous membrane, the periosteum. 



True bone is always made up of lamella, and these again are com- 

 posed of fine fibres lying in a calcified ground-substance. Between 

 the lamellae are branched cells, the bone-corpuscles, which lie in cell- 

 spaces or lacuna. The ramified passages which contain the cell-pro- 

 cesses are termed canaliculi. 



In cancellated bone the blood-vessels run in the interstices supported 

 by the marrow. In compact bone they are contained in little canals 

 the Haversian canals which everywhere pervade the bone. These 

 canals are about 0'05 mm. (^^ inch) in diameter, but some are 

 smaller, others larger than this. Their general direction is longitudinal, 

 i.e. parallel to the long axis of the bone, but they are constantly united 

 by transversely and obliquely running passages. In a section across 



FIG. 58. TRANSVERSE SECTION OF A BONE (ULNA). (Magnified 20 diameters.) 



The openings of the Haversian canals are seen encircled by concentric lamellae. Other lamellse 

 run parallel with the surface (a). 



the shaft of a long bone they are seen as small rounded or irregular 

 holes (fig. 58). When the section has been made by grinding, the 

 holes get filled up with air and debris, and they then look black by 



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