58 THE ESSENTIALS OF HISTOLOGY. 



cellated, but the shaft of a long bone is almost entirely made up of 

 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 lacunce. The ramified passages which contain the cell-processes 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 O05 mm. (-^ inch) in diameter, but some are 



FIG. 66. TRANSVERSE SECTION OF A BONE (ULNA). (Sharpey.) (Magnified 

 20 diameters.) 



The openings of the Haversian canals are seen encircled by concentric lamellae. Other lamella-: 

 run parallel with the surface (a). 



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 



