48 ELEMENTARY TISSUES. 



are called Haversian canals, after the name of the physician, 

 Clopton Havers, who first accurately described them. 



The Haversian canals, the average diameter of which 

 is _i^. of an inch, contain blood-vessels, and by means of 

 them, blood is conveyed to all, even the densest parts 

 of the bone ; the minute canaliculi and lacuna? absorbing 

 nutrient matter from the Haversian blood-vessels, and con- 

 veying it still more intimately to the very substance of the 

 bone which they traverse. The blood-vessels enter the 

 Haversian canals both from without, by traversing the 

 small holes which exist on the surface of all bones beneath 

 the periosteum, and from within by means of small channels, 

 which extend from the medullary cavity, or from the can- 

 cellous tissue. According to Todd and Bowman, the arteries 

 and veins usually occupy separate canals, and the veins 

 which are the larger, often present, at irregular intervals, 

 small pouch-like dilatations (fig. 17). 



The lacuna are occupied by nucleated cells, or, as Dr. 

 Beale expresses it, minute portions of protoplasm or 

 germinal matter ; and there is every reason to believe that 

 the lacunar cells are homologous with the corpuscles of 

 the connective tissue, each little particle of protoplasm 

 ministering to the nutrition of the bone immediately 

 surrounding it, and one lacunar particle communicating 

 with another, and with its surrounding district, and with 

 the blood-vessels of the Haversian canals, by means of 

 the minute streams of fluid nutrient matter which occupy 

 the canaliculi. 



Besides the concentric lamella of bone tissue which 

 surround the Haversian canal in the shaft of a long bone, 

 are others, especially near the circumference, which 

 surround the whole bone, and are arranged concentrically 

 with regard to the medullary canal. 



The ultimate structure of the lamella appears to be 

 reticular. If a thin film be peeled off the surface of a bone 

 from which the earthy matter has been removed by acid, 



