BONE. 57 



are called Haversian canals, after the name of the phy- 

 sician, Clopton Havers, who first accurately described them. 



The Haversian canals, the average diameter of 

 which is -g-J-g- 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 

 lacunae absorbing nutrient matter from the Haversian 

 blood-vessels, and conveying 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 cancellous 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, 



