MICROSCOPIC STRUCTURE 161 



to the bodies of all members of the vertebrate 

 kingdom ; it protects many delicate organs ; 

 and it affords points of attachment for muscles. 



On examining a thin transverse section of 

 human bone under the microscope with a 

 low power it is found to exhibit a number 

 of round or oval apertures. These are trans- 

 verse sections of the Haversian canals. Each 

 Haversian canal contains an artery, vein, nerve, 

 and lymphatic vessels, all for nourishing the 

 bone. They mostly run parallel with the axis 

 in long bones, but in flat bones they are 

 parallel to the surfaces. 



Still keeping to our transverse section, but 

 using a higher power in the microscope, we 

 find it exhibits numerous dark spots with fine 

 lines branching from them in all directions. 

 The dark spots are the lacunae, and the fine 

 lines branching from them are the canaliculae, 

 or calcigerous canals. The dark appearance 

 of both lacunae and canaliculae is owing, in 

 dried bone, to the air they contain. 



If the air be removed by immersion in 

 oil of turpentine they become white or 



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