Ground sections of bone are prepared by using a saw to cut thin sections that are then ground to a thinness to allow viewing with a microscope. Organic material is lost leaving only the mineral matrix of bone. Open spaces filled with air appear darker than the mineralized matrix.
Volkmann's Canals - openings that run perpendicular to Haversian canals connecting them with the surface (#1 and #2)
and each other (#3 and #4). They carry blood vessels and nerve fibers
into the Haversian system.