Normal adult bone is seen here in cross section at high power in an undecalcified ground bone section, demonstrating the cylindrical osteons composed of concentric rings of osteocytes with their dark nuclei in lacunae and thin processes called canaliculi. Each osteon has a central haversion canal with the vascular supply that reaches osteocytes via the canaliculi. Haversion canals of ajacent osteons communicate via Volkmann canals. As the bone slowly remodels over time, the osteons appear with variable size and orientation, as seen below at medium power. Most of the bone is composed of hydroxyapatite crystal (hydrated calcium phosphate). Serum calcium, vitamin D, and parathormone levels control the metabolism of bone.