Compact or cortical bone is a hard, solid osseous tissue found in the cortex of all bones. It also forms the bulk of the diaphysis in long bones. The structural unit of compact bone is the osteon, also called the haversian system. They are long, cylindrical units that are arranged in parallel to withstand compression forces along the long axis. Each osteon contains concentric lamellae surrounding a central osteonic or haversian canal. The collagen fibers of each lamellar ring are oriented alternately, thus resisting twisting forces. Between the lamellae are spaces called lacunae, which harbor osteocytes — the mature bone cells. The haversian canal encloses blood vessels, lymphatic vessels, and nerves connected to the osteocytes via tiny canals called canaliculi. The transverse Volkmann's canal connect the osteons to the main blood and nerve supply to the bone. Interstitial lamellae are found between the osteons. They are incomplete rings of bone matrix that are the remnants of older osteons being recycled via bone remodeling. The circumferential lamellae surround the osteons and are present around the entire circumference of the bone.