Illustration of bone tissue microstructure. Bones have two types of structure, a compact form called cortical bone (foreground) and a lattice-like form called spongy bone (background). Cortical bone comprises structural units called osteons (concentric structures, foreground). Each osteon is formed from a central canal containing arteries (red), veins (blue) and nerves (white), surrounded by concentric rings of a mineral matrix. In this matrix are living osteocytes, (bone cells, pink) connected by canaliculi (microscopic canals) which transport blood. The osteocytes and canaliculi are aligned parallel to the long axis of the bone. Spongy, or cancellous, bone is found in the interior of a bone. It is characterised by a honeycomb arrangement, comprising a network of trabeculae. These structures provide support and strength to the bone. The spaces within this tissue contain bone marrow (not seen), a blood forming substance.