The strong, hard layer under the periosteum is called the compact bone, also known as the cortical bone. It forms the dense outer layer of bones and provides strength and rigidity. The compact bone consists of tightly packed osteons (Haversian systems), which are cylindrical structures containing concentric lamellae of bone matrix and a central Haversian canal. The Haversian canals house blood vessels and nerves that supply nutrients and oxygen to the bone tissue.