Bone cells that are high in calcium and low in marrow and are strong are called rigid bones. Haversian system is present in smooth bones. At the center of the Haversian system is the Haversian groove. Haversian ducts are surrounded by circular lamellae. The lamellae contain lacunae of tiny cavities. The lacunae are surrounded by tiny canaliculi. Lacunae communicate with each other by canaliculi. Haversian ducts are interconnected by transverse ducts. These transverse canals are called Volkmann’s canal. The ossified cavity is called the medullary cavity. The marrow cavity is filled with red or yellow marrow. Fixed bones like femur, tibia-fibula, humerus, radius-ulna etc. 80% of the total weight of the human body is bone.