Bones are divided into two categories. namely-
1. Compact bone: The unstable cells which have more calcium and less marrow and are strong are called compact bone. Pure bones have the Haversian system. 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 channels are called Volkmann’s canal. The unstable cavity is called the marrow 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.
2. Spongy bone: The inelastic cells that have less calcium and more marrow and are soft like a sponge are called spongy bones. Its structural and functional unit is called trabeculae. It has numerous kuthuri or voids. They are like sponge or honeycomb called cancellous or trabecular bone. Each trabecula is composed of osteocytes, lamellae, lacunae and canaliculi. Blood vessels from the unstable periosteum enter the trabeculae and supply nutrients to the bone cells. Haversianism does not exist. Carotid bones of mammals, large bony extremities, flat bones and all bones of birds are spongy in nature. Almost all bones in children are spongy. 20% of the total weight of the human body is spongy bone.