The bone cells which contain 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 and are 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.