Inner structure of bones, historical anatomical artwork. Bone can be compact or cancellous (spongy). Cancellous bone has an open, honeycomb-like structure and is found on the inside of many bones, as shown here. Compact bone forms a dense layer on the outside surface of the bone. Some long bones, such as the femur (left) and tibia (second from right), have a hollow diaphysis (shaft) filled with bone marrow (not seen). The combination of open and dense tissue means bones are both strong and light. This illustration is taken from the 19th century French textbook The Atlas of Human Anatomy and Surgery by J. M. Bourgery and N. H. Jacob.