dense bone in which the bony matrix is solidly filled with organic ground substance and inorganic salts, leaving only tiny spaces (lacunae) that contain the osteocytes (osteocyte), or bone cells. Compact bone makes up 80 percent of the human skeleton; the remainder is cancellous bone, which has a spongelike appearance with numerous large spaces and is found in the marrow space (medullary cavity) of a bone. Both types are found in most bones. Compact bone forms a shell around cancellous bone and is the primary component of the long bones of the arm and leg and other bones, where its greater strength and rigidity are needed.