Light micrograph of spongy bone. This bone type is also called cancellous bone. The bone matrix is composed of collagen fibers with lacunae containing osteocytes, which maintain the bone structure. Spongy bone has trabeculae which act as a series of girders, which transfer the forces received at the end of the bone, to the stronger outer walls of compact bone. The cavities in the spongy bone and central bone marrow are the site of blood cell formation and full of blood vessels.