Flat bones are made up of a layer of spongy bone between two thin layers of compact bone. Their shape is flattened, not rounded. Examples include the skull and ribs. Flat bones have marrow, but not a bone marrow cavity.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.