There are three different cavities, the thoracic, the abdominal and the pelvic cavity. Out of these three cavities the abdominal cavity has no bony protection. The thoracic cavity is protected by your ribs and the pelvic bones protect the pelvic cavity. If you think about your abdominal region its fairly obvious that there are no bones to protect it, the only bones in that area is your spine and that does not protect anything.