The diaphragm is a thin musculotendinous structure that fills the inferior thoracic aperture and separates the thoracic cavity from the abdominal cavity (Fig. 3.34 and see Chapter 4). It is attached peripherally to the: xiphoid process of the sternum, costal margin of the thoracic wall, ends of ribs XI and XII, ligaments that span across structures of the posterior abdominal wall, and vertebrae of the lumbar region.