That portion of the inferior aperture of the thorax formed by the articulated cartilages of the seventh to tenth (false) ribs. ... Synonym: arcus costalis, arcus costarum. ... (05 Mar 2000) ... 
the anterior portion of the thoracic outlet, consisting of the costal cartilages of ribs 7 to 10. Called also costal margin.
Type: Term Synonyms: costal margin