Pronunciation: (stûr'num),[key] —n., —pl.-naPronunciation: (-nu),[key]-nums. 1. Anat.,Zool.a bone or series of bones extending along the middle line of the ventral portion of the body of most vertebrates, consisting in humans of a flat, narrow bone connected with the clavicles and the true ribs; breastbone. See diag. under skeleton. 2. the ventral surface of a body segment of an arthropod.