sternocleidomastoid muscle

(noun) one of two thick muscles running from the sternum and clavicle to the mastoid and occipital bone; turns head obliquely to the opposite side; when acting together they flex the neck and extend the head. Synonyms: sternocleidomastoid, sternocleidomastoid muscle, sternocleido mastoideus, musculus sternocleidomastoideus.