what is sternomastoid muscle
Noun. 1. sternocleidomastoid muscle-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.musculus sternocleidomastoideus, sternocleido mastoideus, sternocleidomastoid.oun. 1. sternocleidomastoid muscle-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.