The sternocleidomastoid is a long muscle in the side of the neck that extends up from the thorax to the base of the skull behind the ear. When the sternocleidomastoid on one side contracts, the face is turned to the opposite side. When both muscles contract, the head is bent toward the chest. If the immovable end is fixed in position by other muscles, it can raise the sternum (breastbone) - an actionmycontentbreak which aids in forceful inhalation of air.