The sternocleidomastoid muscle is a two-headed neck muscle, which true to its name bears attachments to the manubrium of sternum (sterno-), the clavicle (-cleido-), and the mastoid process of the temporal bone (-mastoid). It is a long, bilateral muscle of the neck, which functions to flex the neck both laterally and anteriorly, as well as rotate the head contralaterally to the side of contraction. The muscle is closely related to certain neurovascular structures that pass through the neck on the