Multiple neck muscles help to balance the head and facilitate its movement at the atlanto-occipital joint. For example, the bilateral sternocleidomastoid, or SCM, is a major head flexor along with the suprahyoid and infrahyoid muscles. Its bellies originate at the sternum and clavicle and insert into the mastoid process of the temporal bone. When the SCM contracts bilaterally, it helps to bend the head forward. In contrast, unilateral contraction facilitates lateral flexion on the same side or head rotation to the opposite side. The lateral neck muscles — the anterior, middle, and posterior scalenes — run diagonally along the vertebral column. They connect the cervical vertebrae to the upper two ribs and also facilitate lateral flexion. The splenius capitis, a posterior neck muscle, originates from the nuchal ligament and the C7 to T3 spinous processes and inserts onto the occipital and temporal bones. When contracted bilaterally, it aids head extension, whereas unilateral contraction assists lateral flexion and rotation to the same side.