A muscle with origin from the superior nuchal line, the external occipital protuberance, the nuchal ligament, the spinous processes of the seventh cervical and thoracic vertebrae, with insertion into the lateral third of the posterior surface of the clavicle, the medial side of the acromion, and the upper border of the spine of the scapula, with nerve supply from the accessory nerve and the cervical plexus, and whose action draws the head to one side or backward and rotates the scapula.