- Divides anterior and posterior triangles
- Origin
- Sternal head: anterior surface of the manubrium of the sternum
- Clavicular head: superior surface of the medial third of the clavicle - Insertion
- Lateral surface of the mastoid process of the temporal bone
- Lateral half of the superior nuchal line of the occipital bone - Innervation
- Motor: Spinal accessory nerve (CN XI) - Blood Supply
- Occipital artery (from external carotid)
- Superior thyroid and suprascapular arteries may also contribute - Actions
- Unilateral contraction:
- Ipsilateral side bending of the neck
- Contralateral rotation of the head
- Bilateral contraction:
- Neck flexion
- Accessory muscle of inspiration (elevates sternum and clavicle)
