On each side, the sternocleidomastoid muscle divides the neck into anterior and posterior triangles. It originates from two heads:
- sternal head: superior and anterior surface of manubrium sterni
- clavicular head: superior surface of medial third of clavicle From this continuous origin, the muscle passes superiorly, laterally and posteriorly. It inserts along a continuous line between the lateral surface of the mastoid process of the temporal bone and the lateral half of the superior nuchal line of the occipital bone.
The actions of sternocleidomastoid are manifold:
- acting unilaterally, sternocleidomastoid:
- flexes the cervical spine ipsilaterally
- laterally rotates the cervical spine to the contralateral side
- acting bilaterally, both muscles:
- protract the head
- extend the incompletely extended cervical spine
- provide protection for deeper vital structures throughout some of their course, e.g. the great vessels
Sternocleidomastoid is innervated by the:
- spinal accessory nerve (XI)
- ventral ramus of second cervical nerve (C2)