The sternocleidomastoid muscle is a paired, relatively long superficial muscle that lies on each anterolateral side of the neck. It extends from the sternum and clavicle to the base of the skull.

The sternocleidomastoid is composed of two heads that are named after their origin sites - sternal head and clavicular head. Both heads merge into a single muscle belly at the lower aspect of the muscle. Further, the muscle belly passes upward and laterally toward the mastoid process of the temporal bone, where it inserts.

Moreover, the sternocleidomastoid muscle is easily palpable, and it serves as a topographic structure. This muscle subdivides the anterolateral aspect of the neck into two major neck triangles - anterior neck triangle and posterior (lateral) neck triangle.

Origin: sternal head - superior aspect of the manubrium of the sternum; clavicular head - superior aspect of the sternal end of the clavicle.

Insertion: mastoid process of the temporal bone, lateral half of the superior nuchal line of the occipital bone.

Action: unilateral contractions - rotation of the head to the contralateral (opposite) side, lateral flexion of the neck to the ipsilateral (same) side; bilateral contractions (if the cervical spine is rigid due to contractions of the paravertebral muscles) - flexion of the neck and flexion of the head forward; bilateral contractions (if the cervical spine is not fixed) - extension of the head, drawing the head ventrally and elevating the chin; the sternocleidomastoid muscles are also classified as accessory inspiration muscles, and they act during forced inspiration by elevating the sternum.

Innervation: accessory nerve (CN XI), nerves arising from the cervical plexus (C2 - C3).

Blood supply: sternocleidomastoid branch of the superior thyroid artery and the sternocleidomastoid branch of the occipital artery.

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Sternocleidomastoid muscle