The sternocleidomastoid (also sternocleidomastoid muscle, latin: musculus sternocleidomastoideus) is a paired superficial muscle that lies on each side of the neck, extending from the sternum and clavicle to the base of the skull.
The sternocleidomastoid originates from the manubrium of the sternum and the clavicle.
The fibers of the sternocleidomastoid insert on the mastoid process of the temporal bone and on the superior nuchal line of the occipital bone.
The main action provided by the sternocleidomastoid is the lateral flexion of the head. A unilateral contraction the sternocleidomastoid elevates the chin and rotates the head it to the opposite side, while a bilateral contraction extends the upper vertebral joints, thus elevating the head.
The nerve supply to the sternocleidomastoid is provided by the accessory nerve (CN XI) and by nerves arising from the cervical plexus (C1 - C2).
The arterial blood to the sternocleidomastoid is mostly supplied by the branches of the external carotid artery, such as the sternocleidomastoid branch of the superior thyroid artery and the sternocleidomastoid branch of occipital artery. The venous drainage from the sternocleidomastoid is provided by the sternocleidomastoid vein, which flows into the internal jugular or the superior thyroid vein.