The sternocleidomastoid 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.
Origin: manubrium of the sternum, and the clavicle.
Insertion: mastoid process of the temporal bone and the superior nuchal line of the occipital bone.
Action: lateral flexion of the head; upon unilateral contraction, the sternocleidomastoid elevates the chin and rotates the head to the opposite side, while a bilateral contraction extends the upper vertebral joints, thus elevating the head.
Innervation: accessory nerve (CN XI), also nerves arising from the cervical plexus (C2 - C3).
Blood supply: arteries originating from the external carotid artery, such as the sternocleidomastoid branch of the superior thyroid artery and the sternocleidomastoid branch of the occipital artery.
Venous drainage: by the sternocleidomastoid vein, which flows into the internal jugular vein or the superior thyroid vein.