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 clavicle.

Insertion: mastoid process of the temporal bone and 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: sternocleidomastoid vein, which flows into the internal jugular vein or the superior thyroid vein.