The sternocleidomastoid muscle is a long, paired muscle of the neck. Anchored by two heads at the sternon and clavicle bones, it runs up the front of the neck and then obliquely backward to insert by a tendon in the mastoid process of the temporal bone. It is supplied by the occipital and superior thyroid arteries.
It turns the head from side to side; it also pulls the head down. When the body is in the supine position (lying on the back), it raises the head up and forward.
Motor: accessory nerve (CN XI); sensory: clavical plexus.