The sternocleidomastoid muscle is a two-headed neck muscle. It is innervated by the accessory nerve (cranial nerve XI) and direct branches of the cervical plexus (C1-C2). The sternal head originates from the manubrium sterni, the clavicular head from the middle part of the clavicle. The insertion is the mastoid process of the temporal bone and the superior nuchal line of the occipital bone. (Kinopya ko lang ito sa nakasulat na definition sa notebook ko. I hope it helps to you) :)