A case of the human sternocleidomastoid muscle additionally innervated by the hypoglossal nerve.

An anomalous nerve supply from the hypoglossal nerve (XII) to the sternocleidomastoid muscle (SM) was observed in the right neck of an 82-year-old Japanese female. This nerve branch arose from the hypoglossal nerve at the origin of the superior root of the ansa cervicalis. The nerve fiber analysis revealed that this branch consisted of fibers from the hypoglossal nerve, the first and the second cervical nerves and had the same component as the superior root of the ansa cervicalis. SM appeared quite normal and the most part was innervated by the accessory nerve and a branch from the cervical plexus. The anomalous branch from XII supplied the small deep area near the anterior margin of the middle of the sternomastoid portion of SM. It is reasonable to think that the small deep area of SM, which was innervated by the anomalous branch from XII, occurs as the result of fusion of the muscular component from infrahyoid muscles. If the muscular component does not fuse with SM, it is thought to appear as an aberrant muscle such as the superior sternoclavicular muscle (Hyrtl) which is also supplied from a branch of the superior root of the ansa cervicalis.