Bilateral four heads of the sternocleidomastoid muscle.

The sternocleidomastoid muscle shows a wide range of variations including supernumerary muscular heads. We found a rare variation in the sternocleidomastoid muscle with bilateral supernumerary heads in a 67-year-old Korean male cadaver. Bilateral four muscle bellies were recorded: two sternomastoids, one cleido-occipital and one cleido-mastoid occipital on the right side, and one sternomastoid, one cleido-occipital and two cleido-mastoids on the left side. The variation of bilateral four heads on sternocleidomastoid muscle is important to surgeons and anesthetists for clinical using.