Third head of sternocleidomastoid muscle

During the routine cadaveric dissection, a third origin of sternocleidomastoid muscle was noted; i.e., an additional belly of its clavicular head was observed on the left side of the neck of a 65-year-old female. This additional slip was innervated by the spinal accessory nerve and vascularized by a branch of suprascapular artery. However, the sternal head on the left side and both the heads on the right side were as usual. This additional slip could have been formed due to unusual splitting in the mesoderm of post-sixth branchial arch during organogenesis.
The awareness of variations of sternocleidomastoid muscle is important for surgeons as the additional slips can be used as myocutaneous flaps in head and neck surgeries. This type of variation is also important for radiologists while interpreting MR images of this region.