First report of a “latissimo‐cleidocostal” muscle variant on the anterolateral thoracic wall (915.12)

During dissection of the upper limb in a gross anatomy course for first year dental students, we identified an unusual, supernumerary muscle in the right torso of a 91 year‐old, Caucasian male cadaver. This consisted of a substantial muscle bundle continuous with the antero‐inferior border of the latissimus dorsi, bearing a lengthy, strap‐like tendon that passed superomedially, deep to the pectoralis minor, to insert broadly on the neck of the clavicle and first costal cartilage. Although this muscle bore some partial resemblance to reported anterior supracostal muscle variants on the anterior thoracic skeleton, which are typically innervated by intercostal nerves, it was unique in its: 1) continuity with the latissimus dorsi, 2) bifid insertion on both clavicle and first rib, and 3) innervation by branches of the thoracodorsal nerve. Furthermore, there was no resemblance to previously described axillary arch muscles and other rare variations of latissimus dorsi muscle slips and insertions. Based on its demonstrated insertions and innervation, we name this unusual variant a “latissimo‐cleidocostal muscle”. This recognition should now serve as a working hypothesis, to be tested with future observation and study, that this muscle is distinct from currently recognized supracostal or other latissimus dorsi variants.