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During routine cadaveric dissection of the thorax and upper limb in a clinical gross anatomy course, we identified an unusual, supernumerary muscle in the right torso of a 91 year-old, Caucasian male. 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. It was innervated by branches of the thoracodorsal nerve. 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 showed a unique pattern of origin, insertions, and innervation. Furthermore, there was no resemblance to previously describe axillary arch muscles and other rare variations of latissimus dorsi muscle slips and insertions. We therefore name this unusual variant a “latissimo-cleidocostal muscle”. Future observation and study of this muscle variant should further test whether it is truly distinct from currently recognized supracostal or other latissimus dorsi variants.
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