RESUMO
During a dissection of the brachial plexus we found a rare variation of left posterior cord branching coexisting with an unusual intercalated ectopic muscle. This muscle originated from the shoulder joint capsule at the lesser tubercle on insertion of the subscapularis then pierced between the brachial plexus, enclosed by two roots of the radial nerve, and inserted into the upper part of the latissimus dorsi muscle. The variant posterior cord divided into two roots; a thin lateral and thick medial root. The lateral root gave off the thoracodorsal nerve that penetrated and also innervated the ectopic muscle. The medial root gave off five nerve branches; two upper subscapular, one lower subscapular, one axillary and one terminal branch. A terminal branch fused with the lateral root to form a loop enclosing the ectopic muscle then continued as the radial nerve. This type of variation may be useful to interpret unexplained clinical signs and symptoms and provided additional knowledge to surgeons who perform brachial plexus surgery.