Bilateral variation of subclavius muscle resembling subclavius posticus.

During routine anatomical dissection, a bilateral variation of the muscle subclavius was discovered with additional morphological features consistent with the muscle subclavius posticus. Attached medially to the first rib by tendon and to the clavicle by fibrous bands, the long triangular shaped muscle ran dorsal-laterally to attach onto the transverse scapular ligament and the coracoid process. The scapular insertion of the omohyoid muscle was just medial to this muscle, with some intermingling of fibers close to their insertions. In addition, the muscle on the left was split into two heads before inserting onto the scapula. Innervation was supplied by the nerve to the subclavius. We believe this muscle to be a hybrid of subclavius and subclavius posticus muscles, and we discuss the developmental origin of this rare subclavius variant, as well as the potential role this muscle may play in the development of thoracic outlet syndrome.