Supernumerary rib which arises from the seventh cervical vertebra. It is a congenital abnormalitys present in 1 in 200 (0.5%) of people. The presence of a cervical rib can cause a form of thoracic outlet syndrome due to compression of the brachial plexus or subclavian artery. Compression of the brachial plexus may be identified by weakness of the muscles around the muscles in the hand, near the base of the thumb. Compression of the subclavian artery is often diagnosed by finding a positive Adson's sign on examination, where the radial pulse in the arm is lost during abduction and external rotation of the shoulder.