Extra rib. Coloured frontal X-ray (top) of a section through the chest of a 27-year-old patient with a right cervical rib, with an angiogram (bottom) showing the right subclavian artery (thick, white). A cervical rib is a congenital extra rib arising from the seventh cervical vertebra. Here, it is causing thoracic outlet syndrome as it presses on the scalene muscles of the neck, compressing the right subclavian artery and leading to a reduction in blood flow.