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A cervical rib in humans is a supernumerary (or extra) rib which arises from the seventh cervical vertebra. Sometimes known as "neck ribs",〔(【引用サイトリンク】url=http://discovermagazine.com/2004/jun/useless-body-parts )〕 their presence is a congenital abnormality located above the normal first rib. A cervical rib is estimated to occur in 0.6% (1 in 150 people) to 0.8% of the population. Individuals may have a cervical rib on the right, left or both sides. Most cases of cervical ribs are not clinically relevant and do not have symptoms; cervical ribs are generally discovered incidentally.〔〔 However, they vary widely in size and shape,〔 and in rare cases, they may cause problems such as contributing to thoracic outlet syndrome, because of pressure on the nerves that may be caused by the presence of the rib. A cervical rib represents a persistent ossification of the C7 lateral costal element. During early development, this ossified costal element typically becomes re-absorbed. Failure of this process results in a variably elongated transverse process or complete rib that can be anteriorly fused with the T1 first rib below.〔 〕 ==Associated conditions== The presence of a cervical rib can cause a form of thoracic outlet syndrome due to compression of the lower trunk of the brachial plexus or subclavian artery. These structures become encroached upon by the cervical rib and scalene muscles. 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. A positive Adson's sign is non-specific for the presence of a cervical rib however, as many individuals without a cervical rib will have a positive test. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cervical rib」の詳細全文を読む スポンサード リンク
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