|Classification and external resources|
A cervical rib in humans is a supernumerary (or extra) rib which arises from the seventh cervical vertebra. Sometimes known as "neck ribs", their presence is a congenital abnormality located above the normal first rib. A cervical rib is present in only about 1 in 500 (0.2%) of people; in even rarer cases, an individual may have two cervical ribs.
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 are entrapped between the cervical rib and scalenus muscle.
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.
It is under scientific debate to what degree children born with cervical ribs develop early childhood cancer at a higher rate than the general population. The Hox genes that control the development of cervical vertebrae are believed to play a role in suppressing cancer.
In other vertebrates 
Many vertebrates, especially reptiles, have cervical ribs as a normal part of their anatomy rather than a pathological condition. Some sauropods had exceptionally long cervical ribs; those of Mamenchisaurus hochuanensis were nearly 4 meters long.
In birds, the cervical ribs are small and completely fused to the vertebrae.
Anatomy diagrams 
- Selim, Jocelyn. "Useless Body Parts".
- Galis F (1999). "Why do almost all mammals have seven cervical vertebrae? Developmental constraints, Hox genes, and cancer". J. Exp. Zool. 285 (1): 19–26. doi:10.1002/(SICI)1097-010X(19990415)285:1<19::AID-JEZ3>3.0.CO;2-Z. PMID 10327647.
- Myers, PZ. "Debate on Vertebral variation, Hox genes, development, and cancer". Retrieved 2008-04-16.
- Myers, PZ. "Vertebral variation, Hox genes, development, and cancer". Retrieved 2008-04-16.
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