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Rib 12




Ribs are long, slender flat bones. They fall under the classification of flat bone rather than long bone because they do not have an epiphysis with a medullary cavity. The bone consists of an outer shell of compact bone with a trabecular center. It has a bow-like shape, but is much less arched than the ribs superior to it. The twelfth rib can be longer than the first rib but it shows much variation. Often it is the shortest of the ribs. The proximal aspect of the rib is much simpler and less distinct than the more superior ribs. The proximal end forms a distinct flat surface for articulation with the twelfth thoracic vertebra. It lacks a distinct tubercle and does not articulate with the transverse process of the vertebra. The neck and angle are not distinct. The body ends simply as a blunt point to which a short costal cartilage is attached. A costal groove is absent. This rib is one of the false ribs and is called a floating rib because its cartilage does not attach to the cartilage of the rib above.


The word comes from the Old English word ribbe and this derives from the Anglo-Saxon word ribb. The words had the original meaning of a beam or strip of something. They are closely related to the term ribbon meaning, a narrow band. These are the beams of the chest wall, similar in appearance to the ribs or beams of a boat's walls. The Latin equivalent of rib is costa. Costa is an ancient term that Celsus first used to describe these bones around 30 A.D.


The rib articulates with one bone: the correspondingly numbered thoracic vertebra. The rib forms one articular surface with the vertebra of the same segment. This surface is where the head joins the posterolateral aspect of the vertebral body, often attaching on the side of the pedicle. Distally, the rib forms a blunt point for the attachment of a short piece of costal cartilage.


Ossification of the rib begins in a primary center during the last portion of the second embryonic month. This center arises near the angle of the rib spreading both proximally and distally. Between the fifteenth and twentieth year one secondary centers arise. This is a small epiphysis corresponding to the head of the rib. It ossifies with the rib proper between the twentieth and twenty-fourth year.


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