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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. The proximal aspect of the rib has more character than the distal aspect of the rib. Proximally, it forms a ridged head with articular surfaces for the vertebral body. Just distal to the head is a tapered neck region that has a prominent tubercle projecting posterolaterally from its base. Just beyond the neck the rib angles sharply acquiring a flatter profile. This long, flat, gently arching portion of the rib is the costal body. Beginning on the angle of the rib and continuing onto the proximal two-thirds of its body is a noticeable groove. This groove is the distinguishing landmark for the ventral surface of the rib. The body ends simply, forming a flattened end with a rough, oval surface for the attachment of the costal cartilage. This rib is one of the true ribs or vertebrosternal ribs. It receives this name because the costal cartilage unites the rib directly to the sternum.
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 three bones: the correspondingly numbered thoracic vertebra, the next superior thoracic vertebra, and the sternum. The rib forms two articular surfaces with the vertebra of the same segment. One surface is where the head joins the posterolateral aspect of the superior border of the vertebral body, the other is at the tubercle's junction with the anterior apex of the transverse process. The head also forms a second articular surface with the lower margin of the superiorly adjacent vertebral body. Distally, the rib forms a rough, oval surface that articulates with the sternum via the 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 three secondary centers arise. These are small epiphyses corresponding to the head, the articular portion of the tubercle, and the nonarticular aspect of the tubercle. They ossify with the rib proper between the twentieth and twenty-fourth year.