1. There are seven pairs of true ribs. They are the most superior of the thoracic ribs. They are sometimes called vertebrosternal ribs. They differ from false and floating ribs because they directly articulate with the sternum by means of their costal cartilages. The shortest true rib is rib 1 and their length increases all the way to rib 7. Also, the radius of their curvature increases progressing inferiorly.
2. There are three pairs of false ribs. They are intermediate between the true ribs and the floating ribs. They are sometimes called vertebrochondral ribs. They differ from the true ribs because they do not directly articulate with the sternum. Their costal cartilages join together, however. This fused structure articulates with the costal cartilage of rib seven, and thereby indirectly articulates with the sternum. Many authors include the floating ribs in this group, thereby making five pairs of false ribs. We will consider the floating ribs a mutually exclusive group in order to simplify things.
3. There are two pairs of floating ribs and they are the most inferior of the ribs. They are different from the other ribs of the thoracic region because they do not articulate anteriorly with the sternum or the costal cartilage of other ribs. They do articulate with the thoracic vertebrae 11 and 12 posteriorly. They do have costal cartilage that is imbedded in the muscles of the body wall in the lateral region. They are sometimes called vertebral ribs.
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