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Caliper motion is a specific type of rib motion that allows the ribs to move away from and toward each other in a lateral (side-to-side) fashion, resembling the action of calipers. This motion is most prominent in the floating ribs, which are the last two pairs of ribs: the eleventh and twelfth ribs.

These floating ribs have minimal attachment to the sternum and are primarily connected to the vertebrae at the back. They are not anchored to the costal cartilage like the other ribs and thus can exhibit this unique caliper motion effectively. Because of their anatomical structure, ribs 11 and 12 are particularly well-suited to moving independently and providing this lateral motion.

In contrast, the other sets of ribs, such as those in the upper and middle portions, are primarily involved in pump handle and bucket handle movements, where the motion is more about elevation and depression, rather than the lateral spreading characteristic of caliper motion. Therefore, the correct recognition of ribs 11 and 12 as exhibiting caliper motion highlights their unique anatomical features and role in thoracic mechanics.