Definition of Rib, fluctuating

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Rib, fluctuating: One of the last two ribs. A rib is said to be "fluctuating" if it does not attach to the sternum (the breast bone) or to another rib.

There are usually 12 pairs of ribs in all. Each pair of ribs is attached to the building blocks of the spine (the vertebrae) in the back. The 12 pairs of ribs consist of:

• True ribs: The first seven ribs attach to the sternum (the breast bone) in the front and are known as true (or sternal) ribs.
• False ribs: The lower five ribs do not directly connect to the sternum and are known as false ribs.

The upper three false ribs connect to the costal cartilages of the ribs just above them. The last two false ribs, however, usually have no ventral attachment (no anchor at all in front) and are called fluctuating, floating, or vertebral ribs.