Rib

The human rib cage. (Source: *Gray's Anatomy of the Human Body*, 20th ed. 1918.)

In vertebrate anatomy, ribs (Latin costae) are the long curved bones which form the ribcage. In most animals, ribs surround the chest (Latin thorax) and protect the lungs, heart, and other internal organs of the thorax. In some animals, especially snakes, ribs may provide support and protection for the entire body.

Human anatomy

Human beings have 24 ribs (12 pairs). The first seven sets of ribs, known as "true ribs", have their own individual cartilage connections with the sternum. The following three sets are known as "false ribs", these share a common cartilaginous connection to the sternum, while the last two (eleventh and twelfth ribs) are termed floating ribs (costae fluitantes) or vertebral ribs. They are attached to the vertebrae only, and not to the sternum or cartilage coming off of the sternum. Some people are missing one of the two pairs of floating ribs, while others have a third pair. Rib removal is the surgical excision of ribs for therapeutic or cosmetic reasons.

The ribcage is separated from the lower abdomen by the thoracic diaphragm which controls breathing. When the diaphragm contracts, the ribcage and thoracic cavity are expanded, reducing intra-thoracic pressure and drawing air into the lungs.

In other animals

In mammals, one generally thinks of ribs occurring only in the chest. However, during the development of mammalian embryos, fused-on remnants of ribs can be traced in neck vertebrae (cervical ribs) and sacral vertebrae.

In reptiles, ribs sometimes occur in all vertebrae from the neck to the sacrum.

The ribs of turtles are developed into a bony or cartilagenous carapace and plastron.

Fish can have up to four ribs on each vertebra and this can easily be seen in the herring, although not all fish have this many.