rib

rib, one of the slender, elongated, curved bones that compose the chest cage in higher vertebrates. Ribs occur in pairs, and are found in most vertebrates; however, in some lower vertebrates, including fishes, they run along the entire length of the backbone. The ribs of the snake are used in locomotion. In the human there are 12 pairs of ribs. Each rib is connected to the vertebral column by strong ligaments. In the front, a flexible section of cartilage connects the rib to the sternum, or breastbone. Below the 7th rib, the 8th, 9th, and 10th ribs are not attached directly to the sternum, but to the cartilage of the 7th rib. The 11th and 12th pairs of ribs are not attached in front at all, and hence are known as floating ribs. Technically, these ribs do not "float," however, but are attached to the vertebral column in the rear and extend only part of the way around the chest. In birds and mammals, ribs enclose the lungs and heart and assist in the process of breathing. During inhalation the ribs move upward and farther apart, expanding the chest cavity. During exhalation their downward motion aids in expelling air from the lungs. See skeleton.

Ancient city ruins, north-central Yemen. The ancient fortified city of Mahamzahrib was the centre of the pre-Islamic state of Sabahamzah (950–115 BC). It was located on one of the caravan routes between the Mediterranean Sea and the Arabian Peninsula, and it prospered through its trading monopoly on frankincense and myrrh. The ancient Mahamzahrib Dam was built circa 7th century BC to regulate the waters of the Wadi Sadd; some 1,800 ft (550 m) long, it irrigated more than 4,000 acres (1,600 hectares) and supported a densely settled agricultural region. The dam was destroyed by the 7th century AD and possibly earlier.

Learn more about Mahamzahrib with a free trial on Britannica.com.

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", are directly attached to the sternum through the costal cartilage. 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.

See also

• Bone terminology
• Terms for anatomical location
• Ribs (food)

References

• Clinically Oriented Anatomy, 4th ed. Keith L. Moore and Robert F. Dalley. pp. 62-64