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.

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