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the complex of thoracic vertebrae, ribs, and sternum, which provides a firm support in reptiles, birds, mammals, and man for the pectoral girdle and makes possible the use of the intercostal musculature in respiratory movements.
Historically, the rib cage appeared in Amniota in connection with the progressive evolution of their organs of motion and respiration. In mammals the respiratory function of the rib cage is still greater because of the presence of the diaphragm and the formation of the thoracic cavity. In the majority of reptiles, whose trunks touch the ground, the thorax is flattened, its transverse diameter greater than the dorsoven-tral; in mammals and some reptiles (for example, the chameleons), in which the body is elevated by legs, the thorax is flatter laterally and its dorsoventral diameter exceeds the transverse. This shape of the thorax is called primary. In the anthropoid apes, and in man particularly, the primary form changes to secondary, in which the transverse diameter is greater than the dorsoventral. A barrel-shaped thorax with equal transverse and dorsoventral diameters is characteristic of animals that jump on their hind legs (kangaroo and jerboa), fly (birds, bats, and the fossil pterosaurus), or swim (whales and the fossil ichthyosaurs).
In man, the rib cage takes the form of a truncated cone, flattened back to front. The lateral walls are formed by 12 pairs of ribs, separated by intercostal interstices; the front wall includes the ends of the ribs and the sternum; the rear wall has the spine at its center. The rib cage has an opening above called the superior aperture, whose boundaries are the right and left first ribs, the first thoracic vertebra, and the manubrium sterni. Through this opening the trachea, esophagus, vessels, and nerves pass into the thoracic cavity. The inferior aperture is circumscribed by the ends of the ribs. The thorax is separated from the abdominal cavity below by the diaphragm. Rib cages are variously shaped, depending upon the sex, age, and physique of the subject; for example, in men the thorax is more conical and in women, more cylindrical. Rachitic children characteristically present a carinate thorax; in very old persons the thorax is either flattened or, especially with emphysema, it becomes barrel-shaped. Persons of asthenic build have an elongated and flattened thorax; in those of pyknic build the thorax is short and massive. During inhalation the rib cage expands; this is accompanied by an increase in its longitudinal, anteroposterior, and transverse dimensions.
V. V. KUPRIIANOV