Thorax (Gr. θώραξ), the chest, or upper part of the trunk of the body, situated between the neck and the abdomen. The osseous framework of the thorax consists of the 12 dorsal vertebrae behind, the sternum or breast bone in front, and the ribs upon each side. The spaces between the ribs are occupied by the intercostal muscles, external and internal. The floor or inferior wall of the thorax is constituted by the diaphragm, a vaulted muscular sheet, with a central tendinous expansion, the convexity of which looks upward toward the chest, while its concavity looks downward toward the abdomen. The general form of the thorax is conical, with a broad base below, and presenting at its upper extremity a comparatively narrow and nearly circular opening, bounded on each side by the curved borders of the first pair of ribs, through which the trachea, oesophagus, nerves, and blood vessels pass from the neck into the chest, or vice versa. The framework of the chest thus constituted has a considerable degree of elasticity, owing to the curved form of the ribs themselves, and to the fact that they are articulated with the sternum by elastic cartilages of variable length, oblique in position.

Owing to the mobility of the ribs at their vertebral articulations, they are capable of being rotated outward and elevated by the action of the intercostal muscles, and of thus expanding the cavity of the chest laterally. The expansion also takes place at the same time in an antero-posterior direction, since the sternum itself is elevated by the movement of the ribs and carried further away from the spinal column; in this way is produced the alternate rising and falling of the walls of the chest during respiration. By the contraction of the diaphragm the central tendon of this muscle is drawn downward at the moment of inspiration, thus also expanding the chest vertically. - The cavity of the thorax has a general conoidal form, corresponding with its external configuration. It is partially divided, however, into lateral halves by the forward projection of the bodies of the dorsal ver-tebrte, leaving on each side of the spinal column a tolerably deep rounded groove or furrow. The principal organs contained in the cavity of the thorax are the heart, lungs, and great blood vessels. The heart is situated nearly in the median line, with its point directed downward and toward the left side, and the lower part of its inferior and posterior surface resting upon the central tendon of the diaphragm.

The superior and inferior vena) cava?, the aorta, the pulmonary artery, and the pulmonary veins are connected with the base of the heart, mostly about the level of the junction of the third costal cartilage with the sternum. The lungs are on each side, moulded to the form of the cavity in which they are contained, and partly lapping over the heart and great vessels in an anterior view. The trachea enters the cavity of the chest behind the superior vena cava and the arch of the aorta, and the oesophagus is situated still further backward, in immediate contact with the spinal column. The phrenic nerve passes down on each side, between the subclavian artery and vein, in front of the root of the lung, between the pleura and pericardium, to be distributed to the muscular tissue of the diaphragm. The pneumogastric nerves descend behind the roots of the lungs, where they give off their pulmonary branches, and then accompany the oesophagus through an opening in the diaphragm to the stomach. The thoracic portion of the great sympathetic nerve is on each side of the spinal column, as a chain of ganglia, each ganglion resting upon the head of a rib, and connected by nervous filaments with those above and below.

Cavity of the Thorax in Man, opened anteriorly, showing the internal organs..

Cavity of the Thorax in Man, opened anteriorly, showing the internal organs. a,b, c. Upper, middle, and lower lobes of the right lung, d, e. tipper and lower lobes of the left lung. f. Heart, g. Pulmonary artery, h. Pulmonary vein. i. Aorta, k. Superior vena cava. l Upper surface of the diaphragm, m. Lower extremity of the sternum, n. Trachea.