Thoracic Cavity

The diaphragm, a dome-shaped sheet of striated muscle, divides the anterior body cavity into two parts. The area below the diaphragm, the abdominopelvic cavity, contains the liver, pancreas, gastrointestinal tract, spleen, genitourinary tract, and other organs. Above the diaphragm, the thoracic cavity contains the heart, large blood vessels, trachea, esophagus, and thymus in the central region, and is filled elsewhere by the right and left lungs.

The structures in the central region—or mediastinum—are enveloped by two layers of wet epithelial membrane collectively called the pleural membranes. The superficial layer, or parietal pleura, lines the inside of the thoracic wall. The deep layer, or visceral pleura, covers the surface of the lungs (fig. 16.8).

The lungs normally fill the thoracic cavity so that the visceral pleura covering each lung is pushed against the parietal pleura lining the thoracic wall. There is thus, under normal conditions, little or no air between the visceral and parietal pleura. There is, however, a "potential space"—called the intrapleural space— that can become a real space if the visceral and parietal pleurae separate when a lung collapses. The normal position of the lungs in the thoracic cavity is shown in the radiograph in figure 16.9.

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