The thorax is the body cavity, surrounded by the bony rib cage, that contains the heart and lungs, the great vessels, the esophagus and trachea, the thoracic duct, and the autonomic innervation for these structures. The inferior boundary of the thoracic cavity is the respiratory diaphragm, which separates the thoracic and abdominal cavities. Superiorly, the thorax communicates with the root of the neck and the upper extremity. The wall of the thorax contains the muscles involved with respiration and those connecting the upper extremity to the axial skeleton. The wall of the thorax is responsible for protecting the contents of the thoracic cavity and for generating the negative pressure required for respiration. The thorax is covered by skin and superficial fascia, which contains the mammary tissue.