Chapter 52: Thorax Surface Anatomy

The thorax chapter provides a comprehensive anatomical foundation of the upper trunk, detailing its structure, contents, and clinically relevant surface landmarks. The upper trunk consists of an internal cavity that houses the heart, lungs, esophagus, trachea, main bronchi, thymus, major vasculature, and several critical nerves, including the vagus and phrenic nerves. The external framework is the musculoskeletal thoracic cage, a resilient osseocartilaginous structure comprised of twelve thoracic vertebrae, twelve pairs of ribs with their associated costal cartilages, and the sternum, collectively offering protection to the viscera, including superiorly located abdominal organs like the liver and spleen. Rib articulation varies, with the costal cartilages of the first seven pairs connecting directly to the sternum, while the inferior pairs (eighth through tenth) usually join superior cartilages, and the eleventh and twelfth ribs are free, or floating. The thoracic cavity communicates superiorly via the superior thoracic aperture and is separated inferiorly from the abdominal cavity by the curved musculotendinous respiratory diaphragm. A key anatomical reference is the horizontal sternal plane, which passes through the manubriosternal joint (sternal angle or angle of Louis), serving to divide the mediastinum into superior and inferior compartments, often intersecting the vertebral column between the fourth and fifth thoracic vertebrae. The mediastinum, the central visceral compartment, is further subdivided into anterior, middle (containing the heart within the pericardial coverings, the ascending aorta, and the tracheal bifurcation), and posterior regions (containing the descending thoracic aorta, esophagus, and sympathetic trunks). The thoracic wall musculature includes intrinsic groups, such as the layered intercostal muscles, which are segmentally innervated by the ventral rami of the thoracic spinal nerves. Vascular structures supplying the thoracic wall include the internal thoracic artery, the supreme intercostal artery, and posterior intercostal arteries, the anastomoses of which provide a route for collateral circulation. Innervation is managed by the segmental intercostal nerves and the autonomic nervous system, which includes the sympathetic trunks and vagus nerves forming cardiac, pulmonary, and esophageal plexuses. The surface anatomy of the thorax is emphasized for clinical procedures, noting that contemporary computed tomography (CT) studies frequently reveal significant positional variability in key deep structures, such as the aortic arch and the diaphragmatic domes, compared to conventional anatomical descriptions. The pleural cavities are potential spaces surrounding the lungs, and the reflection of the parietal pleura creates an area adjacent to the sternum, known as the pericardial triangle, which is free of pleura and allows for safe access to the pericardium during certain procedures.