The thorax is a bony cage that keeps heart and lungs working
Between neck and abdomen, the thorax—named from a Greek word for breastplate—houses heart, lungs, and thymus behind a rib cage braced by spine and shoulder girdle. In arthropods the same label names a mid-body tagma, but in humans it is the chest cavity whose commonest complaint is pain.
Contents include major vessels such as aorta, venae cavae, and pulmonary artery; airways and esophagus; diaphragm and xiphoid; pectorals and trapezius; and bones from sternum and twelve ribs to scapula and clavicle. Shoulder breadth comes from the girdle more than the thoracic skeleton that cups the viscera. The sternal angle, roughly three fingers below the jugular notch, marks where the second ribs meet and helps count the rest.
Ribs one through seven join the sternum via their own cartilage; eight through ten hitch to the cartilage above as false ribs; eleven and twelve float without anterior sternum ties. Landmarks such as the male nipple near the fourth rib aid clinical mapping. Chest trauma causes up to a quarter of trauma deaths in the United States, often by disrupting air or blood flow or by painful fractures that blunt ventilation.
Pleurisy, flail chest, atelectasis, and cardiac ischemia can all announce themselves as chest discomfort—crushing pressure radiating to arm and neck when myocardium is failing, or sharper musculoskeletal and digestive mimics otherwise. The thorax is therefore both fortress and sensor: a cuirass of bone that still leaves breathing and circulation vulnerable when walls crack or vessels fail.
Across the animal kingdom the word stretches. In mammals the region is framed by sternum, thoracic vertebrae and ribs, running from neck to diaphragm without the arms; birds also have distinct thoracic vertebrae, whereas reptiles do not. In most insects the middle body section splits into three segments: prothorax, mesothorax and metathorax. Adult legs always sprout from the first, yet it never carries wings, which are confined to the second and usually the third. Each segment divides again into a dorsal notum, paired side pleura and a ventral sternum, often built from separate plates called sclerites. Back in humans, one hidden hazard is pneumothorax, where gas collects in the pleural space; if pressure builds unchecked, blood flow can falter.
Source: Thorax
