The dome-shaped muscle that separates the chest from the abdomen and is essential for breathing is the diaphragm.
More precisely called the thoracic diaphragm, it is a sheet of skeletal muscle stretching across the bottom of the thoracic cavity. Its upper surface forms the floor of the chest, while its lower surface forms the roof of the abdominal cavity. The heart and lungs sit above it; many digestive organs lie below it.
When the diaphragm contracts, it flattens and increases the volume of the chest. This lowers pressure inside the thoracic cavity, drawing air into the lungs. When it relaxes, it rises back into a dome and helps push air out. The phrenic nerves control its movement, which is why damage to these nerves can impair breathing.
The diaphragm is often confused with the intercostal muscles, which lie between the ribs and assist with breathing. The diaphragm is the main muscle of respiration, while the intercostals help expand and stabilize the rib cage. It also contains openings for structures including the esophagus, aorta, and inferior vena cava.