The diaphragm is a thin musculomembranous dome-shaped muscle that separates the thoracic and abdominal cavities. It is attached to the lower ribs at each side, and to the breast bone and the backbone at the front and back. It bulges upwards against the heart and lungs, arching over the stomach, liver, and spleen. There are openings in the diaphragm through which the oesophagus, blood vessels, and nerves pass.

The diaphragm plays an important part in breathing. It contracts with each inspiration, becoming flattened downwards and increasing the volume of the thoracic cavity. With each expiration the diaphragm relaxes and is restored to its dome shape.