Diaphragm

The diaphragm is a domed, internal muscular sheet that separates the abdomen from the thoracic cavity, which houses the heart, lungs, and the ribs. The diaphragm has a very important function in respiration. When the diaphragm contracts, the thoracic cavity increases in volume and air moves into the lungs. At rest, the diaphragm lies at level with the lowest part of the breastbone, also known as the sternum.