Which is the sheet-like muscle at the bottom of a human chest?

The sheet-like muscle at the bottom of the human chest is the diaphragm. It forms a curved boundary between the thoracic cavity, which contains the heart and lungs, and the abdominal cavity below.

The diaphragm is the main muscle of breathing. When it contracts, it moves downward and increases the space inside the chest, lowering pressure so air flows into the lungs. When it relaxes, it rises again, helping push air out. The rib muscles also assist this process.

Although often pictured as a flat sheet, the diaphragm is actually dome-shaped, with a central tendon and muscle fibers attached around the lower ribs, breastbone, and lumbar vertebrae. Its right side usually sits slightly higher because the liver lies beneath it.

A common mix-up is confusing the diaphragm with the pectoral muscles, which lie across the front of the chest and move the arms. The diaphragm is deeper inside the body. Its name can also refer to other structures, but in ordinary anatomy and quiz questions, “the diaphragm” means the thoracic diaphragm.