The thoracic cavity is separated from the abdominal cavity by a sheet of muscle called

The sheet of muscle that separates the thoracic cavity from the abdominal cavity is called the diaphragm. The diaphragm is a large, dome-shaped muscle that sits at the bottom of the rib cage and helps with breathing by contracting and relaxing. When it contracts, it moves downward, which increases the volume of the thoracic cavity and allows the lungs to expand and take in air. When it relaxes, it moves upward, which decreases the volume of the thoracic cavity and helps to expel air from the lungs. Therefore, the diaphragm plays an essential role in the respiratory system and is crucial for breathing.