What structure separates the thorax from the abdomen?

The diaphragm is the muscular structure that separates the thoracic cavity, which houses the lungs and heart, from the abdominal cavity that contains various organs such as the stomach, liver, and intestines. It plays a crucial role in respiration; when the diaphragm contracts, it moves downward, increasing the volume of the thoracic cavity and allowing air to be drawn into the lungs.

While the sternum, ribs, and spine offer protection and structural support for the upper body and help define the thoracic cavity, they do not provide the functional separation between thorax and abdomen like the diaphragm does. The sternum and ribs are part of the rib cage that protects thoracic organs, and the spine serves as a structural foundation for the back, but they do not directly separate the two cavities. Therefore, the diaphragm is the correct answer as it is the primary structure responsible for separating the thorax from the abdomen.