Which muscle acts as a separator between the thorax and abdomen and plays a role in breathing?

The diaphragm is a dome-shaped muscle that serves as a crucial separator between the thoracic cavity, which houses the lungs and heart, and the abdominal cavity, which contains the digestive organs. Its primary role in breathing is to facilitate inhalation and exhalation. When the diaphragm contracts, it moves downward, increasing the volume of the thoracic cavity and allowing air to flow into the lungs. Conversely, when it relaxes, the diaphragm rises, reducing the thoracic volume and helping to expel air from the lungs. This unique action makes the diaphragm the main muscle involved in respiration, particularly during quiet breathing.

While other muscles, such as the intercostal muscles, assist in the breathing process, they do not provide the same level of separation between the thorax and abdomen or play the primary role in inhalation and exhalation as the diaphragm does. The sternocleidomastoid and abdominal muscles also contribute to various movements and functions in the body, but they are not primarily responsible for the separation of the thorax and abdomen or the mechanics of respiration like the diaphragm is.