Which muscle is primarily responsible for expanding the thoracic cavity upon contraction?

The thoracic diaphragm is the primary muscle responsible for expanding the thoracic cavity upon contraction. When the diaphragm contracts, it moves downward and flattens, increasing the vertical dimension of the thoracic cavity. This creates a negative pressure within the cavity, allowing air to be drawn into the lungs during inhalation.

Other muscles, such as the external intercostals, assist in the process of inhalation by elevating the ribs, which also contributes to expanding the thoracic cavity. However, the diaphragm plays a crucial role as the main driver of the inhalation process due to its significant ability to alter the volume of the thoracic cavity. The transversus thoracis primarily aids in forced expiration, and the internal intercostals are mainly involved in exhalation, further distinguishing the diaphragm's role in expanding the thoracic cavity.