The muscular dome that forms the inferior boundary of the thorax, separating the thorax from the abdomen.

• A. Costovertebral angle
• B. Costal arch
• C. Diaphragm
• D. Mediastinum

Answer: (C)

The diaphragm is the muscular dome that marks the inferior boundary between the thorax and the abdomen. It sits as the floor of the thoracic cavity and the ceiling of the abdominal cavity, with a central tendon and surrounding muscular portions. When it contracts, the dome flattens and moves downward, increasing the vertical dimension of the thoracic cavity to draw air into the lungs. The other terms describe non-muscular borders or different thoracic structures: the costovertebral angle is a posterior-lateral landmark at the junction of rib and vertebra, the costal arch is the curved lower edge of the rib cage, and the mediastinum is the central compartment within the thoracic cavity. Thus, the muscular dome separating thorax from abdomen is the diaphragm.