What muscle forms a sheet lining the bottom of the thoracic cavity and aids in breathing?

What muscle forms a sheet lining the bottom of the thoracic cavity and aids in breathing?

• A. Intercostal muscles
• B. Diaphragm
• C. Larynx
• D. Pharynx

Answer: (B)

The diaphragm is the muscle that forms a sheet across the bottom of the thoracic cavity, separating it from the abdomen. When it contracts, it flattens and moves the central tendon downward, increasing the vertical dimension of the chest. This expansion lowers intrathoracic pressure and draws air into the lungs, making the diaphragm the main muscle of breathing. It also serves as the floor of the thorax, with fibers attaching to the lower ribs, sternum, and spine. The other options don’t form that bottom sheet or drive the primary inspiratory effort: intercostal muscles sit between the ribs to move the chest wall, while the larynx and pharynx are parts of the airway and swallowing pathway rather than bottom-of-thorax muscles. Expiration is mostly passive as the diaphragm relaxes back to its dome shape.

The diaphragm is the muscle that forms a sheet across the bottom of the thoracic cavity, separating it from the abdomen. When it contracts, it flattens and moves the central tendon downward, increasing the vertical dimension of the chest. This expansion lowers intrathoracic pressure and draws air into the lungs, making the diaphragm the main muscle of breathing. It also serves as the floor of the thorax, with fibers attaching to the lower ribs, sternum, and spine. The other options don’t form that bottom sheet or drive the primary inspiratory effort: intercostal muscles sit between the ribs to move the chest wall, while the larynx and pharynx are parts of the airway and swallowing pathway rather than bottom-of-thorax muscles. Expiration is mostly passive as the diaphragm relaxes back to its dome shape.