Final answer:
During inspiration, the diaphragm and external intercostal muscles contract, resulting in the rib cage expanding and moving upward, which allows air to enter the lungs due to the lower pressure inside.
Step-by-step explanation:
During inspiration, the inspiratory muscles of the rib cage contract, causing the rib cage to expand and move upward. This process involves the contraction of the diaphragm and the external intercostal muscles. The diaphragm contracts and moves downward while the external intercostal muscles raise the ribs up and outward, increasing the thoracic cavity's volume. As the thoracic cavity expands, the lung volume also increases and the air pressure within the lungs drops below the atmospheric pressure, causing air to flow into the lungs.