Final answer:
The external intercostal muscles raise the rib cage and expand the thoracic cavity during inspiration, allowing air to flow into the lungs. The internal intercostal muscles contract during expiration, decreasing the volume of the lungs and forcing air out.
Step-by-step explanation:
The external and internal intercostal muscles play an important role in respiration. During inspiration, the external intercostal muscles contract, raising the rib cage and expanding the thoracic cavity. This increases the volume of the lungs and creates a pressure gradient that allows air to flow into the lungs. During expiration, the internal intercostal muscles contract, drawing the ribs together and decreasing the volume of the lungs, causing air to be forced out.