Final answer:
Expiration involves the internal intercostal muscles and the transversus thoracis, which work together to decrease thoracic cavity volume and increase pressure, forcing air out of the lungs. The external oblique abdominis assists in forced expiration. The diaphragm, while crucial for inspiration, is not a muscle of expiration as it relaxes during this phase.
Step-by-step explanation:
The muscles involved in expiration, which is the process that causes air to leave the lungs, include the internal intercostal muscles and the transversus thoracis. The internal intercostal muscles help in expiration by drawing the ribs together to constrict the rib cage, while the transversus thoracis assists in depressing the ribs. This action effectively decreases the volume of the thoracic cavity, thereby increasing the pressure within the pleural cavities and forcing air out of the lungs. Another set of muscles, the external oblique abdominis, helps with forced expiration by compressing the abdomen, which also pushes against the diaphragm, causing further decrease in thoracic cavity volume and aiding in expelling air from the lungs. Conversely, the diaphragm, which flattens and contracts during inspiration, does not serve as a muscle of expiration as it relaxes to resume its dome shape, allowing the elastic recoil of the lungs and rib cage to push air out during regular, passive exhalation.