Final answer:
The thoracic muscles involved in breathing are the external and internal intercostal muscles, which aid in inspiration and expiration by changing the volume of the thoracic cavity, along with the diaphragm, which controls the pressure within the pleural cavities.
Step-by-step explanation:
The muscles of the thorax that play a significant role in breathing include the external intercostal muscles, the internal intercostal muscles, and the diaphragm. The external intercostal muscles are located laterally on the sides of the body and are responsible for lifting the rib cage upon contraction, which aids in inspiration by expanding the thoracic cavity.
Contrary to that, the internal intercostal muscles are located medially near the sternum, and they draw the ribs together upon contraction, assisting in expiration by constricting the rib cage. Lastly, the diaphragm is a dome-shaped muscle that contracts and flattens to increase the internal volume of the pleural cavities, thus decreasing internal pressure and allowing air to flow into the lungs.