Final answer:
The primary muscles of inspiration are the diaphragm and the external intercostal muscles, which work together to increase the thoracic cavity's volume and allow air to flow into the lungs. The internal intercostal muscles and the abdominal muscles are more involved in expiration, particularly during forced exhalation.
Step-by-step explanation:
The primary muscles responsible for inspiration, or the inhalation phase during breathing, are the diaphragm and the external intercostal muscles. The diaphragm contracts and flattens, increasing the volume of the thoracic cavity and thereby decreasing the pressure within, which allows air to flow into the lungs. The external intercostal muscles help in this process by contracting to raise and expand the rib cage. Contrary to inspiration, the internal intercostal muscles (except for their chondral portion) and abdominal muscles are more involved in the process of expiration, not inspiration.
Pulmonary ventilation is a rhythmic process comprising of inspiration and expiration. During forced exhalation, not normal inspiration, the internal intercostal muscles and abdominal muscles may contract to help push air out of the lungs. The muscles of the pelvic floor and perineum, while important for other functions, do not have a direct role in the process of air inspiration.