Final answer:
Inspiration involves the intercostal muscles raising the rib cage and decreasing pressure in the pleural cavities, leading to air flowing into the lungs as a result of the created pressure gradient.
Step-by-step explanation:
Inspiration occurs when the intercostal muscles move the rib cage upward and outward, which flattens the diaphragm. As the external and internal intercostal muscles contract, they not only raise the rib cage but also increase the volume inside the pleural cavities. This action results in a decrease of pressure within the cavities. Consequently, due to the pressure gradient established between the inside of the lungs and the outside atmosphere, air will flow into the lungs. The external intercostals are responsible for elevating the ribs during normal inspiration while the internal intercostals are primarily involved in expiration but can aid in forced inspiration as well.