Final answer:
The external intercostal muscles run from infero-lateral to supero-medial, aiding in inspiration by elevating the rib cage. The internal intercostals run in the opposite direction, infero-medial to supero-lateral, and help in expiration by contracting to draw the ribs together.
Step-by-step explanation:
The external intercostal muscles run from the lower border of one rib infero-lateral to the upper border of the rib below in a direction that is supero-medial (from the bottom and outside closer to the front and upwards towards the midline). Conversely, the internal intercostal muscles run from the lower border of a rib infero-medial to the upper border of the rib below in a more supero-lateral direction (from the bottom and towards the midline, upward and out toward the sides). The external intercostals are integral for inspiration as they help raise the rib cage and expand the thoracic cavity, while the internal intercostals assist in expiration by drawing the ribs together, thus constricting the rib cage. The intercostal muscles, including the innermost intercostals, span the intercostal spaces and are vital for efficient breathing by adjusting the volume and pressure within the pleural cavities.