Now, as the external intercostal muscles contract, they also increase the space inside our chest cavity. And that's because as these muscles contract, they essentially increase our rib cage size. They move the ribcage outward, and that increases the overall space inside our thoracic cavity, inside the chest cavity. Now, in the same time that the space inside the chest cavity increases, what also increases is the space inside the intraplural space. So the volume inside this entire region that is found between the parietal and the visceral plural, basically increases. Now, let's recall Boils Law.