The Thoracic Diaphragm is an extremely important part of the respiration system. It is a mostly-solid convex sheet of muscle stretching across the entire length and breadth of the torso, inside the ribcage. In its resting state, the diaphragm is stretched upward, allowing the internal organs more space below, with the lungs at a certain level of compression. When the diaphragm is expanded, either unconsciously or deliberately, it causes the space of the thoracic cavity to be increased, which helps pull air into the lungs via negative pressure. This movement pushes down on the internal organs, and then is reversed into a contraction to shrink the thoracic cavity and forcefully expel air from the lungs.
The lungs can also be filled by movement of the intercostal muscles, which fill the space between the ribs; this expands the thoracic cavity as well, but horizontally instead of vertically.