The diaphragm is the main muscle that controls breathing. It is a sheet of muscle that runs along the bottom of the rib cage. When the diaphragm is relaxed, it is shaped like a dome (like a half circle). When the medulla tells the diaphragm to make the body breathe in, the diaphragm pulls down and straightens out. This creates more room inside the chest, and more room for the lungs to fill up with air. Air comes into the lungs (this is inhalation). When it is time to breathe out, the diaphragm relaxes again and air leaves the lungs.