Anterior view of the muscles involved in respiration. The primary job of the thorax is to promote the movements necessary for breathing. Three muscle groups of the thorax assist in this function: the external intercostals, internal intercostals and the diaphragm. The intercostals lift the ribs up and pull them outward, which enlarges the lungs. As the lungs expand, the pressure inside them is reduced, and they suck in air. During inhalation, the diaphragm contracts and pushes downward; during exhalation, it relaxes and is pushed up into a dome shape by the lower digestive organs, compressing the lungs.