During expiration the radial muscles relax to assume the normal form and the diaphragm acquires the original dome shaped appearance. Simulta­neously, the external intercostal muscles relax and the internal intercostal muscles contract so that the ribs are pulled back to their normal position. Hence the volume of the thoracic cavity is reduced. The reduction in the volume of thoracic cavity exerts pressure on the lungs and air is squeezed out of the lungs to the exterior through the respiratory passage.