Breathing is caused by the actions of the muscles between the ribs, the external intercostal muscles, and the diaphragm. When air is breathed in, the intercostal muscles move the ribs upward and outward, and the diaphragm is pushed downward, thus taking air into the expanded lungs. If a person needs to take a deeper than normal breath, the diaphragm and external intercostal muscles contract more forcefully. Additional muscles, such as the pectoralis minor and sternocleidomastoid, can also pull the thoracic cage further upward and outward, enlarging the thoracic cavity and decreasing internal pressure. Breathing out is a passive process. The intercostal muscles return to their resting position, returning the thoracic cage to its original size and expelling air from the lungs as a result.