Intercostal muscles are muscle groups that are situated in between the ribs that create and move the chest wall. The muscles are broken down into three layers and are primarily used to assist with the breathing process. The three layers are: external intercostal muscles, internal intercostal muscles, and the innermost intercostal muscles. These muscles are innervated and supplied with blood by the intercostal nerves, intercostal veins, and intercostal arteries. The other muscles are the scaleni, subcostal, and transversus thoracis. The external intercostal muscles are responsible for forced and quiet inhalation. They raise the ribs and expand the chest cavity, and originate from ribs one through 11, with insertion from ribs two to 12. The internal intercostal muscles are responsible for forced exhalation. They depress the ribs and decrease space in the chest cavity. They originate from ribs two through 12, with insertion from ribs one to 11. The innermost intercostal muscles are the deepest of the intercostal muscles. They are separated from the internal intercostal muscles by a grouping of nerves and blood vessels known as the nuerovascular bundle.