Thoracic Walls and Muscles of respiration Copynight e The McGraw-Hill Companies. inc Permission requred for re End of expiration Sternocleido End of mastoid inspiration Scalene Labored breathing Additional muscles Muscles Pectoralis Clavicle contract, causing of minor additional expansion Inspira- (cut) of the thorax tion External inter. Internal costals inter- costals Muscles Di phragr Ab expiration dominal uscles Diaphrag Quiet breathing: relaxed The external Ab intercostal dominal muscles contract, muscles elevating the relax ribs and moving the sternum The diaphragm contracts, increasing the superior-inferior b dimension of the thoracic cavity9 Thoracic Walls and Muscles of Respiration