(a) Ribs — The ribs move upwards and outwards due to the contraction of the external intercostal muscles stretched between them thus enlarging the chest cavity all around. (The internal intercostal muscles are relaxed).
(b) Diaphragm — The diaphragm contracts and flattens from the dome shaped outline to an almost horizontal plane and thus contributes to the enlargement of the chest cavity lengthwise.
(c) Abdominal muscles — It helps to increase the size of the thoracic cavity through the motion of the diaphragm which assists in inspiration.