134. Fill in the blank spaces for the following paragraph related to the mechanics of breathing Normal inspiration is caused by of the and the external intercostal muscles. This causes on in the intrathoratic volume and therefore a reduction in the intrathoracic The reduction in intrathoracic pressure creates a pressure gradient between the atmospheric air and the thus resulting in air movement into the Expiration is basically the reverse process of which occurs upon relaxation of the inspiratory muscles due to the "elastic recoil" of the and