Fig. 1. Schematic diagram of the apparatus used to measure the mechanics of the respiratory system. The joey fitted with a facemask was placed in a water-jacketed chamber to maintain temperature and humidity; the facemask opened to room air. A pneumotachograph (P) was inserted directly into the open end of the facemask and the corresponding flow trace electronically integrated for volume. A vacuum was applied to the compartment enclosing the body to produce negative pressures of 981 Pa, adjusted by altering the size of a leak and monitored by a water-filled manometer. Abdominal displacement was measured using a force transducer with a wire arm resting laterally on the abdominal surface just below the final rib. A moist swab maintained humidity in the chamber.