The complex pattern of air flow in the respiratory system of birds suggests that certain sites function as valves. To examine the possibility of mechanical valving, rather than aerodynamic valving, we recorded radiographic images of the orifices where the medioventral secondary bronchi branch from the primary bronchus in resting Pekin ducks. Analysis of the images indicated that the orifices do not change size or shape during the respiratory cycle, suggesting that they function as aerodynamic rather than mechanical valves in directing air flow through the lung. © 1981.
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