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Perry's (1992) model for the origin of bird lungs, showing the relationship to the crocodilian lung

Archosaur and theropod lungs are hypothetical constructs. Extinct groups are marked with a "+", and perforations between chambers are marked with "*." Perry proposes that these perforations play a crucial role in the stepwise evolution of the avian parabronchi, as indicated in the detail sketches of theropod and avian-grade lungs. CrC and CaC are cranial [forward part of the trunk] and caudal [rearward part of the trunk] chambers, which are connected with the respective regions of the intrapulmonary bronchus. MvB and MdB are avian medioventral bronchi and mediodorsal bronchi, which are proposed to evolve from CrC and CaC, respectively, as indicated by small arrows.

From Figure 6, p. 161 of: Perry, Steven F. (1992). "Gas exchange strategies in reptiles and the origin of the avian lung". Physiological Adaptations in Vertebrates. Wood, S. C., Weber, R. E., Hargens, A. R. and Millard, R. W., Eds. New York, Marcel Dekker: 149–167.