Fig. 6. Carrier's constraint. (A, B) Reptiles such as lizards can only breathe or walk, but not both: air is shunted from lung to lung as the torso swings laterally during a stride. (C, D) Modern mammals, such as this greyhound, pump air in and out of their lungs as they run because they have constrained lateral movements of the rib cage, and the concave-convex bowing of the backbone as the limbs come together (C) and spread (D) during the stride pumps air out (C) and in (D). (Image modified from work of David Carrier.)
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