Paradoxical breathing means the deflation of a lung or of a portion of a lung during the phase of inspiration and the inflation of the lung during the phase of expiration. Paradoxical breathing occurs in all air-breathing vertebrates. It depends on the same anatomic factors as the residual air and seems to serve the same purpose. DATA FROM COMPARATIVE PHYSIOLOGY During the evolution of the air-breathing apparatus, provisions were necessary for the dilution of the oxygen of the air,1 and for its saturation with moisture. In the water, where the amphibian ancestors of modern man lived, the concentration of oxygen is less than 1 per cent; in the atmosphere the concentration is nearly 20 per cent. Among the morphologic adaptations to air breathing, the following are of interest: 1. The respiratory passages become long and narrow; a constriction develops in the air tube (the glottis), which narrows in expiration.