Study of the spatial organization of the gas exchange components of a snake lung – the sandboa Eryx colubrinus (Reptilia: Ophidia: Colubridae) – by latex casting

Abstract

The vasculature and the air ways of the lung of the sandboa Eryx colubrinus were cast using latex rubber, corroded, and studied with a scanning electron microscope to determine the shape, topographic configurations, and relative sizes of the gas exchange components. The sandboa had a right lung and a vestigeal left one. The lung, which terminated close to the anus, consisted of two distinctive anatomical regions. The exchange tissue was located in the cranial half of the lung while the caudal one consisted of a transparent avascular ‘air sac’. The right pulmonary artery, which was found on the laterodorsal aspect of the lung, gave rise to branches which supplied blood to the pleura and the faveolar septal walls. The geometric relationship between the flow of the venous blood (from the pulmonary artery) into the parenchymal zone of the lung and the convective/diffusive outwards air flow from the central air duct into the parenchyma is essentially counter-current: the air moves centrifugally and the blood centripetally. However, the arrangement between the air flow in central air duct and that of the venous blood is cross-current (i.e. the two media run in directions perpendicular to each other). These architectural schemes are similar to those that have developed in the avian lung. In fact, in its simplest form, the parenchymal region of the snake's lung corresponds with a single tertiary bronchus (parabronchus) of a bird lung. Further investigations are necessary to identify the factors that enforced this morphological convergence and to verify whether these congruent features are analogous, as they would seem to be, or from a phylogenetic perspective possibly homologous.