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

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 electr on microscope to determine the shape, topographic configurations, and relat ive 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, consist ed of two distinctive anatomical regions. The exchange tissue was located i n the cranial half of the lung while the caudal one consisted of a transpar ent avascular 'air sac'. The right pulmonary artery, which was found on the laterodorsal aspect of the lung, gave rise to branches which supplied bloo d to the pleura and the faveolar septal walls. The geometric relationship b etween the flow of the venous blood (from the pulmonary artery) into the pa renchymal zone of the lung and the convective/diffusive outwards air flow f rom the central air duct into the parenchyma is essentially counter-current : the air moves centrifugally and the blood centripetally. However, the arr angement between the air flow in central air duct and that of the venous bl ood is cross-current (i.e. the two media run in directions perpendicular to each other). These architectural schemes are similar to those that have de veloped in the avian lung. In fact, in its simplest form, the parenchymal r egion of the snake's lung corresponds with a single tertiary bronchus (para bronchus) of a bird lung. Further investigations are necessary to identify the factors that enforced this morphological convergence and to verify whet her these congruent features are analogous, as they would seem to be, or fr om a phylogenetic perspective possibly homologous.