Morphological characterization of the muscular ridge in non-crocodilian reptiles

Abstract: The cardiovascular systems in ectothermic reptiles have long been studied and its development represent a transition state between a single- and a double circulation. This literature review focus on the typical-reptilian heart and therefore distinguishes between orders within the class of Reptilia. The typical-reptilian heart is found in the non-crocodilian reptiles referring to the orders Chelonia and Squamata. The heart is composed of two atria and one common ventricle, compartmentalized into three interconnected ventricular cava. However, there is some species-specific variation in gross anatomy where the monitor lizard Varanus and Python pose a striking exception to the typical-reptilian heart. The cardiac building plan exhibits variation in size and relative dimensions, in particular the relationship between cava and the degree of ventricular septation. Varanus and Python have a larger and more muscular systemic side of the ventricle, a highly developed vertical septum and muscular ridge. Today, it is known that a ventricle with a less developed muscular ridge acts as a single pressure source, subsequently allowing admixture of oxygenated- and deoxygenated blood within the ventricle. In contrast, a ventricle with a more developed muscular ridge and vertical septum may function as a dual pressure source during parts of systole, temporarily separating the ventricle in a high-pressure systemic side and a low-pressure pulmonary side. Septal development provides different haemodynamic conditions and yield diverse functions. This literature review is a combination of practical laboratory work and a literature study. Dissections of the typical-reptilian hearts provide information about exterior and interior morphology, and findings are then compared to existing literature.