The so-called membranous septum is the fibrous component of the septal structures within the heart. It is relatively subtle in its appearance, but of considerable significance to the understanding of cardiac function and cardiac disease, both congenital and acquired. Surprisingly, its existence was seemingly unknown until the early decades of the 19th century. At this time, those writing in the English language described it as the "undefended space," recognizing its importance in the setting of its aneurysmal dilation, and as the site of septal defects. By the initial decade of the 20th century, it had come to be recognized as the landmark to the site of atrioventricular bundle. Over the first decade of the 21st century, its clinical significance has been emphasized in the context of transcutaneous replacement of the aortic valve. In this review, we describe our own recent investigations of this fibrous part of the septal structures. At the same time, we provide a glimpse of our anatomic past, explaining how its initial description relied on the observations of young physicians taking their first steps in the investigation of cardiac anatomy.
Keywords: atrioventricular bundle; heart aneurysms; infero-septal recess; septal defects; undefended space.
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