The normal submucosa of the anorectal junction is markedly thickened, and in section has the appearance of a cavernous (erectile) tissue, owing to the presence of the sacculated veins of the internal rectal venous plexus. The vascular submucosa is especially thickened in the left lateral, right anterolateral, and right posterolateral positions, forming anal cushions, or threshold pads, at the point of closure of the anal canal. Because these cushions contain plexuses of saccular veins capable of directly receiving arterial blood via multiple arteriovenous anastomoses (AVAs), they are variably pliable and turgid, and form a sort of flutter valve that contributes to the normally water- and gas-tight closure of the anal canal.