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It has been suggested that anal fissure is an ischaemic ulcer caused by a combination of poor blood supply to the posterior midline of the anal canal and spasm of the internal anal sphincter. This study investigated the topographical distribution of blood supply to quadrants of the anal canal above and below the dentate line.

Cadaveric anal canals were removed and 1‐cm blocks were cut above and below the dentate line. Blocks were sectioned at 10 µm and every 25th section was mounted. Using the technique of systematic random sampling, fields in the subanodermal space and the internal anal sphincter in posterior, lateral and anterior quadrants of the anal canal were chosen. The numbers of small arterioles in each field were counted. Mean counts were compared for both subanodermal space and internal anal sphincter between quadrants and levels above and below the dentate line …