AbstractThe anatomy of the internal anal sphincter and surrounding structures was investigated in 24 cadavers using a surgical microscope (6–25× magnification). An understanding of the anatomy of the internal anal sphincter is helpful in avoiding complications during surgical procedures in the anorectal region. The external anal sphincter was composed of three ellipsoid rings of skeletal muscle (subcutaneous, superficial, and deep) that encircle the anal canal; in contrast, we found that the internal anal sphincter was composed of flat rings of smooth muscle bundles stacked one on top of the other, like the slats of a Venetian blind. In each anal canal, the average number of ring‐like slats observed was 26.33 ± 2.93 (range = 20–30) and each was covered by its own fascia. The smooth muscle fibers and fascia coalesced at three equidistant points around the anal canal to form three columns that extended distally into the lumen and differed in form from the other anal columns. When viewed from an anterior position, the columns were located anteriorly at the observer's right (5 o'clock position), posteriorly at the right (1 o'clock position), and laterally at the left (9 o'clock position). This heretofore unreported anatomy of the internal anal sphincter may play an important role in closing off the lumen of the anal canal and maintaining bowel continence. Clin. Anat. 17:17–20, 2004. © 2003 Wiley‐Liss, Inc.