The external anal sphincter forms a thick band of muscular fibers arranged in three layers running from the perineal body to the anococcygeal ligament. The subcutaneous fibers are thin and surround the anus and, without bony attachment, decussate in front of it. The superficial fibers sweep forward from the anococcygeal ligament, and the tip of the coccyx around the anus inserts into the perineal body. The deep fibers arise from the perineal body to encircle the lower half of the anal canal to form a true sphincter muscle, which fuses with the puborectalis portion of the levator ani.
The ischiorectal fossa is mainly occupied by fat and separates the ischium laterally from the median structures of the anal triangle. It is a fascia-lined space located inferiorly between the perineal skin and the pelvic diaphragm superiorly; it communicates with the contralateral ischiorectal fossa over the anococcygeal ligament. Superiorly, its apex is at the origin of the levator ani muscle from the obturator fascia. It is bound medially by the levator ani and the external sphincter with their fascial covering, laterally by the obturator internus muscle with its fascia, posteriorly by the sacrotuberous ligament and the lower border of the gluteus maximus muscle, and anteriorly by the base of the urogenital diaphragm. It is widest and deepest posteriorly and weakest medially.
An ischiorectal abscess should be drained without delay, or it will extend into the anal canal.
The cavity is filled with fat that cushions the anal canal and is traversed by many fibrous bands, vessels, and nerves, including the pudendal and the inferior rectal nerves. The perforating branch of S2 and S3 and the perineal branch of S4 also run through this space.
The pudendal (Alcock) canal is a tunnel formed by a splitting of the inferior portion of the obturator fascia running anteromedially from the ischial spine to the posterior edge of the urogenital diaphragm. It contains the pudendal artery, vein, and nerve in their traverse from the pelvic cavity to the perineum.
Blood Supply The blood supply to the anal triangle is from the inferior rectal (hemorrhoidal) artery and vein.
Innervation The innervation to the anal triangle is from the perineal branch of the fourth sacral nerve and the inferior rectal (hemorrhoidal) nerve.
Retroperitoneum and The subperitoneal area of the true pelvis is partitioned into potential spaces by the var-
Retroperitoneal Spaces ious organs and their respective fascial coverings and by the selective thickenings of the endopelvic fascia into ligaments and septa (Fig. 5.24). It is imperative that surgeons operating in the pelvis be familiar with these spaces, as discussed below.
Prevesical Space The prevesical (Retzius) space is a fat-filled potential space bound anteriorly by the pubic bone, covered by the transversalis fascia, and extending to the umbilicus between the medial umbilical ligaments (obliterated umbilical arteries); posteriorly, the space extends to the