Beneath the anal canal mucosa (above the pectinate line) lies the internal hemorrhoidal plexus of veins, which drains into the portal system of veins. 4 cm (range 3. The average length of the surgical anal canal was 4. The middle rectal artery (a branch of the internal iliac artery) and the inferior rectal artery (a branch of the internal pudendal artery) supply the lower anal canal. The anal canal just above the pectinate line for about 1-2 cm is called the anal pecten or transitional zone.
The anorectal flexure is formed by the puborectalis (the innermost fibers of levator ani muscle, which extends from the pubic bone, obturator fascia, and ischial spine to the coccyx and anococcygeal ligament) and the upper ends of the external and internal anal sphincters. During posterior or lateral sphincterotomy for fissure in ano, it is only the internal sphincter that is divided.