→Macroscopic examination of the lateral wall of the anal canal. (A) The lateral wall of the anal canal is shown, highlighting the rectal and anal canal mucosa and the identification of the pectinate line (PL; arrows). (B) After the removal of the mucosa, the circular muscle (CM) is visible in the rectum, and Treitz muscle (asterisks) is observed in the anal canal. Treitz muscle originates from the CM (internal anal sphincter [IAS]) at the level of the anorectal angle, above the pectinate line (dashed arrows), as a result of a directional shift in the muscle bundles. This shift occurs in the anterolateral wall of the anorectal canal at the level of the anorectal angle. (C) Following removal of Treitz muscle, the full extent of the IAS is revealed, with no longitudinal fibers penetrating the IAS toward the lumen. EAS, external anal sphincter; LM, longitudinal muscle.