In transanal excision, the relevant anatomy consists of the anal canal and rectum within 10 cm of the anal verge, as more proximal tumors are very difficult to remove by conventional transanal techniques. Important anatomical landmarks include the anal verge, which is the distal end of the anal canal with the buttocks effaced. The dentate line is a visible irregular line that separates the columnar epithelium of the rectum from the stratified epithelium of the anal canal whose location within the anal canal is variable. The anal canal refers to the area from the anal verge to the top of the anal sphincter complex. The upper edge of the anal canal is defined by the anorectal ring. Above the anorectal ring, the rectum becomes much more capacious. The anal canal is of varying length depending on the habitus of the patient and can vary in length from 2 to 4 cm. The muscularis propria of the rectum consists of inner circular smooth muscle fibers and an outer layer of longitudinal smooth muscle fibers. The internal anal sphincter is an extension of the circular smooth muscle of the rectum. The distal end of the internal anal sphincter is palpable at the anal verge and defines the inner aspect of the intersphincter plane.

Posteriorly and laterally, the mesorectal fat surrounds the rectum. In women, the vagina is immediately anterior to the muscularis propria above the anal canal. In men, the prostate gland and seminal vesicles are encountered anteriorly above the anal verge. The anterior peritoneal reflexion varies between men and women and can vary according to the habitus of the patient. In general, the anterior peritoneal reflexion lies somewhere between 6 and 9 cm above the anal verge anteriorly and anterolaterally to the rectum.