The anus consists of a mucosa-lined anal canal and an epidermis-lined anal margin. The proximal end of the anal canal begins anatomically at the junction of the puborectalis portion of the levator ani muscle and the external anal sphincter, and extends distally to the anal verge, a distance of approximately 4 cm. The anal canal is divided by the dentate line, which overlies the transition from glandular (columnar) to squamous mucosa that is often referred to as the transitional zone. The anal margin begins approximately at the anal verge, which corresponds to the introitus of the anal orifice. It represents the transition from the squamous mucosa to the epidermis-lined perianal skin, and extends to the perianal skin.

anatomy images (here, here, here, here, here, male, female)

PET scans are very good at showing the cancer and particularly the lymph node spread (here, here, here, here, here)