Rectum, anatomy of the terminal part of the large intestine. On this cut, we see the structures of the rectum and anus. These structures are formed at the top of folds (rectal valves) and bulbs, lower of columns delimiting the valves limited at the bottom by a horizontal line, the ano-cutaneous line which delimits the anus at the bottom. Demonstration of the different muscular structures constituting the rectum with the outermost at the bottom the levator ani below the external sphincter muscle of the anus and more inside the internal sphincter of the anus. The entire rectum is largely supplied by the rectal venous plexus and down the lower rectal veins. This is where hemorrhoids form.