The walls of the rectum are very vascular and hold a large number of blood vessels. It is mediated by intrinsic neural pathways in the wall of the rectum and anus, mediated by VIP and NO. Stop Wrecking Your Teeth. If the internal opening is very high in the anal canal a rectal advancement flap is performed using a flap of mucosa and the underlying muscle to cover the internal opening. In this procedure, the internal anal sphincter is cut starting at its distal most end at the anal verge and extending into the anal canal for a distance equal to that of the fissure. This relieves spasm of the muscle and also may increase the flow of blood, both of which promote healing of fissures.