Flexible endoscopy has significantly decreased the incidence of perforation of the rectum. However, because perforations still occur, one should be aware of the anatomy and relationships

of the rectum and anal canal. As the terminal rectum penetrates the pelvic diaphragm, it makes an approximate right-angled bend. From the standpoint of the endoscopist inserting an instrument into the anus, this bend occurs about 4 cm proximal to the anal verge (here defined as the transition zone where the dry, hirsute, perianal skin changes to the moist, squamous epithelium lining the anal canal). This necessitates directing the tip of the instrument toward the concavity of the sacrum. Immediately anterior to this point of angulation are the median prostate gland and paramedian seminal vesicle in male patients, and the vagina in female patients. In male patients, more proximally, the anterior rectal wall is in contact with the urinary bladder. Still further from the anal verge (about 7.5 cm in males and 5.5 cm in females), the peritoneum is reflected from the anterior surface of the rectum to the posterior surface of the urinary bladder (in males) or the uterus (in females), forming the rectovesical or rectouterine pouch (cul-de-sac of Douglas), respectively. This is the most dependent recess of the peritoneal cavity; thus, it can fill with peritoneal fluid, pus, or loops of bowel.