The colon extends from the ileocecal valve to the proximal rectum and is approximately 3–5 f. in length. The terminal ileum enters the cecum on its posteromedial border at the ileocecal valve. The cecum is a large blind pouch approximately 7.5–8.5 cm in diameter which projects from the ascending colon below the ileocecal valve. It is covered by peritoneum and has a narrow diverticulum (the appendix) located in the lower portion. The ascending colon is approximately 20–25 cm in length and is retroperitoneal. The ascending colon ends at the hepatic flexure just under the liver. The transverse colon extends from the hepatic flexure to the splenic flexure and is attached to the gastrocolic ligament. The omentum is attached to the transverse colon on its anterior superior edge. The descending colon is approximately 10–15 cm in length. It is a retroperitoneal structure which begins at the splenic flexure and ends at the sigmoid colon. The sigmoid colon is a redundant loop of colon covered with peritoneum which can be variable in length and location.

The colon progressively diminishes in size from the cecum to the sigmoid colon. The sigmoid colon is approximately 2.5 cm in diameter and is the narrowest portion of the colon. This size discrepancy accounts for the frequent finding that cecal tumors can grow to be large and bulky prior to the onset of symptoms while sigmoid tumors are symptomatic at smaller sizes. In addition, tension on the bowel wall is directly proportional to the diameter of the bowel as explained by Laplace’s Law (T = PR; T, tension in the wall of the bowel; P, internal pressure; R, radius of the bowel). Since the cecum has the largest diameter it is usually the first part of the bowel to rupture due to distal obstruction.

The ascending colon, descending colon, rectum, and posterior surface of the hepatic and splenic flexures are fixed retroperitoneal structures. The cecum, transverse, and sigmoid colon are intraperitoneal and are prone to volvulus because of their location and relative lack of fixation.

The colonic wall consists of mucosa, submucosa, inner circular muscle, outer longitudinal muscle, and serosa. The longitudinal muscle is an incomplete layer and is seen as three bands of muscle called taenia coli . The taenia coli are located 120° apart around the circumference of the colon. They converge proximally at the appendix and disappear as distinct bands at the level of the sacral promontory. Haustra coli are sacculations between the taenia and are separated by crescent shaped folds called plicae semilunares . Appendices epiploicae are fatty appendages covered by peritoneum and have no anatomical or pathologic significance.

The rectum is the last segment of the large intestine and is approximately 12–15 cm in length (Fig. 2.2). It begins at the rectosigmoid junction at the level of the sacral promontory (S3), follows the curve of the sacrum, and ends at the anal canal. Intra-abdominally the rectosigmoid junction is identified as the point where the discrete taenia coli of the sigmoid splay out to form a complete layer of longitudinal muscle in the upper rectum. The rectal wall consists of mucosal, submucosal, inner circular, and outer longitudinal muscular layers. There is no serosal layer in the rectum nor are there haustra which are present in the proximal colon. Folds of the rectal wall are seen endoscopically and are called Valves of Houston . They serve no physiologic function but are useful as landmarks for endoscopy. There are typically three valves and their location within the rectum can be variable. The upper valve is usually found on the left side of the rectum 8–16 cm from the anal verge. The middle valve is typically on the right side of the rectum 7–12 cm and is at the level of the peritoneal reflection. The lower valve is located in the left side 5–10 cm from the anal verge (Shafik et al. 2001).

The mesorectum is a fascial structure which surrounds the perirectal fat, blood vessels, and lymphatics of the rectum. Posterior to the mesorectum is the presacral fascia which covers the presacral venous plexus. The presacral fascial layer fuses with the posterior mesorectal fascia forming the rectosacral fascia or Waldeyer’s fascia below S2-S4. Preservation of these layers is vital to proper surgical technique for proctectomies done for rectal cancer. The peritoneum extends along the anterior wall of the rectum covering one third to two thirds of the anterior rectal wall. Denonvilliers’ fascia is just caudal to the peritoneal reflection. It separates the mesorectum from the prostate and seminal vesicles in men and the vagina in women (Lindsey et al. 2000; Chapuis et al. 2002; Lin et al. 2011).

The anal canal is the terminal structure of the gastrointestinal tract (Fig. 2.2). The surgical anal canal extends from the proximal internal anal sphincter to the anal verge and is approximately 4 cm in length. It is surrounded by the internal anal sphincter and the external anal sphincter. The surgical anal canal extends from the anal verge to the anorectal ring and is more of a functional description than anatomic. The anatomic anal canal extends from the dentate line to the anal verge. The dentate line is an important landmark and is a true mucocutaneous junction located 1–1.5 cm above the anal verge. A 6–12 mm transitional zone exists above the dentate line where the squamous epithelium of the anoderm becomes cuboidal and then columnar epithelium. Columns of Morgagni are 8–14 mucosal folds located just above the dentate line and surround anal crypts. Small rudimentary glands open into some of the crypts. These glands go through the internal sphincter into the intersphincteric groove but do not penetrate the external sphincter. Knowledge of this anatomy is useful in describing the origins of perianal abscesses. The dentate line is a transition point for the blood supply and innervation of the anal canal. The anus is innervated by sympathetic and parasympathetic nerves with no somatic pain fibers proximal to the dentate line. Distal to the dentate line there is somatic innervation which is particularly useful to explain the pain sensation for excisional hemorrhoidectomies and the relative lack of pain for elastic ligation done above the dentate line. The anal verge is the junction between anal and perianal skin. Anal epithelium (anoderm) lacks hair follicles, sebaceous glands, or sweat glands. The anal margin is identified at the border of the anal verge where hair follicles and keratinized epithelium begin.

The anorectal ring is 1–1.5 cm above the dentate line and is the palpable upper border of the anal sphincter complex. The internal anal sphincter, external anal sphincter, and levator ani make up the anal sphincter complex and are vital for maintenance of fecal continence. The internal anal sphincter is a continuation of the inner circular layer of the rectum. It extends beyond the external anal sphincter approximately 1 cm from the dentate line. It is a smooth muscle which is contracted at rest contributing to anal tone. The sympathetic innervation (T11-L2) and parasympathetic innervation (S2-S4) of the internal anal sphincter cause the internal anal sphincter to be contracted at rest (Freckner and Ihre 1976). The external anal sphincter is skeletal muscle innervated by branches of the pudendal nerve and the perineal branch of S4. It too is contracted at rest but it is a voluntary muscle which can be contracted further if necessary. The external anal sphincter wraps around the internal anal sphincter and is a continuation of the levator ani. The levator ani is actually three muscles (the puborectalis, pubococcygeus, and iliococcygeus) which make up the pelvic floor. The puborectalis is a continuation of the external anal sphincter and it creates a U-shaped sling around the anal canal. The muscle is not contiguous in the anterior quadrant but is fixed anteriorly to the pubis. Contraction of the puborectalis pulls the rectum anteriorly and contributes to fecal continence.

The superior mesenteric artery arises from the ventral surface of the aorta below the celiac axis, passes behind the pancreas, and crosses in front of the third portion of the duodenum (Fig. 2.3). The SMA is the central axis around which the midgut rotated during embryogenesis and gives off right and left branches which form the blood supply to the midgut. The right branches include the inferior pancreaticoduodenal artery, the middle colic artery, the right colic artery, and the ileocolic artery. The left branches are the jejunal and ileal branches and number 15–20 (Sagar and Pemberton 1997). The middle colic artery is the first branch of the SMA arising just below the uncinate process of the pancreas. It supplies the transverse colon and communicates with branches of the IMA. The middle colic artery typically branches into a right and left branch in the center of the transverse colon. The left branch forms collaterals with the left colic artery via the marginal artery. The right colic artery supplies the ascending colon but is the most variable of the colic arteries and is present in only 10–40 % of cadavers (Michels et al. 1965; Wenk 1995; Garcia-Ruiz et al. 1996). It may arise directly from the SMA, the middle colic, or the ileocolic artery. The ileocolic artery is the terminal branch of the SMA and supplies blood to the terminal ileum, cecum, and ascending colon.

The inferior mesenteric artery arises from the infrarenal aorta and supplies the descending colon, sigmoid colon, and upper rectum via its left colic, sigmoidal, and superior rectal branches (Fig. 2.3). The left colic artery is the first branch of the IMA and runs upward reaching the splenic flexure where it forms collaterals with the left branch of the middle colic artery. The sigmoid arteries can arise from the left colic artery or the descending branch of the IMA (Goligher 1967). The terminal branch of the inferior mesenteric artery becomes the superior rectal artery which descends in the sigmoid mesocolon and bifurcates at the level of S3. Branches of the superior rectal artery supply the upper and middle rectum. Ligation above the origin of the left colic artery has been considered “high ligation” and below this level “low ligation” (Surtees et al. 1990). It is probably more accurate to describe the artery proximal to the left colic artery as the IMA and caudal as the superior hemorrhoidal artery.