Anatomy of the Colon and Rectum
The colon can be divided embryologically into the midgut (the transverse colon and portions proximal to it) and the hindgut (the distal half of the colon). The length of the colon is approximately 150 cm, and its diameter gradually diminishes from the cecum to the rectosigmoid junction. In appearance it is distinguished from the small bowel by its size and by its saccular or haustral look, a function of its unique arrangement of muscular layers. It has a complete inner circular muscle coat, but the longitudinal outer layer is condensed into three bands, the taenia coli. These bands run from the base of the appendix and merge in the distal sigmoid colon, and thus the rectum has a complete longitudinal muscle coat. The colon is also distinguished by the appendices epiploicae, a series of fatty appendages located on its antimesenteric surface.
Course and Peritoneal Coverings
The general topography of the colon varies from person to person ( Fig. 1-1 ). The colon begins in the right lower quadrant of the abdomen where the terminal ileum enters its medial surface at the ileocecal valve. Below the level of the valve is the cecum, the broadest portion of the colon. The vermiform appendix projects from the lowermost part of the cecum, and the appendix orifice is the apex of the large bowel. The cecum is often entirely enveloped by peritoneum. The superior and inferior ileocecal ligaments help maintain the angulation between the ileum and the cecum at the ileocecal valve.
From the ileocecal junction, the colon ascends on the right side of the abdomen to overlie the lower pole of the right kidney, a distance of about 20 cm. The ascending colon is invested by peritoneum on its anterior, lateral, and medial surfaces. The colon then turns acutely medially, downward, and forward, at the hepatic flexure. The transverse colon is the longest segment of the colon (40 to 50 cm), extending from the hepatic to the splenic flexure. It is invested with peritoneum and is mobile on its mesentery, the root of which overlies the right kidney, the second portion of the duodenum, the pancreas, and the left kidney. This posterior relationship is important because these structures are liable to injury during a right hemicolectomy. The stomach is immediately above and the spleen is to the left. The greater omentum arises from the greater curvature of the stomach and descends in front of the transverse colon to which it is attached. To mobilize the greater omentum or to enter the lesser sac, the fusion of the omentum to the transverse colon mesentery must be dissected. Because the omental bursa becomes obliterated caudal to the transverse colon and toward the right side, this dissection should be started on the left side of the transverse colon.
The splenic flexure takes an acute angle high in the left upper quadrant and lies anterior to the midportion of the left kidney. The descending colon (approximately 30 cm) passes downward and posteriorly from the splenic flexure, over the lateral border of the left kidney. The anterior, medial, and lateral portions of its circumference are covered by peritoneum. The sigmoid colon extends from just above the pelvis in the left lower quadrant of the abdomen, forms a loop in or above the pelvis, and becomes the rectum at the level of the sacral promontory. Its length varies dramatically from 15 to 50 cm. The lateral surface of the sigmoid mesentery is fused to the parietal peritoneum of the lateral abdominal wall, and the fusion plane is generally known as the ‘‘white line of Toldt.” The sigmoid is completely covered with peritoneum. The posterior surface is attached to the posterior wall of the abdomen by a fan-shaped mesentery, the base of which extends from the left iliac fossa, along the pelvic brim, and across the sacroiliac joint to the second or third sacral segment; in so doing, it forms the intersigmoid fossa, which serves as a valuable guide to the left ureter, lying just beneath.
The rectum begins at the level of the sacral promontory and ends by passing through the levator ani muscles. The rectum differs from the colon in that the outer longitudinal muscle layer is complete. The rectum measures 12 to 15 cm in length and lacks sacculations and appendices epiploicae. The rectum has three lateral curves: the upper and lower curves are convex to the right, and the middle is convex to the left. On their inner aspect, these infoldings into the lumen are known as the valves of Houston. The middle fold is the internal landmark corresponding to the anterior peritoneal reflection.
Peritoneal Relations and Fascial Attachments
The rectum is divided into upper, middle, and lower thirds. The upper third is covered by peritoneum anteriorly and laterally, the middle third is covered only anteriorly, and the lower third is extraperitoneal. The location of the peritoneal reflection shows considerable variation between individuals and between men and women. In men, it is usually 7 to 9 cm from the anal verge, whereas in women it is 5 to 7.5 cm above the anal verge. The posterior and sometimes the lateral aspects of the rectum are covered by a layer of fat contained by the fascia propria of the rectum. This area is known as the mesorectum, and it contains arteries and veins of the rectal blood supply, along with lymphatic channels and nodes. It is continuous with the sigmoid mesentery and peters out just above the levator muscles. Total mesorectal excision implies the complete excision of all fat enclosed within the fascia propria.
The sacrum and coccyx are covered with a strong fascia that is part of the parietal pelvic fascia, known as Waldeyer fascia. The rectosacral fascia is a component of Waldeyer fascia that runs from the level of the fourth sacral segment to the fascia propria of the rectum. This fascia tethers the posterior rectum to the hollow of the sacrum and should be sharply divided for full mobilization ( Fig. 1-2 ). Anteriorly, the extraperitoneal portion of the rectum is covered with a visceral pelvic fascia, the fascia propria, or investing fascia. Anterior to the fascia propria is a filmy delicate layer of connective tissue known as Denonvilliers fascia. It separates the rectum from the seminal vesicles and the prostate or vagina ( Fig. 1-2 ). The distal rectum, which is extraperitoneal, is attached to the pelvic side wall on each side by the lateral ligaments, which are composed of the pelvic plexus, connective tissues, and minor branches of the middle rectal artery.
The arterial supply of the colon arises from the superior and inferior mesenteric arteries.
Superior Mesenteric Artery
The ileocolic artery is the last branch of the superior mesenteric artery (SMA), arising from its right side and running diagonally around the mesentery to the ileocecal junction. It is always present and has two chief branches: the ascending branch and the descending branch. The origin of the right colic artery varies greatly: it may arise from the SMA, the middle colic artery, or the ileocolic artery, and it is absent in some people ( Fig. 1-3, A ). The middle colic artery normally arises from the SMA either behind the pancreas or at its lower border ( Fig. 1-3, A ). The artery curves toward the hepatic flexure and divides into a right branch that anastomoses with the ascending branch of the right colic artery and a left branch that anastomoses with the ascending branch of the left colic artery.
Inferior Mesenteric Artery
The inferior mesenteric artery (IMA) arises from the abdominal aorta approximately 3 to 4 cm above the aortic bifurcation, about 10 cm above the sacral promontory. The first branch is the left colic artery, arising 2.5 to 3 cm from its origin ( Fig. 1-3, A ). It bifurcates, and its ascending branch courses directly toward the splenic flexure and anastomoses with the left branch of the middle colic artery, whereas the descending branch anastomoses with the sigmoid arteries. The sigmoid arteries usually originate from the IMA; the first sigmoid artery may arise from the left colic artery. The number of sigmoidal branches may vary up to six. The IMA proceeds downward, crossing the left common iliac artery and vein to the base of the sigmoid mesocolon to become the superior rectal artery ( Fig. 1-3, B ). It forms a rectosigmoid branch and an upper rectal branch and then divides into left and right terminal branches. The terminal branches extend downward and forward around the lower two thirds of the rectum to the level of the levator ani muscle. Most middle rectal arteries arise from the internal pudendal arteries (67%). The remainder come from inferior gluteal arteries (17%) and internal iliac arteries (17%). The inferior rectal arteries , which are branches of the internal iliac arteries, arise from the pudendal artery (in the Alcock canal). They traverse the ischioanal fossa and supply the anal canal and the external sphincter muscles.
The marginal artery, generally known as the marginal artery of Drummond, is a series of arcades of arteries along the mesenteric border of the entire colon ( Fig. 1-4 ). The arcades begin with the ascending colic branch of the ileocolic artery and continue distally to the sigmoid arteries ( Fig. 1-4 ). A truly critical point exists at the splenic flexure, where the marginal artery is often small. This ‘‘weak point’’ has the potential to cause compromised blood supply. Another more proximal collateral is the “arc of Riolan,” found in about 7% of the population. It is a short loop connecting the left branch of the middle colic artery and the trunk of the IMA ( Fig. 1-4 ) that serves as critical collateral for patients with a diseased SMA or IMA.
The veins of the intestine follow their corresponding arteries and bear the same terminology.
Superior Mesenteric Vein
The veins from the right colon and transverse colon drain into the superior mesenteric vein (SMV), which lies to the right and in front of the SMA. All ileocolic veins drain into the SMV. The right colic vein, if present, joins the SMV in 56% of persons and the gastrocolic trunk in 44% of persons. The middle colic vein (which is the most variable) and the right colic vein occasionally form a common trunk with the right gastroepiploic vein and/or the pancreaticoduodeinal vein. This common trunk is called the gastrocolic trunk. The middle colic vein drains directly into the SMV in 85% of persons, and the remainder drains into the SMV via the gastrocolic trunk.
Inferior Mesenteric Vein
The inferior mesenteric vein (IMV) is a continuation of the superior rectal vein. It receives blood from the left colon, the rectum, and the upper part of the anal canal. All the tributaries of the IMV closely follow the corresponding arteries but are slightly to the left of them. At the level of the left colic artery, the IMV follows a course of its own and ascends in the extraperitoneal plane over the psoas muscle to the left of the ligament of Treitz. It continues behind the body of the pancreas to enter the splenic vein ( Fig. 1-5 ).
The extramural lymphatic vessels and lymph nodes follow the regional arteries and can be classified into four groups: epicolic, paracolic, intermediate, and main (principal) glands ( Fig. 1-6 ).