94 Colorectal Anatomy

The colon originates in the right lower quadrant as a mucosa, muscularis, and serosa. The mucosa contains the blind pouch, the cecum, into which the ileum dumps its glands of Lieberkühn, lamina propria, and muscularis effluent via the ileocecal valve. The cecum is also identi- mucosa. The submucosa contains lymphatics, blood fied by the appendix, present at the junction of the three vessels, and Meissner’s plexus. The muscular layer is taenia. The colon continues its clockwise course through divided into the inner circular layer, where Auerbach’s the as the ascending colon, hepatic flexure, plexus lies, and the outer longitudinal layer. In the colon transverse colon, splenic flexure, descending colon, and the outer longitudinal muscle layer is condensed into sigmoid colon, which transitions into the . The three bundles, the taenia coli. These bundles spread out total length of the colon is 150cm. Although continuous, to become continuous in the rectum. The outer layer, the the proximal colon to the splenic flexure is embryologi- serosa, is absent in the rectum. The colon is also identi- cally derived from the midgut, and the splenic flexure and fied by the presence of haustra and epiploic appendages. distal colon are hindgut derivatives. This corresponds to The cecum is the widest part of the colon, and the remain- its vascular supply, with the midgut portion supplied by ing colon tapers throughout its length until becoming the the superior mesenteric and the hindgut portion rectum, where it again widens. by the inferior mesenteric artery. The blood supply of the colon is highly variable. The The cecum is an intraperitoneal organ. The ascending first colonic branch of the superior mesenteric artery is colon, however, is fixed to the abdominal wall and has the , which enters the transverse meso- peritoneum on its anterior surface only. The transverse colon and divides into right and left branches. The last colon is the longest segment of colon and is again branch of the superior mesenteric artery (SMA) is the intraperitoneal. Because of its mobility, the positioning of , which divides into an ascending and a the transverse colon within the abdomen is variable. The descending (ileal) branch. More variable is the right colic transverse colon is positioned between the layers of the artery, which may or may not be present. It classically transverse mesocolon, which contains the middle colic arises from the SMA, but it may be a branch of the middle artery. The transverse mesocolon also attaches to the colic or ileocolic .The left colon is supplied by the lower border of the pancreas.The greater omentum fuses inferior mesenteric artery (IMA), which gives rise to the to the anterosuperior portion of the transverse colon.The and multiple sigmoidal arteries.All of the splenic flexure is markedly superior to the hepatic flexure major colic arteries are connected via an anastomosing and is tethered to the diaphragm via the phrenocolic lig- network; the colic arteries all feed the marginal artery of ament. The descending colon is again fixed in position Drummond, which runs along the mesenteric border of and does not have peritoneum on its posterior surface. the entire colon.The arc of Riolan, or meandering mesen- The descending colon then becomes the sigmoid colon, teric artery, is present in 7% of individuals and is an which is again intraperitoneal. Classically described as S arterial loop connecting the superior mesenteric system shaped, the course and length of the sigmoid colon is to the inferior mesenteric system. It may be an important highly variable. Its length can range from 15 to 50cm, and collateral in instances of arterial disease. The superior its mobility allows it to occupy any location in the mesenteric follows the course of the SMA. The infe- or lower abdomen. It runs from the pelvic brim to the rior mesenteric vein receives tributaries from the left sacral promontory, at which point it becomes the rectum. colon, sigmoid, and superior rectal vein. It joins the Like other areas of the gastrointestinal tract, the layers splenic vein, which then joins the superior mesenteric of the colon wall can be divided into the mucosa, sub- vein to become the portal vein.

223 224 Part XII. Gastrointestinal Disorders

Lymphatic drainage follows the arterial supply of the fascia lies anteriorly and separates the rectum from either colon. Colonic lymph nodes are classified as follows: the or . The lateral ligaments of the rectum run from the peritoneal reflection to the levator Epicolic: on the colon or within the epiploices ani and may contain the and pelvic Paracolic: between the colon and the marginal artery splanchnic nerves. Blood supply is via the superior rectal Intermediate: along the major colic arteries artery, which is the terminal branch of the IMA, and the Main/principal: along the SMA and IMA middle and inferior rectal arteries, which arise from the The rectum begins at the sacral promontory and is pudendal artery. characterized by the merging of the taenia. It is 12 to 15 The anal canal runs from the anorectal junction (at the cm in length and has no mesentery, epiploic appendages, level of the levator ani) to the anal verge, approximately or haustra. It contains the valves of Houston at the points 4cm. The musculature of the anal canal can be thought where the rectum curves. The valves of Houston are pro- of as two cylinders. The innermost cylinder is made up of jections of the bowel wall including the circular but not the smooth muscle of the intestinal wall and contains the longitudinal muscle layers. Because the projections are internal sphincter, while the outermost cylinder is skele- not full thickness, biopsy specimens taken from these tal muscle under voluntary control and contains the areas have a lower risk of perforation. The rectum external sphincter. The dentate line is located at 2cm and becomes the anal canal at the level of the levator ani mus- is characterized by the columns of Morgagni and colum- culature. Waldeyer’s fascia is the rectosacral fascia and nar epithelium proximal and squamous epithelium distal. contains branches of the sacral splanchnic nerves and This demarcation is also significant, because sensation is may have branches of the sacral vessels. Denonvillier’s present distal to the dentate line.