The large intestine is a hollow muscular organ that begins at the ileocecal valve and ends at the anus (Fig. 13.1). It includes the cecum, with the attached vermiform appendix; the ascending, transverse, descending, and sigmoid colon; and the rectum. It measures approximately 150 cm (5 feet) in length. The diameter of the large intestine is greater than the small intestine. The cecum has the largest diameter and is nestled in the right iliac fossa. The diameter of the colon decreases as it proceeds distally so that the lumen of the sigmoid is considerably smaller than that of the cecum. In the rectum, the diameter widens again slightly.

The junction of the colon with the rectum is not a precise anatomic point, and it is loosely described as the rectosigmoid. This area lies just below the sacral promontory, approximately 15 cm from the anal verge. The rectum is divided into two parts: An upper part that extends from the third sacral vertebrae to the pelvic diaphragm, and a lower part, or anal canal, that continues down to the anus. The latter is discussed further in Chapter 15.

The ascending colon extends from the cecum to the hepatic flexure and lies retroperitoneally against the right
posterior abdominal wall. It abuts the right lobe of the liver at the hepatic flexure and lies lateral to the gallbladder. The transverse colon is the longest segment of the large intestine, extending across the abdomen from the hepatic flexure to the splenic flexure. It attaches to the stomach by the gastrocolic ligament and contacts the second part of the duodenum, the pancreas, and the spleen. The omentum is attached to its anterior. The descending colon begins at the splenic flexure and lies retroperitoneally along the left posterior abdominal wall, abuts the lateral border of the left kidney, and may have a partial mesentery as it approaches the sigmoid colon in the left iliac fossa. The sigmoid colon lies within the peritoneal cavity and possesses a mesentery that is sometimes called the mesosigmoid or sigmoid mesocolon. The sigmoid colon may rest on the urinary bladder or uterus. As it passes through the peritoneal reflection, the sigmoid colon becomes the rectum. The rectum curves gently downward and anteriorly along the sacrococcygeal concavity onto the pelvic diaphragm, for a distance of about 12 cm. It abuts the prostate or the vagina inferiorly before turning posteriorly and caudad through the pelvic floor to become the anal canal at the dentate line. For most of its path, the colon lies against the posterior abdominal wall, forming a frame around the loops of small intestine.