Anatomy of the Colon, Rectum, and Anus

The Structure and Histology of the Colon and Sigmoid Colon

The large intestine varies in caliber depending on its functional state. Haustra form sacculations that are separated by constricting furrows, so that the lumen bulges and contracts alternately. The caliber is greatest at the commencement of the large intestine (cecum) and narrows toward the rectum. Viewed as a whole, the various parts of the large intestine describe a horseshoe-shaped arc ( Fig. 76.1 ). The total length of the large intestine is approximately 120 to 150 cm (4–5 ft). The four segments of the colon are known as the ascending, transverse, descending, and sigmoid colon. The ascending and descending colon are situated retroperitoneally, and the transverse and sigmoid colon are situated intraperitoneally.

The ascending colon averages approximately 15 to 20 cm (6–8 inches) in length and runs in a more or less straight course from the upper lip of the ileocecal valve to the right colic or hepatic flexure, where it passes into the transverse colon. The right colic flexure is usually on the undersurface of the right lobe of the liver. The transverse colon, varying from 30 to 60 cm in length, extends from the hepatic flexure to the left colic or splenic flexure, situated slightly more cranially. It lies intraperitoneally and thus is attached to the posterior abdominal wall by a peritoneal fold (mesentery) and the transverse mesocolon, which is very short in the region of the flexures and longest in the middle of the transverse colon. The retroperitoneal descending colon, approximately 20 to 25 cm in length, extends downward from the left colic flexure to the iliac crest or beyond it into the left iliac fossa. After running from the angle between the lateral edge of the kidney and the quadratus lumborum muscle and then over the iliac muscle, the colon finally passes in front of the psoas major, crossing the femoral and genitofemoral nerves, and continues with no sharp dividing line into the pelvic colon, or sigmoid colon, at which point the colon becomes intraperitoneal again. On its anterior surface, the descending colon is overlapped by the greater omentum and generally by coils of the small intestine.

Corresponding to the structure of the entire intestinal tract, the wall of the colon and cecum consists of a mucosa, a submucosa, a double-layered muscularis, and—depending on its relation to the peritoneum—a serosa and subserosa or an adventitia. The external aspects of the colon, however, differ from those of the small intestine, not only because of its greater caliber but also owing to the appearance of three typical formations: (1) the three taeniae, (2) the haustra, and (3) the appendices epiploicae. The three taeniae are longitudinal bands, approximately 8 mm in width, running along the total length of the colon; they exist because the outer muscle layer (i.e., longitudinal muscle) does not constitute a uniform coat. In the region of these three bands, the longitudinal musculature is conspicuous by its thickness, whereas in the spaces between them, it consists merely of a very thin coating. Each taenia is named by reference to its topographic situation in relation to the transverse colon. The taenia mesocolica is situated dorsal to the transverse colon at the line of attachment of the transverse mesocolon and comes to lie dorsomedially on the ascending and descending colon. The taenia omentalis is related to the line of attachment of the greater omentum on the ventrocranial surface of the transverse colon and runs along the dorsolateral aspect of the ascending and descending portions. The taenia libera is free (not related to any mesenteric or omental attachment) and generally found on the caudal (inferior) surface of the transverse and on the interior aspect of the ascending and descending colon. Where the appendix joins the cecum and where the sigmoid passes into the rectum, the three taeniae merge into one uniform muscle coat, which in the proximal rectum is more strongly developed at its anterior and posterior parts than laterally. Generally the posterior, lateral, and anterior taeniae coalesce into a broad longitudinal band in the region of the middle and lower sigmoid.

The haustra are more or less prominent sacculations formed in the spaces between the taeniae. They are separated from each other by constricting circular furrows of varying lengths. The degree of their prominence depends on contraction of the taeniae; the more the taeniae contract, the more marked the haustra intestine becomes, whereas it is almost completely absent when the taeniae are totally relaxed.

The third structural characteristic, the appendices epiploicae, consists of grape-shaped subserosal pockets filled with fat and varying in size according to the patient's nutritional state. On the ascending and descending colon, the epiploic appendices are generally distributed in two rows, whereas on the transverse colon, they form only one row along the line of the taenia libera. These fat pads can become extremely large in obese patients.

Corresponding to the furrows between the haustra, visible on the outer surface, the mucous membrane of the large intestine forms crescent-shaped transverse folds known as the plicae semilunares. As a rule, the lengths of these semilunar folds correspond to the distance between two taeniae, although they may be longer. Whereas Kerckring folds in the small intestine consist merely of mucosa and submucosa, the plicae semilunares also include the circular muscle layer.

In contrast to the small intestine, the mucosa of the large intestine is not covered with villous projections but contains deep tubular pits that increase in depth toward the rectum and extend as far as the muscularis mucosae. In the submucosa, in addition to the usual structures (blood vessels, lymphatics, Meissner submucosal plexus), numerous solitary lymphatic nodules are present, originating in the reticular tissue with the tunica (lamina) propria and penetrating through the muscularis mucosae into the submucosa. The mucosal epithelium of the large intestine comprises one layer built of tall prismatic cells that, when fixed in a fresh state, display a cuticular border on their surface. Goblet cells are numerous, especially at the base of the pits.

In contrast to the small intestine, the colonic epithelium is relatively simple, but enterochromaffin cells are present, producing serotonin; as are L cells, producing GLI/PYY; and rare D cells, producing somatostatin as well as numerous other substances when studied in detail.

The sigmoid colon is a specialized section of the colon that has a specific motility function, as described in Chapter 83 . In populations of the Western world, the sigmoid typically develops diverticula and then the pathology of diverticulitis (see Chapter 90 ). Colonoscopists are frequently challenged by passage of the instrument through this area because of chronic diverticulosis. Therefore the sigmoid's anatomic structure is of particular importance ( Fig. 76.2 ).

The exact point of commencement of the sigmoid colon—in other words, the transition of descending colon to sigmoid colon—is indefinite. The sigmoid is generally considered to be the part of the large bowel between the descending colon and the rectum that, as a result of its attachment to the mesentery, is freely movable. Because this mesentery is subject to great variations, the extent of the sigmoid also becomes variable. It has been described as beginning at the left iliac crest and the margin of the left psoas muscle or brim of the pelvis minor. Other authorities regard the sigmoid colon as comprising the iliac colon (an iliac portion that has no mesentery) and the pelvic portion (pelvic colon), with a mesentery beginning at the brim of the pelvis minor. The mesenteriolized sigmoid colon generally assumes an omega-shaped flexure arching over the pelvic inlet toward the first or second sacral vertebra (S1 or S2) or toward the right side of the pelvis. It finally joins the rectum at an acute angle at about the S3 level. This typical shape of the sigmoid is not a constant finding.

The sigmoid colon may be short, running straight and obliquely into the pelvis, or it may be so long that the loop extends far to the right or, in extreme cases, high into the abdomen. Its average length is approximately 40 cm (16 inches) in adults and 18 cm in children. With the variations mentioned, it may reach 84 cm (and even longer).

The root of the mesentery (i.e., of the mesosigmoid) is variable but, characteristically, starts in the upper left iliac fossa, proceeds downward a few inches, and proceeds mesially and again upward to a point on the psoas muscle, slightly to the left of the fourth lumbar vertebra (L4), where it turns downward into the pelvis. The line of mesenteric attachment takes the shape of an irregular and blunted inverted V. Turning caudally after having reached its highest point, the attachment line of the mesosigmoid courses over the left common iliac artery and vein just above the division of the artery. The length of the mesosigmoid (i.e., distance from root of bowel wall) is extremely variable. A small peritoneal fossa, the intersigmoid fossa or recess, is formed by the mesosigmoid while twisting around the vascular pedicle. Rarely, this causes retroperitoneal hernia. Nevertheless, the fossa is a valuable guide to the vascular stalk. The left ureter passes retroperitoneally behind the intersigmoid recess. These relationships are variable but still important when disease occurs in this area.

The looping, arching variations in the sigmoid colon complicate the passage of instruments and make diagnosis difficult when disease occurs in the sigmoid. Diverticulitis can be confused with appendicitis, and diverticulitis with perforation can affect the ureter and pelvic organs.

The mucosa and submucosa of the sigmoid colon are almost identical to the corresponding structures of other parts of the colon. The same holds true for the arrangement of the circular and longitudinal muscle layers except for the most distal parts of the sigmoid colon, where the three flat, longitudinal muscle bands (taeniae), typical of the large intestine, spread out to form the completely encircling longitudinal muscle layer of the rectosigmoid junction. In the same region, the circular layer thickens, in some cases to such an extent that its prominence is alluded to as the “sphincter muscle” of the junction. It is questionable, however, whether this thickening has a true sphincteric function. Throughout the course of the sigmoid colon, the appendices epiploicae of the serous coat diminish gradually in number and size.

The importance of anatomic variations in the sigmoid colon is discussed under colonoscopy (see Chapter 79 ).