Anatomy of the Large Intestine

Because of its embryologic development from both the midgut and hindgut, the colon has two main sources of blood supply: the superior mesenteric (1) and the inferior mesenteric arteries (2). The superior mesenteric artery (1) supplies the right colon, the appendix, and small intestine. The middle colic artery (3) is the most prominent branch of the superior mesenteric artery. The middle colic artery branches into a right and left division. The right division anastomoses with the right colic (4) and the ileocolic (5) arteries. The left branch communicates with the marginal artery of Drummond (6). The middle and right colic and ileocolic arteries are doubly ligated near their origin when a right colectomy is performed for malignancy. The ileocolic artery reaches the mesentery of the appendix from beneath the terminal ileum. Angulation or obstruction of the terminal ileum should be avoided following the ligation of the appendiceal artery (7) in the presence of a short mesentery.

The inferior mesenteric artery arises from the aorta just below the ligament of Treitz. Its major branches include the left colic (8), one or more sigmoid branches (9, 10), and the superior hemorrhoidal artery (11). Following ligation of the inferior mesenteric artery, viability of the colon is maintained through the marginal artery of Drummond (6) by way of the left branch of the middle colic artery.

The third blood supply to the large intestine arises from the middle and inferior hemorrhoidal vessels. The middle hemorrhoidal vessels (12) arise from the internal iliac (hypogastric) (13), either directly or from one of its major branches. They enter the rectum along with the suspensory ligament on either side. These are relatively small vessels, but they should be ligated.

The blood supply to the anus is from the inferior hemorrhoidal (14) vessels, a branch of the internal pudendal artery (15). In low-lying lesions wide excision of the area is necessary with ligation of the individual bleeders as they are encountered.

The venous drainage of the right colon parallels the arterial supply and drains directly into the superior mesenteric vein (1). The inferior mesenteric vein, in the region of the bifurcation of the aorta, deviates to the left and upward as it courses beneath the pancreas to join the splenic vein. High ligation of the inferior mesenteric vein (16) should be carried out before extensive manipulation of a malignant tumor of the left colon or sigmoid in order to avoid the vascular spread of tumor cells.

The right colon can be extensively mobilized and derotated to the left side without interference with its blood supply. The mobilization is accomplished by dividing the avascular lateral peritoneal attachments of the mesentery of the appendix, cecum, and ascending colon. Blood vessels of a size requiring ligation are usually present only at the peritoneal attachments of the hepatic and splenic flexures. The transverse colon and splenic flexure can be mobilized by separating the greater omentum from its loose attachment to the transverse colon (see Plate 24). Traction on the splenic flexure should be avoided lest troublesome bleeding result from a tear in the adjacent splenic capsule. The abdominal incision should be extended high enough to allow direct visualization of the splenic flexure when it is necessary to mobilize the entire left colon. The left colon can be mobilized toward the midline by division of the lateral peritoneal attachment. There are few, if any, vessels that will require ligation in this area.

The descending colon and sigmoid can be mobilized medially by division of the avascular peritoneal reflection in the left lumbar gutter. The sigmoid is commonly quite closely adherent to the peritoneum in the left iliac fossa. The peritoneal attachment is avascular, but because of the proximity of the spermatic or ovarian vessels, as well as the left ureter, careful identification of these structures is required. Following the division of the peritoneal attachment and the greater omentum, further mobilization and elongation of the colon can be accomplished by division of the individual branches (8, 9, 10) of the inferior mesenteric artery. This ligation must not encroach on the marginal vessels of Drummond (6).

The posterior wall of the rectum can be bluntly dissected from the hollow of the sacrum without dividing important vessels. The blood supply of the rectum is in the mesentery adjacent to the posterior rectal wall. Following division of the peritoneal attachment to the rectum and division of the suspensory ligaments on either side, the rectum can be straightened with the resultant gain of considerable distance (Plate 80). The pouch of Douglas, which may initially appear to be quite deep in the pelvis, can be mobilized well up into the operative field.

The lymphatic supply follows the vascular channels, especially the venous system. Accordingly, all of the major blood supplies of the colon should be ligated near their points of origin. These vessels should be ligated before a malignant tumor is manipulated. Complete removal of the lymphatic drainage from lesions of the left colon requires ligation of the inferior mesenteric artery (2) near its point of origin from the aorta.

Low-lying malignant rectal lesions may extend laterally along the middle hemorrhoidal vessels (12) as well as along the levator ani muscles. They may also extend cephalad along the superior hemorrhoidal vessels (11). The lymphatic drainage of the anus follows the same pathway but may include spread to the superficial inguinal lymph nodes (17). The lower the lesion, the greater the danger of multiple spread from the several lymphatic systems involved.