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SURGICAL ANATOMY OF LARGE INTESTINE

Several important anatomic facts influence the technique of surgery in the large intestine. As

a consequence of its embryologic development, the colon has two main sources of blood

supply. The cecum, ascending colon, and proximal portion of the transverse colon are

supplied with blood from the superior mesenteric artery, while the distal transverse colon,

splenic flexure, descending colon, sigmoid, and upper rectum are supplied by branches of the

inferior mesenteric artery (see Figure 1).

Advantage may be taken of the free anastomotic blood supply along the medial border of the

bowel by dividing either the inferior mesenteric artery or the middle colic artery and by

depending upon the collateral circulation through the marginal artery of Drummond to

maintain the viability of a long segment of intestine. The peritoneal reflection on the lateral

aspect of the colon is practically bloodless, except at the flexures or in the presence of

ulcerative colitis or portal hypertension, and may be completely incised without causing

bleeding or jeopardizing the viability of the bowel. When the lateral peritoneum is divided and

the greater omentum freed from the transverse colon, extensive mobilization is possible,

including derotation of the cecum into the right or left upper quadrant. Care should be taken to

avoid undue traction on the splenic flexure lest attachments to the capsule of the spleen be

torn and troublesome bleeding occur. In the presence of malignancy of the transverse colon,

the omentum is usually resected adjacent to the blood supply of the greater curvature of the

stomach.

After the colon has been freed from its attachments to the peritoneum of the abdominal wall,

the flexures, and the greater omentum, it can be drawn toward the midline through the

surgical incision limited only by the length of its mesentery. This mobility of the colon renders

the blood supply more accessible and often permits a procedure to be performed outside the

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peritoneal cavity. The most mobile part of the large bowel is the sigmoid, because it normally

possesses a long mesentery, whereas the descending colon and right half of the colon are

fixed to the lateral abdominal wall.

The lymphatic distribution of the large bowel conforms to the vascular supply. A knowledge of

this is of great surgical importance, especially in the treatment of malignant neoplasm,

because an adequate extirpation of potentially involved lymph nodes requires the sacrifice of

a much larger portion of the blood supply than would at first seem essential. The lymphatic

spread of carcinoma of the large intestine along the major vascular supply has been

responsible for the development of classic resections. Local "sleeve" resection for malignancy

may be indicated in the presence of metastasis or because of the patient's poor general

condition.

When a curative resection is planned, the tumor and adjacent bowel must be sufficiently

mobilized to permit removal of the immediate lymphatic drainage area.

Basically, the resections of the colon should include either the lymphatic drainage area of the

superior mesenteric vessels or that of the inferior mesenteric vessels. While this would

approach the ideal, experience has shown that approximately four types of resections are

commonly performed: right colectomy, left colectomy, anterior resection of the rectosigmoid,

and abdominoperineal resection. For years lesions of the cecum, ascending colon, and

hepatic flexure have been resected by a right colectomy with ligation of the ileocolic, right

colic, and all or part of the middle colic vessels (A). Lesions in the cecal area may be

associated with involved lymph glands along the ileocolic vessels. As a result, a segment of

the terminal ileum is commonly resected along with the right colon. Lesions in the region of

the splenic flexure are in the one area where left colectomy by a sleeve resection may be

performed. Extensive resections can be carried out with good assurance of an adequate

blood supply, since the marginal vessels are divided nearer their points of origin. In addition

to the marginal vessels, the left colic artery near its point of origin and the inferior mesenteric

vein are ligated even before manipulation of the tumor is carried out to minimize the venous

spread of cancer cells. End-to-end anastomosis without tension can be accomplished by

freeing the right colon of its peritoneal attachments and derotating the cecum back to its

embryologic position on the left side. The blood supply is sustained through the middle colic

vessels and the sigmoidal vessels. Although the veins tend to parallel the arteries, this is not

the case with the inferior mesenteric vein. This vein courses to the left before it dips beneath

the body of the pancreas to join the splenic vein (B).

Lesions of the lower descending colon, sigmoid, and rectosigmoid may be removed by an

anterior resection. The inferior mesenteric artery is ligated at its point of origin from the aorta

(C) or just distal to the origin of the left colic artery. The upper segment for anastomosis will

receive its blood supply through the marginal arteries of Drummond from the middle colic

artery. The viability of the rectosigmoid is more uncertain following the ligation of the inferior

mesenteric artery. Accordingly, the resection is carried low enough to ensure a good blood

supply from the middle and inferior hemorrhoidal vessels. This level is usually so low that the

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anastomosis must be carried out in the pelvis anterior to the sacrum. Here again the principle

of mobilizing the flexures as well as the right colon may be required to ensure an anastomosis

without tension.

The most extensive resection involves lesions of the low rectosigmoid, rectum, and anus.

High ligation of the inferior mesenteric vessels and ligation of the middle and inferior

hemorrhoidal vessels, along with wide excision of the rectum and anus, are required. Since

the lymphatic drainage to the anus and lower rectum may drain laterally even to the inguinal

region, wide lateral excision of low-lying rectal and anal neoplasms is mandatory.

In order to minimize the possibilities of tumor spread, the lesion should be covered with gauze

as early in the procedure as possible. Further isolation should be provided by ligation of the

colon above as well as below the tumor with gauze or umbilical tapes. Likewise, early ligation

of the vascular supply should be performed before manipulation of the tumor is carried out.

Since bowel anastomosis must be performed in the absence of tension, it is imperative that

considerable mobilization of the colon, especially of the splenic flexure, be carried out if

continuity is to be restored following extensive resection of the left colon. The presence of

pulsating vessels adjacent to the mesenteric margin, which has been cleared preparatory to

the anastomosis, should be assured. Injection of 1% procaine into the adjacent mesentery will

sometimes enhance arterial pulsation. Occasionally, pulsations are not apparent since the

middle colic artery is compressed as a result of the small bowel's being introduced into a

plastic bag and displaced to the right and outside of the abdominal wall. The Doppler

apparatus may be used to verify the adequacy of the blood supply.

The large intestine bears an important relation to a number of vital structures. Thus, in

operations on the right half of the colon, the right ureter and its accompanying vessels are

encountered behind the mesocolon. The duodenum lies posterior to the mesentery of the

hepatic flexure and is always exposed in mobilizing this portion of the bowel. The spleen is

easily injured in mobilizing the splenic flexure. The left ureter and its accompanying spermatic

or ovarian vessels are always encountered in operations on the sigmoid and descending

colon. In an abdominoperineal resection of the rectum, both ureters are potentially in danger

of injury. The surgeon must not only be aware of these structures, but must positively identify

them before dividing the vessels in the mesentery of the colon.

The anatomic arrangement of the colon that permits mobilization of low-lying segments

sometimes tempts the surgeon to reconstruct the normal continuity of the fecal current

without adequate extirpation of the lymphatic drainage zones. Extensive block excision of the

usual lymphatic drainage areas, combined with excision of a liberal segment of normal-

appearing bowel on either side of a malignant lesion, is mandatory. Primary anastomosis of

the large intestine requires viable intestine, the absence of tension, especially when the bowel

becomes distended postoperatively, and a bowel wall of near-normal consistency. Although

the danger from sepsis has decreased substantially in recent years, the fact remains that the

surgical problems concerned with the large intestine are often complex and require more

seasoned judgment and experience than does almost any other field in general surgery.