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For descriptive purposes the large intestine is divided, successively, into the caecum and appendix, colon, rectum and anal canal. The colon is further subdivided into ascending, transverse, descending and sigmoid segments. The sigmoid colon crosses the pelvic brim to enter the pelvic cavity where it becomes continuous with the rectum. Embryologically, the proximal half of the large intestine is derived from the midgut, while the distal half develops from the hindgut. This dual derivation is reflected in the proximal half of the large intestine being supplied by the superior mesenteric artery and the distal half by the inferior mesenteric artery; the arteries of the embryological midgut and hindgut respectively. The principal physiological role of the large intestine is the absorption of water, sodium and chloride from the fairly large volume of liquid ileal contents delivered to the large intestine daily. The unabsorbed residue is excreted as faeces. Fibreoptic colonoscopy and MRI and CT colonography allow detailed internal inspection of the large intestine with relative ease and speed and without subjecting the patient to too much discomfort. This article emphasizes the clinically and surgically-relevant aspects of the anatomy of the caecum, appendix and colon.
Commencing at the ileocaecal junction, the large intestine comprises successively, the caecum and appendix, colon, rectum and anal canal (Figure 1). The last two are described in detail elsewhere in this issue.
For descriptive purposes, the colon is further subdivided, in proximo-distal sequence, into ascending, transverse, descending and sigmoid segments (Figure 1). The sigmoid colon crosses the pelvic brim to enter the pelvic cavity where it becomes continuous with the rectum. The latter traverses the pelvic floor through the levator hiatus to enter the perineum where it becomes continuous with the anal canal. The large intestine is considerably less mobile than the small intestine owing to substantial lengths of the large intestine lying retroperitoneally and being adherent to the posterior abdominal wall.
Differences between large intestine and small intestine
The caecum and all parts of the colon are readily distinguishable from the small intestine by the presence on their outer surface of three longitudinal bands of muscle termed taeniae coli (Figures 1 and 2). Each of these bands is approximately 1.5 cm wide, and represents a condensation of the longitudinal muscle fibres in the outer layer of the muscularis propria of the bowel wall. Taeniae coli are absent over the rectum and appendix. Another distinguishing feature of the colon is the presence on the serosal surface, of numerous small fat tags. These are termed appendices epiploicae (Figures 1 and 2). They are most abundant on the surface of the distal descending colon and sigmoid colon. Their function is unknown. Incidentally, appendices epiploicae are absent over the caecum, appendix and rectum. Other features which typify the large intestine and allow it to be distinguished from the small intestine are its greater calibre and its sacculated (haustrated) appearance.
Relationship of peritoneum to large intestine
The transverse colon and sigmoid colon are both completely ensleeved in peritoneum and suspended from the posterior abdominal wall by separate peritoneal mesenteries, the transverse and sigmoid mesocolons, respectively. The considerable mobility of the transverse and sigmoid colons within the abdominal cavity is due entirely to the presence of these suspensory mesenteries.
The greater omentum descends from the greater curvature of the stomach and overlaps the anterior aspect of the entire transverse colon. It is attached along the free border of the transverse colon. The strip of greater omentum extending between the greater curvature of stomach and the free border of the transverse colon is referred to as the gastrocolic omentum.
In contrast to the transverse and sigmoid colons, the ascending and descending colons are normally devoid of mesenteries and lie directly on the posterior abdominal wall. Both the ascending and descending segments of the colon are covered on their fronts and sides by the parietal peritoneum of the posterior abdominal wall. The adherence of the ascending and descending colons to the posterior abdominal wall is relatively avascular, and enables the surgeon to mobilize these segments of the large bowel relatively easily. (Note, however, that in 5–10% of individuals the ascending and/or descending colons may possess a sizeable mesentery. The explanation for this lies in embryology and is due to failure of the normal, partial dissolution of the dorsal mesentery of the midgut and hindgut that occurs during development.)
Unlike the ascending colon, the caecum is normally completely enveloped in peritoneum; a feature which affords the caecum a certain degree of mobility. The appendix usually hangs free on its own mesentery, although it may occasionally be tucked extraperitoneally behind the ascending colon or it may adhere to the back of the caecum.
General morphology and topographical relations of the caecum and colon (Figure 1)
The large intestine in the adult is about 1.5 m in length.
The caecum (which means a blind-ended pouch) lies in the right iliac fossa and projects downwards below the level of the ileocaecal junction. It is about 5–7 cm in length.
The three taeniae coli of the caecum (one on the anterior surface, one on the posterior surface and one on the medial aspect) converge on the base of the vermiform appendix.
The vermiform appendix is a blind diverticulum that arises from the posteromedial aspect of the caecum, about 2.5 cm inferior to the ileocaecal valve. Its length is, on average, 7–8 cm (range: 1–20 cm). The appendix is also highly variable in position. Most commonly it lies behind the caecum (retrocaecal), but a long appendix may extend behind the ascending colon and even abut the lower pole of the right kidney or the duodenum. In other instances it dangles in the subcaecal position (abdominal), hangs down into the pelvis (pelvic), or tucks itself behind the terminal ileum (retroileal).
The appendix derives its blood supply from the appendicular artery (see Figure 4) which arises from the posterior caecal artery, in turn a branch of the ileocolic artery. The appendicular artery passes behind the terminal ileum to reach the appendix via the appendicular mesentery (mesoappendix). It is an end-artery and represents the entire blood supply of that organ. Acute infection of the appendix may result in thrombosis of this vessel, with resultant gangrene and then perforation of the appendix. This is in contrast to acute cholecystitis, where the rich collateral blood supply to the gall bladder, via vessels passing from the right hepatic artery in the gall bladder bed, ensures the rarity of gangrene of the gall bladder even if the cystic artery becomes thrombosed as a result of the inflammatory process.
The ascending colon is 15 cm in length and extends vertically upwards from the ileocaecal junction to the inferior (visceral) surface of the right lobe of the liver, where it bends sharply anteroinferiorly and medially forming the right colic flexure (hepatic flexure) before becoming continuous with the transverse colon. The hepatic flexure is an immobile segment of the colon immediately behind which are the fascial coverings surrounding the lower part of the right kidney. The transverse colon extends across the abdomen from the hepatic to the left colic flexure (splenic flexure). It is normally the most mobile segment of the large intestine owing to its being suspended by a generous mesenteric sheet, the transverse mesocolon. Typically, the transverse colon measures 45 cm in length.
At its left extremity the transverse colon turns inferiorly and somewhat posteriorly at the left colic flexure (splenic flexure) to become continuous with the descending colon. The splenic flexure is usually at a noticeably higher level than the hepatic flexure. Extending laterally, superiorly and posteriorly from the splenic flexure is a short, double-layered fold of peritoneum, the phrenico-colic ligament, that blends with the peritoneum lining the inferior surface of the left dome of the diaphragm. The splenic flexure and phrenico-colic ligament are in contact superiorly with the anterior pole of the spleen (Figure 1). Mobilization of the splenic flexure during left hemicolectomy necessitates division of the phrenico-colic ligament with the attendant risk of inadvertent injury to the spleen.
The descending colon is about 25 cm in length and extends vertically from the splenic flexure to the left iliac fossa where it becomes continuous with the sigmoid colon (formerly known as the pelvic colon) (Figure 1).
The sigmoid colon is 35–40 cm in length. Commencing in the left iliac fossa at its junction with the descending colon, the sigmoid colon crosses the pelvic brim to enter the pelvic cavity where it becomes continuous with the rectum opposite the third piece of the sacrum.
The sigmoid colon is suspended by the sigmoid mesocolon, which like all mesenteries is a double-layered peritoneal sheet. Dorsally, the sigmoid mesocolon is attached along a line that crosses the pelvic brim from the left iliac fossa to the presacral region.
The proximal half of the large intestine (comprising caecum, appendix, ascending colon, hepatic flexure and more than half of the transverse colon) derives its blood supply from branches of the superior mesenteric artery (Figure 3). The distal third of the transverse colon, splenic flexure, descending colon, sigmoid colon and rectum derive their blood supply from branches of the inferior mesenteric artery (Figure 3). Arising from the right side of the superior mesenteric artery, the ileocolic artery runs towards the ileocaecal junction and gives off an ileal branch which supplies the terminal ileum, and a colic branch which runs superiorly along the left side of the ascending colon behind the peritoneum to anastomose with the right colic artery. The ileocolic artery ends by dividing into anterior and posterior caecal branches. The latter is the usual source of the appendicular artery (Figure 4). The right colic artery arises from the right side of the superior mesenteric artery considerably proximal to the origin of the ileocolic artery. It runs to the right behind the peritoneal floor of the infracolic compartment and supplies the distal half of the ascending colon and hepatic flexure. The middle colic artery is the most proximal of the colic branches to arise from the superior mesenteric artery (Figure 3). Soon after its origin it enters the transverse mesocolon, where it divides into right and left branches. Together these two branches supply the proximal two-thirds of the transverse colon.
The remainder of the colon and rectum are supplied by branches of the inferior mesenteric artery. The inferior mesenteric artery arises from the anterior aspect of the abdominal aorta, either just below (or, at times, overlapped by) the lower border of the third part of the duodenum. It runs obliquely, inferolaterally behind the peritoneal floor of the infracolic compartment. Within a few centimetres of its origin, the inferior mesenteric artery gives off the left colic artery which runs to the left retroperitoneally before dividing into upper and lower branches. The former supplies the distal transverse colon, splenic flexure and proximal descending colon. The lower branch supplies the distal descending colon. Arising directly from the inferior mesenteric artery are three or four sigmoid arteries. These enter the sigmoid mesocolon and thereby reach the sigmoid colon. Anatomical variations in the course and origins of colic arteries are common. For example, the ileocolic and right colic arteries may arise from a common trunk. In another fairly common variation the upper and lower branches of the left colic artery may come off as separate vessels directly from the inferior mesenteric artery.
Lying within the inner margin of the colon close to the bowel wall is an extremely effective anastomotic arcade, the marginal artery (of Drummond), which links the colic branches of the superior mesenteric and inferior mesenteric arteries (Figure 3). It is entirely due to the presence of an effective marginal artery that the inferior mesenteric artery is sacrificed in patients undergoing repair of an abdominal aortic aneurysm without jeopardizing the blood supply to the distal colon. Conversely, in some very elderly patients with advanced atheromatous disease, the marginal artery may be severely compromised, and this may lead to anastomotic ischaemia/dehiscence following bowel resection.
A relative, natural, vascular watershed exists in the very distal part of the transverse colon between the territories of the middle colic and left colic arteries, resulting in the splenic flexure being somewhat vulnerable to ischaemia.
Generally speaking, the veins draining the colon correspond in name, location and course to the colic arteries (Figure 5). Thus the ileocolic, right colic and middle colic veins accompany the corresponding arteries and drain directly into the superior mesenteric vein, while the left colic and sigmoid veins drain into the inferior mesenteric vein. The superior mesenteric vein courses upwards within the root of the small intestinal mesentery in front of the third part of the duodenum and uncinate process of the pancreas to reach the back of the neck of pancreas, where it is joined by the splenic vein to form the portal vein (Figure 5).
The inferior mesenteric vein ascends in the retroperitoneal floor of the infracolic compartment to join the splenic vein behind the body of the pancreas (Figure 5), and thereby drains indirectly to the portal vein.
The lymph nodes and lymph vessels draining the colon are closely related to the arteries which supply the colon. The lymph nodes are arranged in tiered fashion forming four distinct groups. The first tier comprises numerous small lymph nodes lying on the surface of the colon. These are the epicolic lymph nodes. The second tier is made up of lymph nodes lying along the medial borders of the ascending and descending colon and along the inner (mesenteric) borders of the transverse and sigmoid colons. These are the paracolic lymph nodes. They receive afferent vessels from the epicolic nodes and also directly from the gut wall. Efferent vessels from the paracolic nodes pass on to the third tier, the intermediate group of lymph nodes. The intermediate nodes lie alongside the major colic branches of the superior mesenteric artery (ileocolic, right colic and middle colic arteries) and inferior mesenteric artery (left colic and sigmoid arteries). Efferent vessels from the intermediate nodes, in turn, drain to the fourth tier, the aortic group of nodes, situated near the origins of the superior and inferior mesenteric arteries. From the latter nodes, efferent vessels drain into the cisterna chyli. The lymphatic drainage of each segment of bowel corresponds fairly accurately to its arterial supply.
The arrangement of vessels and lymphatics supplying the colon determines the extent of resection to be carried out when undertaking partial colectomies for malignant disease. High ligation of the vessels supplying the involved portion of large intestine along with resection of a generous wedge of corresponding mesocolon will remove the lymph nodes draining that area.
For a right hemicolectomy, the terminal few centimetres of ileum, caecum, appendix, ascending colon, hepatic flexure and proximal third of transverse colon are removed en bloc after preliminary ligation of ileocolic, right colic and right branch of middle colic arteries. For transverse colectomy, the entire transverse colon is removed including the hepatic and splenic flexures following preliminary ligation of the middle colic artery. For a left hemicolectomy, the distal transverse colon, splenic flexure, the entire descending colon and proximal sigmoid colon are remove en bloc after preliminary ligation of the left branch of middle colic artery, left colic artery and some of the sigmoid arteries.
The colon consists of several concentric layers that are easily seen through a light microscope. From the lumen outwards these layers are: columnar epithelium, lamina propria, muscularis mucosae, submucosa, muscularis propria (comprising an inner circular layer and outer longitudinal layer of smooth muscle) and finally, serosa.
The adult configuration of the colon and the relationship of the transverse colon to the duodenum are best understood by considering the embryological development of the midgut and hindgut during the second and third months of intrauterine life.
In the early embryo the gut develops as a midline structure. The midgut (fed by the superior mesenteric artery) adjoins the hindgut (fed by the inferior mesenteric artery); the ‘boundary’ between the two being represented in the adult, approximately, by the junction between the proximal two-thirds and distal one-third of the transverse colon. The embryonic midgut may also be defined as that part of the gut which resides in the physiological umbilical hernia between the 6th and 10th weeks of intrauterine life. In the cord the distal limb (colic limb) of the ‘U’ lies to the left and the proximal limb lies to the right. During ‘reduction’ of the physiological hernia, the small intestine is the first to return to the abdominal cavity. This pushes the caudal part of the colon over to the left of the abdominal cavity. The caecum is the last part of the hernia to be returned to the abdomen, where initially it lies just below the liver in front of the other loops of intestine. With progressive lengthening of the gut the caecum moves downwards and to the right and eventually comes to lie in the right iliac fossa. These changes involving the midgut are collectively referred to as ‘rotation of the gut’, and may be pictured as a U loop of gut rotating anti-clockwise through three right angles; the distal limb of the U turning first to the left, then cephalad and, finally, to the right. This embryological twisting is reflected in the adult by the transverse colon running from right to left in front of the second part of the duodenum, and the superior mesenteric artery and vein crossing in front of the third part of the duodenum.