CHAPTER 67 Large intestine

OVERVIEW

The large intestine extends from the ileocaecal valve to the anus. Broadly speaking, it lies in a curve which tends to form a border around the loops of small intestine that are located centrally within the abdomen (Figs 67.1, 67.2). The large intestine begins in the right iliac fossa as the caecum, from which the vermiform appendix arises. The caecum becomes the ascending colon which passes upwards in the right lumbar region and hypochondrium to the inferior aspect of the liver where it bends to the left forming the hepatic flexure (right colic flexure) and becomes the transverse colon. This loops across the abdomen with an anteroinferior convexity until it reaches the left hypochondrium, where it curves inferiorly to form the splenic flexure (left colic flexure) and becomes the descending colon, which proceeds through the left lumbar and iliac regions to become the sigmoid colon in the left iliac fossa. The sigmoid colon descends deep into the pelvis and becomes the rectum which ends in the anal canal at the level of the pelvic floor. The large intestine is approximately 1.5 m long in adults, although there is considerable variation in its length. Its calibre is greatest near the caecum and gradually diminishes to the level of the sigmoid colon. The rectum is largest in calibre in its lower third and forms the rectal ampulla above the anal canal.

Fig. 67.1 Overview of the abdominal colon and its relations.

Fig. 67.2 Appearance of the abdominal colon on double contrast barium enema examination demonstrating the transverse colon, hepatic and splenic flexures.

The large intestine differs from the small intestine in several ways: it has a greater calibre; for most of its course it is more fixed in position; its longitudinal muscle, though a complete layer, is concentrated into three longitudinal bands, taeniae coli, in all but the distal sigmoid colon and rectum; small adipose projections, appendices epiploicae, are scattered over the free surface of the whole colon (they tend to be absent from the caecum, vermiform appendix and rectum); the colonic wall is puckered into sacculations (haustrations), which may partly be due to the presence of the taeniae coli, and which may be demonstrated on plain radiographs as incomplete septations arising from the bowel wall.

The large intestine develops as a fully mesenteric organ. However, after the rotation of the gut tube in utero, large portions of it come to lie adherent to the retroperitoneum, which means that some parts of the colon are fixed within the retroperitoneum, and other parts are suspended by a mesentery within the peritoneal cavity. Those portions of the colon within the retroperitoneum are separated from other retroperitoneal structures by a thin layer of connective tissue which forms an avascular field during surgical dissection, but which offers little or no barrier to the spread of disease within the retroperitoneum.

The caecum may be within the retroperitoneum, but more frequently is suspended by a short mesentery. The ascending colon is usually a retroperitoneal structure although the hepatic flexure may be suspended by a mesentery. The transverse colon emerges from the retroperitoneum on a rapidly elongating mesentery and lies, often freely mobile, in the upper abdomen. The transverse mesocolon shortens to the left of the upper abdomen and may become retroperitoneal at the splenic flexure. Occasionally the splenic flexure is suspended by a short mesentery. The descending colon is retroperitoneal usually to the level of the left iliac crest. As the colon enters the pelvis it becomes increasingly more mesenteric again at the origin of the sigmoid colon, although the overall length of the sigmoid mesentery is highly variable. The distal sigmoid colon lies on a rapidly shortening mesentery as it approaches the pelvis; by the level of the rectosigmoid junction the mesentery has all but disappeared, so that the rectum enters the pelvis as a retroperitoneal structure. The caecum and proximal ascending colon are often more mobile on a longer mesentery in the neonate and infant than they are in the adult.

The mesenteries of the colon consist of visceral peritoneum enclosing connective and adipose tissues which envelop the vessels, nerves and lymphatics as they course from the retroperitoneum. Where the colonic mesenteries lie in contact with the retroperitoneum, the (potential) space between the retroperitoneum and mesentery is referred to as the subperitoneal space, which allows free tracking of pathological processes in either direction.

EXTERNAL APPEARANCE

The haustrations of the colon are often absent in the caecum proximal to the origin of the ascending colon and are often relatively sparse in the ascending and proximal transverse colon. In these regions the taeniae coli are usually thin and occupy only a small percentage of the circumference of the colon. There are few if any appendices epiploicae on the serosal surface of the caecum, and only a limited number on the surface of the ascending colon. The haustrations become more pronounced from the middle of the transverse colon to the distal portion of the descending colon: the sigmoid colon is often characterized by marked sacculation. The width of the taeniae coli remains fairly constant throughout the length of colon but the number of appendices epiploicae usually increases, becoming in the sigmoid colon where they can be large in the obese individual. The taeniae are located in fairly constant positions beneath the serosal surface of the colon except in the transverse colon. They are oriented anteriorly on the anti-mesenteric aspect of the colon opposite the midline of the mesenteric attachment (taenia libera), posterolaterally (taenia omentalis) and posteromedially (taenia mesocolica) midway between the taenia libera and the mesentery (Fig. 67.3). In the caecum and descending colon, which are partly retroperitoneal structures, the posterolateral taenia is often obscured from view by the peritoneal reflection onto the colonic wall. In the transverse colon, the taeniae are rotated through 90° as a consequence of the mobility and dependent position of this part of the colon, thus anterior becomes inferior, posteromedial becomes posterior and posterolateral becomes superior. The taeniae coli broaden to occupy more of the circumference of the sigmoid colon in its distal portion and by the level of the rectosigmoid junction they form distinct anterior and posterior bands. These bands subsequently unite to form a complete longitudinal muscle covering for the rectum which therefore has no external sacculation. The rectum also lacks serosal appendices epiploicae.

Fig. 67.3 Layers of the colonic wall.

INTERNAL APPEARANCE

Throughout its length, the internal aspect of the colon is characterized by the presence of haustrations. These infoldings of the wall consist of mucosa and submucosa, and may partially span the lumen, but they never form a complete, circumferential ring. The pattern of the haustrations and appearance of the colonic mucosa help the clinician appreciate the level reached during flexible endoscopic examinations of the colon. In the portion of the caecum where haustrations occur, the three longitudinal taeniae coli converge to form a characteristic ‘trefoil’ pattern on the caecal wall (Fig. 67.4). Elsewhere, the wall of the lower pole of the caecum is usually devoid of haustrations, although a spiral mucosal pattern is often seen in the region of the appendix orifice (see Fig. 67.19). The upper caecum and ascending colon possess shallow but long haustrations which may extend across one-third of the lumen (Fig. 67.5). This pattern is most pronounced in the transverse colon where the long haustrations often confer a triangular appearance to the cross section of the lumen when viewed along the axis (such as at colonoscopy) (Fig. 67.6). The wall of the colon is thinnest in the region of the caecum and ascending colon and is most at risk of perforation during therapeutic endoscopic procedures. The haustrations of the descending and sigmoid colon tend to be thicker and shorter than those of the transverse colon, and this gives a more circular cross-section to the lumen. The overall luminal diameter is often smallest in the descending colon (Fig. 67.7). During endoscopy, the pattern of the submucosal vessels becomes more conspicuous in the sigmoid colon (Fig. 67.8). The mobility of the sigmoid colon on its mesentery means that shorter lengths of colon tend to be visible during endoscopy than anywhere else in the colon. The haustrations of the rectum tend to form consistent and recognizable folds: the pattern of the submucosal vessels is the most pronounced in the colon being of largest calibre with multiple visible veins (Fig. 67.9). Distinct veins are usually visible during endoscopy, and they are most marked above the anorectal junction.

Fig. 67.4 Endoscopic appearance of the caecum. The characteristic trefoil appearance of the confluence of the three taeniae is usually obvious.

Fig. 67.19 Endoscopic appearance of the orifice of the appendix. The orifice varies from a small depression to an obvious lumenal structure.

Fig. 67.5 Endoscopic appearance of the ascending colon.

Fig. 67.6 Endoscopic appearance of the transverse colon. Note the characteristic triangular appearance of the haustrations when viewed collectively (see also Fig. 67.10C).

Fig. 67.7 Endoscopic appearance of the descending colon. The lumen tends to look rather more featureless than the more proximal colon.

Fig. 67.8 Endoscopic appearance of the sigmoid colon. Multiple large mucosal folds are characteristic.

Fig. 67.9 Endoscopic appearance of the rectum. Note the large transverse folds, with little else in the way of mucosal folds, characterizes the rectum. Prominent veins are often seen, particularly in the lower third.

RADIOGRAPHIC APPEARANCES

Cross-sectional imaging of the colon can be performed with computerized tomography (CT) and magnetic resonance imaging (MRI). On axial imaging the colon may be filled with particulate faeces and air (Fig. 67.10). The wall in normal individuals is thin. The caecum and ascending colon often contain faecal residue and are easily identified in the right retroperitoneum. The transverse colon may contain faeces or gas, but lies in a variable position suspended by its mesentery. The descending colon in the left retroperitoneum is frequently collapsed and contains little faecal residue.

Fig. 67.10 Appearance of the colon on multislice computerized tomographic examination. A, Axial CT. B, Coronal reformat showing normal calibre and distribution of the abdominal colon (ascending, transverse and descending). C, Volume-rendering of the colonic wall using the axial data set to produce virtual colonoscopic views, showing the triangular lumen of the transverse colon. D, Volume-rendering of the air-filled colon using the axial data set to give an image similar to a double contrast barium enema demonstrating haustrations.

(A and B by courtesy of Dr Louise Moore, Chelsea and Westminster Hospital.) (C and D by courtesy of GE Worldwide Medical Systems.)

The volume data sets produced by modern multislice CT can now produce virtual colonoscopic mucosal images in the distended and cleaned colon, and surface-rendered images of the internal surface of the bowel.

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

Arteries

The arterial supply of the large intestine is derived from both the superior and inferior mesenteric arteries (Fig. 67.11). The caecum, appendix, ascending colon and right two-thirds of the transverse colon (derived from the midgut) are supplied from ileocolic, right colic and middle colic branches of the superior mesenteric artery. The left part of the transverse, descending and sigmoid colon, rectum and upper anal canal (hindgut derivatives) are supplied predominantly from the inferior mesenteric artery via the left colic, sigmoid and superior rectal arteries, with small contributions from branches of the internal iliac artery (the middle and inferior rectal arteries). The larger unnamed branches of these vessels ramify between the muscular layers of the colon which they supply. They subdivide into smaller submucosal rami and enter the mucosa. The terminal branches divide into vasa brevia and vasa longa which either enter the colonic wall directly or run through the subserosa for a short distance before crossing the circular smooth muscle to give off branches to the appendices epiploicae (Fig. 67.12).

Fig. 67.11 The main branches of the superior and inferior mesenteric arteries.

Fig. 67.12 Typical pericolic arrangement of arterial vasculature.

Arterial anastomoses and the marginal artery of the colon

The arterial supply to the large intestine has less profuse anastomoses and ‘cross-over’ of arterial territories than the small intestine but there are several areas and anastomoses which provide for some degree of anatomical collateral supply to areas of the large intestine.

The marginal artery (of Drummond) of the colon is the name given to the vessel which lies closest to and parallels the wall of the large intestine. It is formed by the main trunks, and the arcades arising from, the ileocolic and right colic, middle colic and left colic arteries. It is most apparent in the ascending, transverse and descending colons (Fig. 67.13). The sigmoid colon has little if any marginal artery. Anastomoses form between the main terminal branches which run parallel to the colon within the mesentery and give rise to vasa recta and vasa brevia to supply the colon. The marginal artery in the region of the splenic flexure may be absent or of such small calibre as to be of little clinical relevance. It may hypertrophy significantly when one of the main visceral arteries is compromised, e.g. following stenosis or occlusion of the inferior mesenteric artery, and it then provides a vessel of collateral supply.

Fig. 67.13 The marginal artery running parallel to the colon and anastomosing with the branches of the superior mesenteric artery supplying the right side of the colon, digital subtraction arteriogram.

(By courtesy of Dr J Jackson, Hammersmith Hospital, London.)

The arc of Riolan is the name given to the arterial anastomosis which is sometimes present in the mesentery between the right side of the transverse colon and the upper descending colon. When present it is formed from a large branch of the middle colic artery which runs parallel and posterior to the middle colic artery in the transverse mesocolon and anastomoses with an ascending branch of the left colic artery close to its origin in the root of the mesentery. This provides a direct communication between the superior and inferior mesenteric arteries.

Colic nodes

Lymph node clearance in colorectal cancer resections

Radical lymphadenectomy during resection for colorectal cancer requires removal of the highest possible lymph node draining the area of colon in which the tumour is located. In cases of cancer involving the rectum and sigmoid colon, this usually involves resection of the preterminal colic nodes of the inferior mesenteric artery and thus ligation of the inferior mesenteric artery at its root or just below the origin of the left colic artery. A detailed description of the classification of the lymph nodes with regard to the site of the primary tumour within the colon has been suggested by the Japanese Society for the Cancer of the Colon and Rectum.

INNERVATION

The colon and rectum are innervated by the sympathetic and parasympathetic systems (see Ch. 60; see Figs 60.2–60.4).

The sympathetic supply to the caecum, appendix, ascending colon and right two-thirds of the transverse colon originates in the fifth to the 12th thoracic spinal segments and are conveyed to the coeliac and superior mesenteric plexuses via the greater and lesser splanchnic nerves where they and synapse on ganglionic neurones. Postganglionic axons travel along branches of the superior mesenteric artery, and are distributed to the colon from periarterial plexuses. The parasympathetic supply is derived from the vagus nerve via the coeliac and superior mesenteric plexuses.

The sympathetic supply of the left third of the transverse colon, the descending and sigmoid colon, rectum and upper anal canal originates in the first and second lumbar spinal segments. The fibres are distributed via the lumbar splanchnic nerves through the abdominal aortic and the inferior mesenteric plexuses and via the sacral splanchnic nerves through the superior and inferior hypogastric plexuses. Postganglionic axons reach the gut wall via periarterial plexuses on branches of the inferior mesenteric artery. The parasympathetic supply travels via the pelvic splanchnic nerves (nervi erigentes) from cell bodies in the second to the fourth sacral spinal segments. Those distributed to the rectum and upper anal canal run through the inferior and superior hypogastric plexuses to branches of the inferior mesenteric artery. Some of those distributed to the descending and sigmoid colon run via these plexuses; however, a large number of fibres pass directly through the retroperitoneal tissues to the splenic flexure, descending and sigmoid colon independently of the inferior mesenteric artery.

The ultimate distribution within the wall of the large intestine is similar to the small intestine. The colic sympathetic nerves are motor to the ileocaecal valve musculature and inhibitory to mural muscle in the colon and rectum. Some fibres are vasoconstrictor to the colic vasculature. Parasympathetic nerves are secretomotor to colorectal glands, motor to the colic and rectal musculature and inhibitory to the internal anal sphincter. Afferent impulses mediating sensations of distension are carried by visceral afferent fibres which travel with the parasympathetic nerves; pain impulses pass in visceral afferents travelling with the sympathetic and parasympathetic nerves supplying the rectum and the upper part of the anal canal.

CAECUM, VERMIFORM APPENDIX AND ASCENDING COLON

CAECUM

The caecum is a large blind pouch lying in the right iliac fossa below the ileocaecal valve, continuous proximally with the distal ileum and distally with the ascending colon. The blind-ending vermiform appendix usually arises on its medial side at the level of the ileal opening. Its average axial length is 6 cm and its breadth 7.5 cm. It rests posteriorly on the right iliacus and psoas major, with the lateral cutaneous nerve of the thigh interposed. Posteriorly lies the retrocaecal recess which frequently contains the vermiform appendix. The anterior abdominal wall is immediately anterior to the caecum except when it is empty, in which case the greater omentum and some loops of the small intestine may be interposed. Usually the caecum is entirely covered by peritoneum, but occasionally this is incomplete posterosuperiorly where it lies over the iliac fascia only separated from it by loose connective tissue.

In early fetal life the caecum is usually short, conical and broad based, with an apex turned superomedially towards the ileocaecal junction. As the fetus grows, the caecum increases initially in length more than in breadth, still with the apex turned superomedially; in the later stages of intrauterine growth, the proximal part of the caecum widens so that the caecum is still a conical shape, but the apex (from which the appendix arises) comes to point inferomedially. This infantile form persists throughout life in only a very small percentage of individuals. Occasionally the originally conical caecum becomes quadrate as a result of the outgrowth of a saccule on each side of the anterior taenia: the saccules are of equal size and the appendix arises from the depression between them instead of from the apex of a cone. In the normal adult form, the right saccule grows more rapidly than the left, forming a new ‘apex’. The original apex, with the appendix attached, is pushed towards the ileocaecal junction (Fig. 67.16).

Fig. 67.16 The caecum and ileocaecal valve, double contrast barium enema appearance.

The process of fluid and electrolyte reabsorption starts in the caecum but takes place largely in the ascending and transverse colon. The distensible nature and ‘sac-like’ morphology of the caecum permit storage of large volumes of semi-liquid chyme that enters from the small bowel via the ileocaecal valve. The large resting diameter of the caecum makes it most liable to distension during periods of increased intracolonic pressure and thus it is the most vulnerable portion of the large intestine to perforation secondary to dilatation (obstructed or otherwise).

Ileocaecal valve

The ileum opens into the posteromedial aspect of the large intestine at the junction of the caecum and colon via the ileocaecal ‘valve’. The orifice has two flaps which project into the lumen of the large intestine (Fig. 67.17). The precise shape and form of the valve varies; in the cadaveric caecum the flaps are often semilunar, but in life they can often be seen to form a rosette or trefoil pattern of mucosa. The upper flap, approximately horizontal, is attached to the junction of the ileum and colon; the lower flap is longer and more concave, and is attached to the junction of the ileum and caecum. At their ends the flaps fuse, continuing as narrow membranous ridges, the frenula of the valve. The orifice may appear in many different shapes depending on the state of contraction or distension of the caecum; it is commonly either a slit or an oval.

Fig. 67.17 Endoscopic appearance of the ileocaecal valve.

The margin of the ileocaecal valve is formed by a reduplication of the intestinal mucosa and circular muscle. Longitudinal muscle fibres are partly reduplicated as they enter the valve, but the more superficial fibres and the peritoneum continue from the small to the large intestine without interruption. The valve may prevent reflux of chyme from the caecum to the ileum, and may slow the passage of ileal contents into the caecum when the circular muscle of the valve is contracted by sympathetic stimulation. Although circular and longitudinal muscle layers of the terminal ileum continue into the valve, there is little evidence that it constitutes a true functional sphincter.

The ileal valvular surfaces are covered with villi and have the structure of the small intestinal mucosa, whereas their caecal surfaces have no villi but display numerous orifices of tubular glands peculiar to the colonic mucosa.

Caecal volvulus

Because the caecum and ascending colon may possess a mesentery to a variable degree, it is possible for the caecum (and lower portion of the ascending colon) to rotate about their mesenteric attachment (mesentero-axial volvulus) such that the lower pole of the caecum comes to lie in the left or right upper quadrant of the abdomen. The fixed apex of this rotation is formed by the attachment of the caecal mesentery to the retroperitoneum. Volvulus occurs most commonly in those individuals where a large portion of the ascending colon lies on a mesentery and the common origin of the caecal and ascending colic mesentery is narrow. It is extremely unlikely in individuals where the caecum and ascending colon possess a short broadly attached mesentery. True isolated volvulus of the caecum is extremely rare.

VERMIFORM APPENDIX

The vermiform appendix is a narrow, vermiform (worm-like) tube which arises from the posteromedial caecal wall, approximately 2 cm below the end of the ileum. It may occupy one of several positions (Fig. 67.18). The commonest positions seen in clinical practice are retrocaecal or retrocolic (behind the caecum or lower ascending colon respectively), pelvic, or descending (when the appendix hangs dependently over the pelvic brim, in close relation to the right uterine tube and ovary in females). Other positions, including subcaecal (below the caecum), and pre- or post-ilial (anterior or posterior to the terminal ileum respectively), are occasionally seen, especially when there is a long appendicular mesentery which allows greater mobility.

Fig. 67.18 The major positions of the appendix encountered at surgery or postmortem.

The three taeniae coli on the ascending colon and caecum converge on the base of the appendix, and merge into its longitudinal muscle. The anterior caecal taenia is usually distinct and can be traced to the appendix, which affords a guide to its location intra-operatively. The appendix varies from 2–20 cm in length: it is often relatively longer in children and may atrophy and shorten after mid-adult life. It is connected by a short mesoappendix to the lower part of the ileal mesentery. This fold is usually triangular, extending almost to the appendicular tip along the whole viscus.

The lumen of the appendix is small and opens into the caecum by an orifice lying below and slightly posterior to the ileocaecal opening. The orifice is sometimes guarded by a straight mucosal fold forming an asymmetrical ‘valve’ which gives the appendix orifice the appearance of a ‘strung bow’ (Fig. 67.19). This fold tends to lie parallel to the medial wall of the caecum and ileocaecal valve and an imaginary ‘arrow’ placed in the bow usually points in the direction of the ileocaecal valve; this is a useful sign during colonoscopic examinations. The lumen may be widely patent in early childhood but is often partially or wholly obliterated in the later decades of life.

Microstructure

The layers of the wall of the appendix are essentially those of the rest of the large intestine (see below) but some features are notably different and are described here. The serosa forms a complete covering, except along the mesenteric attachment. The longitudinal muscular fibres form a complete layer of uniform thickness, except over a few small areas where both muscular layers are deficient, so that the serosa and submucosa are in contact. At the base of the appendix, the longitudinal muscle thickens to form rudimentary taeniae that are continuous with those of the caecum and colon.

The submucosa typically contains many large lymphoid aggregates that extend from the mucosa and obscure the muscularis mucosae layer, which consequently is discontinuous. These aggregates also cause the mucosa to bulge into the lumen of the appendix, so that it narrows irregularly (Fig. 67.20). The mucosa is covered by a columnar epithelium as it is elsewhere in the large intestine, and the epithelium that overlies the mucosal lymphoid tissue contains M cells. Glands (crypts) are similar to those of the colon but are fewer in number and so are less densely packed. They penetrate deep into the lymphoid tissue of the mucosal lamina propria. The submucosal lymphoid tissue frequently exhibits germinal centres within its follicles, indicative of B-cell activation, as occurs in secondary lymphoid tissue elsewhere. Lymphoid follicles are absent at birth but accumulate over the first 10 years of life to become a prominent feature of the appendix. In adults, the normal layered structure of the appendix is lost and the lymphoid follicles atrophy and are replaced by collagenous tissue, and in the elderly, the appendix may be filled with fibrous scar tissue.

Fig. 67.20 Low-power micrograph of the appendix in transverse section, showing part of its circumference and a faecal pellet lodged in its lumen. Lymphoid tissue (basophilic staining) occupies much of the mucosa between crypts, and part of the submucosa. The muscularis externa and outermost serosal layer are seen at the right of the field.

Acute appendicitis

The genesis of acute appendicitis varies between individuals. It may follow obstruction of the lumen, when it is a consequence of increased intraluminal pressure and retention of (infected) contents which allows acute suppuration to occur. The size of the orifice of the appendix in some individuals may contribute to the risk that this will happen. Appendicoliths may be visible on plain radiographs in 7–15% of the normal population: in those patients with acute abdominal pain, their presence indicates a high risk of appendicitis being present. The increased size of the orifice and lumen at the extremes of life may be a reason why acute appendicitis is relatively uncommon in these age groups. Acute appendicitis may also present as a primary suppuration of the tissues of the appendix itself: the reduction in appendicular lymphoid tissue that occurs in later life may be another reason why the disease is infrequent in the elderly. Although the appendix is well supplied by arterial anastomoses at its base, the appendicular artery is an end artery from the midpoint upwards and its close proximity to the wall makes it susceptible to thrombosis during episodes of acute inflammation. This may render the distal appendix ischaemic and explains the frequency of gangrenous perforation seen in the disease.

The appendix and overlying visceral peritoneum are innervated by sympathetic and parasympathetic nerves from the superior mesenteric plexus. Visceral afferent fibres carrying sensations of distension and pressure mediate the symptoms of ‘pain’ felt during the initial stages of appendicular inflammation. In keeping with other structures derived from the midgut, these sensations are poorly localized initially, and referred to the central (periumbilical) region of the abdomen. It is not until parietal tissues adjacent to the appendix become involved in any inflammatory process that somatic nociceptors are stimulated, and there is an associated change in the nature and localization of pain. The caecum and proximal ascending colon share a common innervation with the appendix (sympathetic and parasympathetic nerves via the superior mesenteric plexus), which means that early inflammation in the caecum (typhlitis) results in similar visceral pain symptoms to those experienced in appendicitis.

ASCENDING COLON

The ascending colon is approximately 15 cm long and narrower than the caecum. It ascends to the inferior surface of the right lobe of the liver, on which it makes a shallow depression, and then turns abruptly forwards and to the left, at the hepatic flexure. It is a retroperitoneal structure covered anteriorly and on both sides by peritoneum. Its posterior surface is separated by loose connective tissue from the iliac fascia, the iliolumbar ligament, quadratus lumborum, the aponeurosis of transversus abdominis, and the anterior peri-renal fascia inferolateral to the right kidney. The lateral femoral cutaneous nerve, usually the fourth lumbar artery, and sometimes the ilioinguinal and iliohypogastric nerves, lie posteriorly as they cross quadratus lumborum. Laterally the peritoneum forms the lateral paracolic gutter and medially the medial paracolic gutter (Fig. 67.21). The ascending colon possesses a narrow mesocolon for part of its course in up to one-third of cases. Anteriorly it is in contact with loops of ileum, the greater omentum and the anterior abdominal wall.

Fig. 67.21 Posterior relations of the ascending colon.

Hepatic flexure

The hepatic flexure forms the junction of the ascending and transverse colon as the latter turns down, forwards and to the left. It is variable in position and usually has a less acute angle than the splenic flexure. The anterior surface of the lower pole of the right kidney is posterior, the right lobe of the liver is superior and anterolateral (Fig. 67.22), the descending (second) part of the duodenum is medial and the fundus of the gallbladder is anteromedial. The posterior aspect of the hepatic flexure is not covered by peritoneum and the gut wall is in direct contact with pararenal fascia. The hepatic flexure is often covered in the greater omentum which may be attached to the anterior surface both of the upper ascending colon and the proximal (right) end of the transverse colon.

Fig. 67.22 Axial CT images of the ascending colon obtained (A) at the level of the mid ascending colon and (B) at the level of the hepatic flexure, showing the relationship of the ascending colon and hepatic flexure to the right lobe of the liver, the right and left kidneys, the second part of the duodenum, and the gallbladder (supplied by Dr Louise Moore, Chelsea and Westminster Hospital, London).

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

Arteries

Superior mesenteric artery

The arteries to the caecum, appendix and ascending colon are somewhat variable, but all are derived from the superior mesenteric artery via the ileocolic and right colic (when present) arteries (see Figs 67.11, 67.13).

Ileocolic artery

The ileocolic artery arises from the right side of the superior mesenteric artery in its upper half in the root of the mesentery. It descends to the right beneath the parietal peritoneum towards the caecum and crosses anterior to the right ureter, gonadal vessels and psoas major to enter the right iliac fossa. Although the terminal distribution varies, the artery usually divides into a superior division which runs superolaterally towards the ascending colon where it anastomoses with the right colic artery (or right branch of the middle colic artery), and an inferior division which anastomoses with the final ileal branch of the distal superior mesenteric artery. The inferior division approaches the superior border of the ileocolic junction. It usually gives rise to the following branches: ascending colic (which passes up on the ascending colon to anastomose with the superior division), anterior caecal, posterior caecal (which usually gives rise to the appendicular artery) and ileal (which ascends to the left on the lower ileum, supplies it and anastomoses with the last ileal branch of the superior mesenteric artery (Fig. 67.23). The caecum is supplied almost entirely from the ileocolic artery.

Fig. 67.23 The arteries of the caecum, vermiform appendix and ascending colon.

The ileocolic artery is the most prominent vessel in the lower right colic mesentery and traction on the caecum in the direction of the anterior superior iliac crest will readily cause the vessel to ‘tent’ the mesentery due to its direct course towards the caecum. This allows easy identification of the vessel during laparoscopic surgery.

The appendicular artery descends behind the terminal ileum to enter the mesoappendix a short distance from the appendicular base (Fig. 67.23). Here it gives off a recurrent branch, which anastomoses at the base of the appendix with a branch of the posterior caecal artery: the anastomosis is sometimes extensive. The main appendicular artery approaches the tip of the organ, at first near to, and then in the edge of, the mesoappendix. The terminal part of the artery lies on the wall of the appendix and may be thrombosed in appendicitis, which results in distal gangrene or necrosis. Accessory arteries are common, and many individuals possess two or more arteries of supply to the appendix.

The arcades of the terminal ileal artery provide a collateral supply to the caecum via anastomoses with the ileal branch of the ileocolic artery.

Right colic artery

The right colic artery is a small vessel that is highly variable in its anatomy (Fig. 67.24). Most commonly it arises as a common trunk with the middle colic artery. Alternatively it may arise as a separate branch from the right side of the superior mesenteric artery, or from the ileocolic artery (when it is referred to as an accessory right colic artery), and occasionally it may be absent. From its origin in common with the middle colic artery it passes towards the ascending colon, deep to the parietal peritoneum and anterior to the right gonadal vessels, right ureter and psoas major. Sometimes it arises more superiorly and crosses the second part of the duodenum and inferior pole of the right kidney. Near the colon it divides into a descending branch, which anastomoses with the ileocolic artery, and an ascending branch which anastomoses with the right branch of the middle colic artery. These form the marginal artery in the area of the hepatic flexure from which vessels are distributed to the upper third of the ascending colon, and the right side of the transverse colon.