Last's Anatomy: Regional and Applied

Part six. Gastrointestinal tract

The alimentary canal has its embryological origin in the yolk sac. It is lined by epithelium derived from the endoderm, and outgrowths from this epithelium (liver and pancreas) are thus endodermal in origin.

Macroscopically the alimentary canal is a tube of muscle lined with mucous membrane and covered, for the most part, with peritoneum.

The muscular wall of the alimentary canal, from the upper end of the oesophagus to the lower end of the rectum, is in two separate layers, an inner circular and an outer longitudinal. This arrangement is characteristic of all tubes that empty by orderly peristalsis and not by a mass contraction. In reality the muscle is in two spiral layers. The inner layer forms a closewound spiral and is the circular layer. The outer layer spirals in such gradual fashion that its fibres are virtually longitudinal, the longitudinal layer. The latter is mostly separated into three discrete bundles, the taeniae coli, in the colon. An innermost third layer of fibres arranged in an oblique manner reinforces the body of the stomach.

The two layers of the upper third of the oesophagus (cervical and superior mediastinal parts) consist of striated muscle. In the lower two-thirds of the oesophagus this is gradually replaced by visceral muscle, which constitutes the two layers of the remainder of the alimentary canal.

The mucous membrane of the alimentary tube consists of three components: epithelium, an underlying connective tissue layer or lamina propria, and a thin layer of smooth muscle, the muscularis mucosae.

A variety of neuroendocrine cells are scattered in the mucosa of the alimentary tract from oesophagus to anal canal. The number of these cells progressively decreases in an anal direction. They all produce peptides and/or amines, which are active as hormones or neurotransmitters. They are a part of the amine precursor uptake and decarboxylation (APUD) cell series, and they modulate autonomic activity as well as each other. The substances produced by these cells include gastrin, vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), motilin, secretin, somatostatin, substance P (SP), serotonin and endorphins.

Oesophagus

The epithelium is stratified squamous non-keratinizing, like that of most of the pharynx and mouth. The muscularis mucosae, which is absent from the uppermost part, is characteristically thicker in the oesophagus than in any other part of the alimentary tract. Small groups of mucus-secreting glands may be present at the upper and lower ends, in both the mucosa and submucosa.

Stomach

At the gastro-oesophageal junction (cardia), or a few centimetres above it, there is an abrupt change along an irregular line from greyish-pink stratified to reddish-pink single-layered columnar epithelium which continues all the way to the anal canal, but with differing cell types and configuration in the different organs. Throughout, the epithelium is not a flat layer but dips down into the connective tissue lamina propria to form myriads of glands. In the main part of the stomach, the body, the mucus-secreting surface cells dip down to form the gastric pits which in turn continue downwards as the straight test-tube-like glands whose cells include the peptic and parietal cells secreting pepsin and hydrochloric acid respectively. The parietal cells also secrete intrinsic factor, which is necessary for the intestinal absorption of vitamin B₁₂. For about a 1cm ring at the cardia, the glands are shorter and the cells all mucus-secreting. In the pyloric region the glands are like coiled test-tubes and again the cells are all mucus-secreting. This part of the stomach contains most of the gastrin-producing endocrine G cells, as well as D cells which secrete somatostatin. Chromaffin endocrine cells in both body and pyloric regions produce serotonin and endorphin.

Small intestine

In the whole length of the small intestine, the columnar epithelium not only dips down to form glands (the crypts of Lieberkühn) but is also thrown up between the gland openings into villi, which consist of finger-like or leaf-like connective tissue cores covered by epithelium. Some of the villous cells are mucus-secreting goblet cells; others are the absorbing cells or enterocytes. Both types are derived from progenitors in the crypts and are constantly being shed and renewed, every few days. (So are other alimentary epithelial cells but at slower rates.) At the bases of the crypts are the granular Paneth cells which secrete lysozyme. Scattered among the other crypt cells are various neuroendocrine cells responsible for the production of intestinal hormones, including secretin, somatostatin and CCK, as well as chromaffin cells.

The duodenum is distinguished from the rest of the small intestine in having mucus-secreting glands (of Brunner) in the submucosa. In the terminal ileum there are groups of lymphoid follicles in the mucosa forming Peyer's patches, in contrast to single follicles which can be found throughout the alimentary canal from mouth to anus.

Large intestine

In the large intestine there are no villi, only glands (crypts) containing a high proportion of goblet cells. In the appendix the glands are rather shallower and less closely packed than in the rest of the large intestine, and there are numerous lymphoid follicles in the mucosa and submucosa.

In the upper part of the anal canal, the columnar epithelium gives place to the stratified squamous type, but here the junction is not as clear cut as the change at the cardia (see above).

The abdominal oesophagus, about 1 to 2cm in length, turns forwards and to the left immediately below the diaphragmatic opening and grooves the posterior surface of the left lobe of the liver. The anterior and posterior vagal trunks are related to the respective oesophageal surfaces here (see p. 250). It is covered on its front and left side by peritoneum which passes from it on the right to the diaphragm as the uppermost part of the lesser omentum, and on the left forms the uppermost part of the greater omentum. The posterior wall of the oesophagus is rather shorter than the anterior, for the orifice in the diaphragm lies very nearly vertical. The left inferior phrenic artery lies behind the oesophagus.

The oesophagus enters the stomach at the cardiac orifice. The right margin is continuous with the lesser curvature, while the left margin makes an acute angle with the gastric fundus, the cardiac notch. The fibres of the right crus that pass to the left of the diaphragmatic opening form a sling around the abdominal oesophagus. Various factors contribute towards guarding against the reflux of gastric contents; these include the contraction of these right crus fibres, the angle of entry of the oesophagus into the stomach, the longitudinal folds of the oesophageal mucosa, a high pressure zone in the lower 3cm of oesophagus and the effect of positive intra-abdominal pressure on the abdominal segment of the oesophagus.

The stomach is the most dilated part of the alimentary tract, interposed between the oesophagus and duodenum in the upper part of the abdominal cavity. Lying mainly in the left hypochondrial, epigastric and umbilical regions, much of it is under cover of the lower ribs. It is a muscular bag, relatively fixed at both ends but otherwise subject to great variations in size depending on the volume of its contents. The gastro-oesophageal junction is the cardia, the most fixed part of the organ, and lies 2.5cm to the left of the midline at the level of T10 vertebra, behind the seventh left costal cartilage. It is 40cm from the incisor teeth. The gastroduodenal junction is the pylorus. In the recumbent position with the stomach empty this is usually a little to the right of the midline at the level of L1 vertebra but may be considerably lower.

The main parts of the stomach are the fundus, body and pyloric part, with the greater and lesser curvatures forming the left and right borders respectively. The stomach is completely invested by peritoneum, which passes in a double layer from the lesser curvature to the liver as the lesser omentum, and hangs down from the fundus and greater curvature as the greater omentum.

The fundus is the part which projects upwards above the level of the cardia, and is in contact with the left dome of the diaphragm. It is usually full of swallowed air.

The largest part of the stomach is the body, extending from the fundus to the notch, the angular incisure, on the lower part of the lesser curvature. Variable amounts of the body will be above and below the costal margin, in contact with the diaphragm and anterior abdominal wall. Likewise the lowest part of the greater curvature may be above or below umbilical level.

The pyloric part extends from the angular notch to the gastroduodenal junction, and consists of the proximal pyloric antrum which narrows distally as the pyloric canal (Fig. 5.25). The circular muscle of the distal end of the canal is palpably thickened to form the pyloric sphincter, whose position is indicated on the anterior surface by the prepyloric vein (of Mayo). The pyloric canal lies on the head and neck of the pancreas.

Behind the stomach are a group of structures comprising the stomach bed (Fig. 5.26). The posterior wall of the stomach is covered by peritoneum of the anterior wall of the lesser sac, and the bed is covered by the lesser sac's posterior wall. Apart from the left crus and dome of the diaphragm the bed consists of the splenic artery, body of pancreas, transverse mesocolon, upper part of left kidney, left suprarenal gland, spleen and left colic flexure. To the right of the lesser curvature in the midline lies the aorta with the coeliac trunk, the coeliac plexus and ganglia, and coeliac lymph nodes.

Blood supply

The stomach is supplied by branches from the coeliac trunk. Along the lesser curvature between the two layers of the lesser omentum the left gastric artery anastomoses with the right gastric; these arteries may be double. The fundus and upper left part of the greater curvature receive about six short gastric arteries from the splenic artery in the gastroplenic ligament. The rest of the curvature is supplied by the left and right gastroepiploic vessels, which run between the two layers of the greater omentum and anastomose with each other. The right gastroepiploic artery is closer to the greater curvature than the left gastroepiploic artery. During partial gastrectomy the greater omentum is divided below the right and above the left gastroepiploic arteries; the blood supply to the omentum usually remains intact as the larger omental branches of the left gastroepiploic artery are preserved. A branch from the splenic artery as it runs along the upper border of the pancreas, the posterior gastric artery, may also supply the stomach.

Veins of the same name accompany the arteries and drain into the portal vein itself or its splenic and superior mesenteric tributaries. The prepyloric vein (unaccompanied by an artery) drains into the right gastric vein.

Lymph drainage

All lymph from the stomach eventually reaches coeliac nodes after passing through various outlying groups (Fig. 5.27A). Lymph vessels anastomose freely in the stomach wall, but there are valves in the vessels that direct lymph in such a way that a line drawn parallel to the greater curvature and two-thirds of the way down the anterior surface indicates a ‘watershed’ (Fig. 5.27B). From the largest zone above and to the right of this line, lymph passes to left and right gastric nodes along the left and right gastric arteries. From the upper left quadrant lymph flows via left gastroepiploic nodes and directly to pancreaticosplenic nodes at the splenic hilum, and at the upper border and posterior surface of the pancreas, accompanying the splenic artery. From the rest of the greater curvature region of the stomach lymph reaches nodes along the right gastroepiploic vessels, which drain to subpyloric nodes near the gastroduodenal artery. The pyloric part of the stomach drains to hepatic nodes in the porta hepatis and to subpyloric and right gastric nodes. In cases of gastric carcinoma, the left supraclavicular nodes may rarely become palpably involved (Troisier's sign), presumably by spread along the thoracic duct.

Nerve supply

Sympathetic fibres (which are vasomotor and also cause pyloric sphincter contraction) accompanied by afferent (pain) fibres run with the various arterial branches to the stomach. The parasympathetic supply from the vagi controls motility and secretion (although 90% of vagal fibres below the diaphragm are afferent for reflex activities, not pain). At the oesophageal opening in the diaphragm the anterior vagal trunk (comprising mainly left vagal fibres from the oesophageal plexus in the posterior mediastinum) lies in contact with the anterior oesophageal wall (Fig. 5.28), usually nearer its right margin; sometimes this trunk is double or triple. Each anterior trunk gives off one or two hepatic branches, which run in the upper part of the lesser omentum to join the plexus on the hepatic artery and portal vein, and then turn down in the anterior wall of the epiploic foramen to reach the pylorus. The anterior vagus also gives off several gastric branches which supply the fundus and body, and one large branch (greater anterior gastric nerve or anterior nerve of Latarget) which runs down in the lesser omentum near the lesser curvature with the left gastric artery, and subdivides in the manner of a crow's foot to supply the antrum and pyloric sphincter. The posterior vagal trunk (comprising mainly right vagal fibres) lies in loose tissue a little behind and to the right, not in contact with the posterior surface of the oesophagus (Fig. 5.28). It gives coeliac branches that run backwards along the left gastric artery to the coeliac plexus. It also gives gastric branches to the fundus and body, and a large branch (greater posterior gastric nerve or posterior nerve of Latarget) which runs in the lesser omentum behind the anterior trunk to reach the antrum but not the pyloric sphincter.

Vagotomy. Truncal vagotomy involves cutting the trunks at the level of the abdominal oesophagus. In selective vagotomy the vagal trunks are cut distal to the hepatic branch of the anterior vagus and the coeliac branches of the posterior vagus. Although effective in diminishing gastric secretion, truncal and selective vagotomy are often accompanied by gastric stasis, so that an antral drainage procedure is required. Highly selective vagotomy (parietal cell vagotomy) attempts to avoid stasis by cutting only the branches to the fundus and body, leaving the antral and pyloric branches intact. Arterial branches run into the lesser curvature transversely but nerve branches approach it obliquely. Ligating vessels will inevitably sever some nerve branches but not all, as not all nerves accompany vessels closely, and any individual nerves that can be identified must be cut also.

Structure

The main histological features are referred to on page 248. Although the angular notch is usually taken as the dividing line between the body and pyloric parts of the stomach, this does not necessarily indicate exactly where the body-type of mucosa with its parietal (acid-secreting) cells gives way to the pyloric-type with its concentration of G cells (producing gastrin). The outer longitudinal and inner circular muscle coats completely invest the stomach. They are reinforced by an incomplete innermost oblique muscle layer; its fibres loop over the fundus, being thickest at the notch between oesophagus and stomach, helping to maintain the angle here.

The small intestine consists of the duodenum, jejunum and ileum, although clinically the term small intestine often excludes the duodenum.

The duodenum is a C-shaped tube lying in front of, and to the right of the inferior vena cava and aorta (Fig. 5.26). The first 2.5cm are contained between the peritoneum of the lesser and greater omenta, but the remainder is retroperitoneal. It is divided into four parts, superior, descending, horizontal and ascending, or more simply first, second, third and fourth. The total length is 25cm (10 in), the lengths of the parts being easily remembered in inches as 2, 3, 4 and 1 but less conveniently in centimetres (5, 7.5, 10 and 2.5).

The duodenum makes its C-shaped loop round the head of the pancreas, which is opposite the body of L2 vertebra; so the first part may be said to lie at the level of L1, the second on the right side of L2, the third crosses in front of L3, and the fourth is on the left of L2 vertebrae.

The first part of the duodenum runs to the right, upwards and backwards from the pylorus; a foreshortened view is consequently obtained in anteroposterior radiographs (Fig. 5.25). The first 2.5cm (i.e. the duodenal cap, see below) lies between the peritoneal folds of the greater and lesser omenta; it forms the lowermost boundary of the opening into the lesser sac (Fig. 5.35A). It lies in front of the gastroduodenal artery, bile duct, and portal vein, and behind these structures lies the inferior vena cava. The gallbladder is anterior to the duodenal cap. The next 2.5cm passes backwards and upwards on the upper part of the head of the pancreas to the medial border of the right kidney. It is covered in front with peritoneum, and the inferior surface of the right lobe of the liver lies over this peritoneum. Its posterior surface is bare of peritoneum.

The second part of the duodenum curves downwards over the hilum of the right kidney (Fig. 5.26). It is covered in front with peritoneum and crossed by the attachment of the transverse mesocolon, so that its upper half lies in the supracolic compartment to the left of the hepatorenal pouch (in contact with the liver) and its lower half lies in the right infracolic compartment medial to the inferior pole of the right kidney (in contact with coils of jejunum). It lies alongside the head of the pancreas, approximately at the level of L2 vertebra.

Its posteromedial wall receives the common opening of the bile duct and main pancreatic duct at the hepatopancreatic ampulla (of Vater), which opens on the summit of the major duodenal papilla, halfway along the second part, 10cm from the pylorus. It is overlapped by a semilunar flap of mucous membrane. Two centimetres proximal is the small opening of the accessory pancreatic duct (on the minor duodenal papilla).

The third part of the duodenum curves forwards from the right paravertebral gutter over the slope of the right psoas muscle (gonadal vessels and ureter intervening) and passes over the forwardly projecting inferior vena cava and aorta to reach the left psoas muscle (Fig. 5.26). As its inferior border crosses the aorta it lies on the commencement of the inferior mesenteric artery. Its upper border hugs the lower border of the pancreas. It is covered by the peritoneum of the posterior abdominal wall just below the transverse mesocolon. It is crossed by the superior mesenteric vessels and by the leaves of the commencement of the mesentery of the small intestine sloping down from the duodenojejunal flexure. It lies, therefore, in both right (mainly) and left infracolic compartments (Fig. 5.14). Its anterior surface is in contact with coils of jejunum.

The fourth part of the duodenum ascends to the left of the aorta, lying on the left psoas muscle and left lumbar sympathetic trunk, to reach the lower border of the body of the pancreas. It is covered in front by the peritoneal floor of the left infracolic compartment and by coils of jejunum. It breaks free from the peritoneum that has plastered it down to the posterior abdominal wall and curves forwards and to the right as the duodenojejunal flexure. This pulls up a double sheet of peritoneum from the posterior abdominal wall, the mesentery of the small intestine, which slopes down to the right across the third part of the duodenum and posterior abdominal wall (Fig. 5.29).

The duodenojejunal flexure is fixed to the left psoas fascia by fibrous tissue and may be further supported by the suspensory muscle of the duodenum (muscle or ligament of Treitz). This is a thin band of connective tissue which may contain muscle—skeletal muscle fibres that run from the right crus of the diaphragm to connective tissue around the coeliac trunk and smooth muscle fibres that run from there, behind the pancreas and in front of the left venal vein, to the muscle coat of the flexure.

Internally the mucous membrane of most of the duodenum, like the rest of the small intestine, is thrown into numerous circular folds (plicae circulares or valvulae conniventes). But the walls of the first 2.5cm are smooth, hence the smooth outline of the full shadow of barium in the ‘duodenal cap’ at radiographic examination (Fig. 5.25). From the duodenal cap onwards the plicae break up the barium and its shadow.

Paraduodenal recesses. To the left of the duodenojejunal flexure certain peritoneal folds may cover recesses or fossae. The paraduodenal recess proper (Fig. 5.29) lies behind a fold raised by the upper end of the inferior mesenteric vein; an incarcerated internal hernia in this fossa may obstruct and thrombose the vein, and there is danger of dividing the vein if the peritoneum has to be divided at operation to free the hernia. Horizontal folds of peritoneum may cover superior and inferior duodenal recesses and a retroduodenal recess may be found behind the flexure. The mouths of these four recesses or fossae all face towards each other.

Blood supply

The duodenum is supplied by the superior and inferior pancreaticoduodenal arteries, but the first 2cm, the usual site of ulceration, receives blood from the hepatic, gastroduodenal, supraduodenal, right gastric and right gastroepiploic arteries. Venous drainage is to tributaries of the superior mesenteric and portal veins.

Lymph drainage

Duodenal lymph drains by channels that accompany the superior and inferior pancreaticoduodenal vessels to coeliac and superior mesenteric nodes.

The jejunum is wider-bored and thicker-walled than the ileum. The thick wall of the jejunum feels double (the mucous membrane can be felt through the muscle wall, ‘a shirt sleeve felt through a coat sleeve’); the thin wall of the ileum feels single.

The lower reaches of the ileum are distinguished by the presence on the antimesenteric border of elongated whitish plaques in the mucous membrane, usually but not always visible through the muscle wall. These are the aggregated lymphoid follicles (Peyer's patches). The jejunum lies coiled in the upper part of the infracolic compartment, the ileum in the lower part thereof and in the pelvis.

The jejunum and ileum together lie in the free margin of the mesentery. Total length varies greatly, from about 4 to 6 metres. The jejunum constitutes rather less than half the total length, say two-fifths, allowing three-fifths for the ileum.

An ileal (Meckel's) diverticulum is present in 2% of individuals, 60cm (2ft) from the caecum, and is 5cm (2 in) long, according to a popular mnemonic but the length of the diverticulum is variable and its site may be more proximal. Its blind end may contain gastric mucosa or liver or pancreatic tissue; ulceration and perforation of the tip can occur. It represents the intestinal end of the vitellointestinal duct, and its apex may be adherent to the umbilicus or connected thereto by a fibrous cord, a further remnant of the duct.

Blood supply

Numerous jejunal and ileal branches arise from the left side of the superior mesenteric artery and enter the mesentery by passing between the two layers of the root. The jejunal branches join each other in a series of anastomosing loops to form arterial arcades: single for the upper jejunum and double lower down. From the arcades, straight arteries pass to the mesenteric border of the gut (Fig. 5.30). These vessels are long and close together, forming high narrow ‘windows’ in the intestinal border of the mesentery, visible because of the scantiness of mesenteric fat here. The straight vessels pass to one or other side of the jejunum and sink into its wall. Occlusion of a straight artery may lead to infarction of the segment supplied because these are end arteries, but occlusion of arcade vessels is usually without effect due to their numerous anastomotic connections.

The ileal arteries are similar but form a larger series of arcades—three to five, the most distal lying near the ileal wall so that the straight vessels branching off the arcades are shorter. There is more fat in this part of the mesentery, so the windows characteristic of the jejunal part are not seen; this is a useful feature in identifying loops of the bowel.

The end of the superior mesenteric artery itself supplies the region of the ileal diverticulum (if present), and anastomoses with the arcades and with the ileocolic branch to supply the terminal ileum.

The veins all correspond to the arteries and thus drain to the superior mesenteric.

Lymph drainage

Jejunal and ileal lymph drains to superior mesenteric nodes via mural and intermediate nodes in the mesentery.

Nerve supply

Autonomic nerves reach the wall of the small intestine with its blood vessels. The parasympathetic vagal supply augments peristaltic activity and intestinal secretion. There are many afferent fibres whose function is uncertain. The sympathetic supply, which is vasoconstrictor and normally inhibits peristalsis, is from the lateral horn cells of spinal segments T9 and 10. Pain impulses use sympathetic pathways mainly and small intestinal pain is usually felt in the umbilical region of the abdomen.

The large intestine consists of the caecum with the (vermiform) appendix, the ascending, transverse, descending and sigmoid parts of the colon, the rectum and the anal canal.

Caecum

This blind pouch of the large intestine projects downwards from the commencement of the ascending colon, below the ileocaecal junction (Fig. 5.31A). It is usually completely covered by peritoneum, which is reflected downwards to the floor of the right iliac fossa, and the retrocaecal peritoneal space may be shallow or deep, according to the distance of this reflection from the lower end of the caecum. Often there are two peritoneal folds from either side of the posterior wall of the caecum, forming between them the retrocaecal recess in which the appendix may lie. As in the rest of the colon, the longitudinal muscle of the caecum is concentrated into three flat bands, the taeniae coli, within which is the circular muscle layer of the sacculated wall. The taeniae lie one anterior, one posteromedial and one posterolateral. All three converge on the base of the appendix, to which they are a useful guide.

Internally the ileocaecal junction is guarded by the ileocaecal valve (Fig. 5.31B), whose almost transverse lips may help to prevent some reflux into the ileum, but any possible sphincteric action is poor.

In the infant the caecum is conical and the appendix extends downwards from its apex. The lateral wall outgrows the medial wall and bulges down below the base of the appendix in the adult; the base of the appendix thus comes to lie in the posteromedial wall of the caecum above its lower end and the three taeniae coverage to this point. The endoscopic view of the site of convergence has a trefoil appearance (Fig. 5.31D).

The caecum lies on the peritoneal floor of the right iliac fossa, over the iliacus and psoas fasciae and the femoral and lateral femoral cutaneous nerves. Its lower end lies at the pelvic brim. When distended its anterior surface touches the parietal peritoneum of the anterior abdominal wall; when collapsed, coils of ileum lie between the two.

Blood supply. Branches of the anterior and posterior caecal arteries (branches of the ileocolic artery) fan out over the respective surfaces of the caecum. The posterior caecal artery is larger and gives a branch to the base of the appendix. There are corresponding veins.

Lymph drainage. Lymph passes to nodes associated with the ileocolic artery.

Appendix

The vermiform (worm-shaped) appendix is a blind-ending tube (Fig. 5.31) varying in length (commonly about 6–9cm), which opens into the posteromedial wall of the caecum 2cm below the ileocaecal valve (Fig. 5.31B). On the surface of the abdomen this point (McBurney's) lies one-third of the way up the oblique line that joins the right anterior superior iliac spine to the umbilicus. While the position of its base is constant in relation to the caecum, the appendix itself may lie in a variety of positions (Fig. 5.31C). The most common, as found at operation, is the retrocaecal position, with the pelvic position next in order of frequency.

The three taeniae of the caecum merge into a complete longitudinal muscle layer for the appendix. The submucosa contains many lymphoid masses and the lumen is thereby irregularly narrowed. This lumen is wider in the young child and may be obliterated in old age.

The appendix has its own short mesentery, the mesoappendix, which is a triangular fold of peritoneum from the left (inferior) layer of the mesentery of the terminal ileum (Fig. 5.31A). A small fold of peritoneum extends from the terminal ileum to the front of the mesoappendix. This is the ileocaecal fold (or ‘bloodless fold of Treves’, although it sometimes contains blood vessels) and the space between it and the mesoappendix is the inferior ileocaecal recess. Another fold lies in front of the terminal ileum, between the base of the mesentery and the anterior wall of the caecum. This fold is raised up by the contained anterior caecal artery and is called the vascular fold of the caecum. The space behind it is the superior ileocaecal recess.

Blood supply. The appendicular artery is normally a branch of the inferior division of the ileocolic artery, which runs behind the terminal ileum to enter the mesoappendix. As it does so it gives off a recurrent branch which anastomoses with a branch of the posterior caecal artery; this may replace the appendicular artery. The appendicular artery runs first in the free margin of the mesoappendix and then close to the appendicular wall, where it may be thrombosed in appendicitis, leading to ischaemic necrosis and perhaps rupture of the appendix; there is no collateral circulation, for the appendicular artery is an end artery. There are corresponding veins.

Lymph drainage. As from the caecum, lymph passes to nodes associated with the ileocolic artery.

Appendicectomy. Exposure of the appendix during appendicectomy is through a McBurney or transverse muscle-splitting incision (see p. 232). If it is not immediately obvious, tracing any of the taeniae down over the caecal wall will lead to the base of the appendix.

Of the four parts of the colon, the transverse and sigmoid parts are suspended in mesenteries—the transverse mesocolon and sigmoid mesocolon respectively—but the ascending and descending colon are plastered on to the posterior abdominal wall so that they have posterior ‘bare areas’ devoid of peritoneum.

Ascending colon

This first part of the colon, about 15cm in length, extends upwards from the ileocaecal junction to the right colic (hepatic) flexure. The latter lies on the inferolateral part of the anterior surface of the right kidney, in contact with the inferior surface of the liver. The ascending colon lies on the iliac fascia and the anterior layer of the lumbar fascia. Its front and both sides possess a serous coat, which runs laterally into the paracolic gutter and medially into the right infracolic compartment. The original embryonic mesentery is retained in about 10% of adults.

The taeniae coli lie, in line with those of the caecum, anteriorly, posterolaterally and posteromedially. These consist of longitudinal muscle fibres and the circular muscle coat is exposed between them. The ascending colon is sacculated, due to the three taeniae being ‘too short’ for the bowel. If the taeniae are divided between the sacculations the latter can be drawn apart and the bowel wall flattened.

Small pouches of peritoneum, distended with fat, the appendices epiploicae, project in places from the serous coat. The blood vessels supplying them from the mucosa perforate the muscle wall. Mucous membrane may herniate through these vascular perforations, a condition known as diverticulosis. Diverticulitis is inflammation of these mucosal herniae.

Transverse colon

This part of the colon, about 50cm long, extends from the hepatic to the splenic flexure in a loop which hangs down to a variable degree between these two fixed points, anterior to coils of jejunum and ileum. The convexity of the greater curvature of the stomach lies in its concavity, the two being connected by the gastrocolic omentum. Because of the fusion between the greater omentum and the transverse colon, the rest of the greater omentum appears to hang down from its lower convexity. The transverse colon is completely invested in peritoneum; it hangs free on the transverse mesocolon, which is attached from the inferior pole of the right kidney across the descending (second) part of the duodenum and the pancreas to the inferior pole of the left kidney. The splenic flexure lies, at a higher level than the hepatic flexure, well up under cover of the left costal margin.

The taeniae coli continue from the ascending colon. Due to the looping downwards and forwards of the transverse colon from the flexures, which lie well back in the paravertebral gutters, some rotation of the gut wall occurs at the flexures, and the anterior taenia of ascending and descending colons lies posteriorly, while the other two lie anteriorly, above and below. The appendices epiploicae are larger and more numerous than on the ascending colon.

Descending colon

Approximately 25cm long, this extends from the splenic flexure to the pelvic brim, and in the whole of its course is plastered to the posterior abdominal wall by peritoneum (like the ascending colon), though a mesentery is present in about 20% of adults. The splenic flexure lies on the lateral surface of the left kidney, below and in contact with the tail of the pancreas and the spleen. A fold of peritoneum, the phrenicocolic ligament (see p. 238), attaches the splenic flexure to the diaphragm at the level of the tenth and eleventh ribs. Surgical mobilization of the splenic flexure requires division of this ligament. During this manoeuvre the spleen needs to be safeguarded as it lies in contact with the upper surface of the ligament. The descending colon lies on the lumbar fascia and the iliac fascia. It ends at the pelvic brim about 5cm above the inguinal ligament. Mobilization of the descending colon is conveniently carried out by dividing the peritoneum along the white line of Toldt (see p. 241).

The three taeniae coli, in continuity with those of the transverse colon, lie one anterior and two posterior (medial and lateral). Appendices epiploicae are numerous and diverticulosis is common in this part of the colon.

Sigmoid colon

The sigmoid colon extends from the descending colon at the pelvic brim to the commencement of the rectum in front of the third piece of the sacrum. It is usually about 40cm long, though great variations in length are common. It is completely invested in peritoneum and hangs free on a mesentery, the sigmoid mesocolon. The attachment of the pelvic mesocolon to the pelvic brim and the sacrum has been described on page 241. Congenital peritoneal adhesions are frequently found between the lateral aspect of the pelvic mesocolon and the parietal peritoneum of the floor of the left iliac fossa; they need to be divided during surgical mobilization of the sigmoid colon.

Like the rest of the large intestine, the commencement of the sigmoid colon is sacculated by three taeniae coli, but these muscular bands are wider than elsewhere in the large gut, and meet to clothe the terminal part of the sigmoid in a complete longitudinal coat. The sigmoid colon possesses well-developed appendices epiploicae, and diverticulosis is most common in this part of the colon. It lies, usually, in the pelvic cavity, coiled in front of the rectum, lying on the peritoneal surface of the bladder (and uterus).

Blood supply of the colon

The ascending colon and the proximal two-thirds of the transverse colon are supplied by the ileocolic, right colic and middle colic branches of the superior mesenteric artery, and the remainder of the colon by the left colic and sigmoid branches of the inferior mesenteric. The anastomotic branches near the inner margin of the whole colon form the marginal artery (of Drummond) from which short vessels run into the gut wall (Figs 5.23 and 5.24). The weakest link in this marginal chain of vessels is near the left colic flexure, between the middle and left colic branches, i.e. between midgut and hindgut vessels. An inner arterial arc (of Riolan) between the ascending branch of the left colic artery and the trunk of the middle colic artery may supplement the blood supply to the colon in this region.

The veins correspond to the arteries, and thus reach the portal vein via the superior or inferior mesenteric veins. There is some anastomosis between portal and systemic venous drainage where the ascending and descending colon are in contact with the posterior abdominal wall.

Lymph drainage

As is usual the lymph channels follow the arteries, so that drainage is to superior or inferior mesenteric nodes.

Nerve supply

Being derived from the midgut (up to near the splenic flexure) and the hindgut (from there onwards), the large intestine receives its parasympathetic supply partly from the vagi and partly from the pelvic splanchnic nerves (see p. 282). The sympathetic supply is derived from spinal cord segments T10–L2. The pain fibres that accompany these vasoconstrictor nerves give rise to periumbilical pain if from midgut derivatives (e.g. the appendix) but to hypogastric pain if from the hindgut. As from the rectum, some pain fibres from the descending and sigmoid colon appear to run with the parasympathetic nerves (see p. 295).

Colectomy

The vessel pattern with the accompanying lymphatics determines the extent of partial resections of the colon for carcinoma. For a right hemicolectomy the resection extends from the terminal ileum to the proximal part of the transverse colon, with ligation of the ileocolic and right colic vessels adjacent to their superior mesenteric parent. In a transverse colectomy the transverse colon and the right and left colic flexures are removed together with the transverse mesocolon and greater omentum with ligation of the middle colic vessels. For a left hemicolectomy the resection is from the left end of the transverse colon to part of the sigmoid colon, with ligation of left colic and upper sigmoid vessels. For sigmoid colectomy the removal extends from the lower descending colon to the rectum, with ligation of lower left colic and sigmoid vessels. Resections for diverticular disease (as opposed to neoplasia) can be more localized.

The rectum is considered with the pelvic organs on page 292 and the anal canal with the perineum on page 313.