On the posterior surface of the anterior abdominal wall the peritoneum is raised into six folds, one above and five below the umbilicus. The falciform ligament consists of two adherent layers of peritoneum connecting the anterior surface of the liver to the supraumbilical part of the anterior abdominal wall, just to the right of the midline, and to the inferior surface of the diaphragm. Its concave, inferior margin, which contains the ligamentum teres (the obliterated remains of the left umbilical vein, see p. 31), deviates to the right and is attached to the notch for this ligament on the inferior border of the liver.

Below the umbilicus there is a central fold with a pair on either side. Centrally is the median umbilical fold, containing the median umbilical ligament (the obliterated remains of the urachus; see p. 298). On each side, and also running as far as the umbilicus, is the medial umbilical fold, containing the medial umbilical ligament (the obliterated remains of the umbilical artery; see p. 31). Farther laterally is the lateral umbilical fold, containing the inferior epigastric vessels, which enter the rectus sheath by passing across the arcuate line; although called umbilical folds, this lateral pair do not reach as far as the umbilicus.

The serous-coated organs fill the abdominal cavity so that visceral surfaces are in contact with one another or with the parietal peritoneum. The space between them is only potential, not actual, and it contains only a few millilitres of tissue fluid which lubricates adjacent surfaces so they can glide over one another. This is the general peritoneal cavity or greater sac.

The omental bursa, or lesser sac, is a subsection or diverticulum of the peritoneal cavity behind the stomach. It opens into the greater sac through a slit-like aperture in front of the inferior vena cava, the epiploic foramen (see p. 236). The anterior wall of the lesser sac is formed by the posterior layer of the lesser omentum, the peritoneum over the posterior aspect of the stomach and the posterior of the anterior two layers of the greater omentum (Fig. 5.13). The posterior wall is formed by the anterior of the two posterior layers of the greater omentum which adheres to, but is surgically separable from, the anterior surface of the transverse colon and the transverse mesocolon. Above the attachment of the transverse mesocolon to the anterior border of the pancreas, the posterior wall is formed by the peritoneum that covers the front of the neck and body of pancreas, upper part of left kidney, left suprarenal gland, commencement of abdominal aorta, coeliac artery (plexus and nodes) and part of the diaphragm (Fig. 5.14). Theoretically the cavity of the lesser sac should extend down between the anterior two layers and the posterior two layers of the greater omentum, but because of fusion of these layers the cavity does not extend much below the transverse colon. The narrow upper border of the lesser sac is at the right side of the abdominal oesophagus, where the peritoneum of the posterior wall is reflected anteriorly on the inferior aspect of the diaphragm to form the posterior layer of the lesser omentum. At the tail end of the pancreas the left border of the lesser sac is formed by the splenorenal and gastrosplenic ligaments (see below and Fig. 5.49).

The greater omentum is a double sheet of peritoneum, folded on itself to form four layers (Fig. 5.13). The anterior two layers descend from the greater curvature of the stomach (where they are continuous with the peritoneum on the anterior and posterior surfaces of the stomach) like an apron, overlying coils of intestine, and then turn round and ascend up to the transverse colon where they loosely blend with the peritoneum on the anterior surfaces of the transverse colon and the transverse mesocolon above it. The four layers of the greater omentum below the transverse colon fuse with each other to form an integral structure. This contains adipose tissue of variable amount, depending on the nutritional status of the patient and numerous macrophages.

The part of the greater omentum between the stomach and the transverse colon is often referred to as the gastrocolic omentum. The right and left gastroepiploic vessels run between the layers of the gastrocolic omentum, close to the greater curvature of the stomach. The lesser sac may be accessed through the gastrocolic omentum. Other routes of surgical access to the lesser sac are through the lesser omentum and through the transverse mesocolon.

Below the stomach the left border of the greater omentum envelops the spleen, except for a small bare area at the hilum. The spleen therefore lies in the general peritoneal cavity. Two double-layered folds of peritoneum, the gastrosplenic and splenorenal ligaments, connect the hilum of the spleen to the greater curvature of the stomach and the anterior surface of the left kidney respectively. The splenic vessels and pancreatic tail lie in the splenorenal ligament and the short gastric and left gastroepiploic vessels run in the gastrosplenic ligament (Fig. 5.49).

The two layers of peritoneum that extend between the liver and the upper border (lesser curvature) of the stomach constitute the lesser omentum or gastrohepatic omentum). It can usually only be seen when the liver is lifted up, away from the stomach. Its attachment to the stomach extends from the right side of the abdominal oesophagus and along the lesser curvature to the first 2cm of the duodenum (Fig. 5.35). The liver attachment is L-shaped (Fig. 5.32B), to the fissure for the ligamentum venosum and the porta hepatis. Between the duodenum and the liver it has a right free margin, where the anterior and posterior layers of peritoneum become continuous. This fold forms the anterior boundary of the epiploic foramen.

The epiploic foramen (of Winslow, or the aditus to the lesser sac, Fig. 5.35) is a vertical slit about 2.5cm at the right border of the lesser sac. Its upper boundary is the caudate process of the liver (Fig. 5.32B). The lower boundary is the first part of the duodenum. The posterior boundary is the inferior vena cava, covered by the parietal peritoneum of the posterior abdominal wall which, continuing to the left through the foramen, becomes the peritoneum of the posterior wall of the lesser sac. Anteriorly the foramen is bounded by the right free margin of the lesser omentum containing between its two peritoneal layers the portal vein, and anterior to it the hepatic artery and bile duct, with the duct to the right of the artery, as well as autonomic nerves, lymphatics and nodes.

Traced downwards over the stomach, the two layers of the lesser omentum become the greater omentum (Fig. 5.13). Traced upwards, the two layers enclose the liver and then spread on to the diaphragm and anterior abdominal wall as the coronary, triangular and falciform ligaments (Fig. 5.14).

The peritoneal cavity is descriptively divided into compartments called supracolic, infracolic and pelvic.

The dividing line between the supracolic and infracolic compartments is the attachment of the transverse mesocolon to the posterior abdominal wall, or rather to the organs that lie on the abdominal wall at this level (Figs 5.13 and 5.14). The transverse mesocolon is a double fold of peritoneum passing from the transverse colon to the front of the second part of the duodenum, and to the anterior aspect of the head and the anterior border of the body of the pancreas. The transverse colon and transverse mesocolon are adherent to the posterior surface of the greater omentum. When the greater omentum is lifted up over the costal margin, the stomach, transverse colon and mesocolon are lifted upwards with it, and the posterior surface of the mesocolon and the infracolic compartment brought into view (Fig. 5.15).

The attachments of the liver to the diaphragm and abdominal wall define the subdivisions of the supracolic compartment (Figs 5.16 and 5.32). To the right and left of the falciform ligament are the right and left subphrenic (subdiaphragmatic) spaces. These two spaces are closed above by the superior layer of the coronary ligament and the anterior layer of the left triangular ligament respectively. Behind the right lobe of the liver and in front of the right kidney is the right subhepatic space or hepatorenal pouch (of Morison). This space is closed above by the inferior layer of the coronary ligament and the small right triangular ligament. To the right it is bounded by the abdominal surface of the diaphragm. On the left side the space communicates through the epiploic foramen with the lesser sac or left subhepatic space. Below it is continuous with the right paracolic gutter (see below).

When lying supine, the hepatorenal pouch is the lowest part of the peritoneal cavity (with the sole exception of the pelvis), and hence is an area where intraperitoneal fluid of any sort is likely to accumulate.

The infracolic compartment, below the level of the transverse mesocolon, is divided into two by the attachment of the root of the mesentery of the small intestine (Fig. 5.14), which passes down from left to right at an angle of about 45°. It begins on the left at the duodenojejunal junction, crosses the third part of the duodenum where the superior mesenteric vessels enter between its two layers, and then continues downwards across the aorta, inferior vena cava, right psoas muscle and ureter to the right iliac fossa. This attachment is 15cm long. The intestinal border of the mesentery is plicated like the hem of a very full skirt and measures about 6m long. The depth of the mesentery (from root to gut) is greatest at the central part, about 20cm.

In the retroperitoneal tissue in the region of the root of the mesentery there are numerous Pacinian corpuscles. Tension and traction on peritoneal folds in the upper abdomen produce a fall of blood pressure by undue stimulation of these encapsulated mechanoreceptors.

To the right of the root of the mesentery is the triangular right infracolic space (Fig. 5.14). Its apex lies below, at the ileocaecal junction. Its right side is the ascending colon, and its upper border is the attachment of the transverse mesocolon.

Lateral to the ascending colon is the right paracolic gutter. It can be traced upwards into the hepatorenal pouch and downwards into the pelvis—pathways for the gravitation of fluid.

The left infracolic space is larger than the right infracolic compartment and is quadrilateral in shape. It widens below where it is continuous across the pelvic brim with the cavity of the pelvis (Fig. 5.14). Its upper border is the attachment of the transverse mesocolon, and its left side is the descending colon.

Lateral to the descending colon is the left paracolic gutter (Fig. 5.14). It is limited above by a small transverse fold of peritoneum between the left (splenic) flexure of the colon and the diaphragm, the phrenicocolic ligament. Traced downwards the gutter leads to the left of the attachment of the lateral limb of the sigmoid mesocolon at the pelvic brim.

At the lower end of the left infracolic compartment is the attachment of the sigmoid mesocolon (Fig. 5.14). It is Λ-shaped and the two limbs diverge from each other at the bifurcation of the common iliac vessels, on the pelvic brim over the left sacroiliac joint. The lateral limb passes forwards along the pelvic brim (over the external iliac vessels) halfway to the inguinal ligament, while the medial limb slopes down into the hollow of the sacrum, where it reaches the midline in front of S3 vertebra, at the commencement of the rectum. At the apex of the attachment of the pelvic mesocolon, just beneath the peritoneum and lying over the bifurcation of the common iliac artery, is the left ureter, with the inferior mesenteric vessels medial to it, the vein lying between the ureter and the artery.

The parietal peritoneum is supplied segmentally by the spinal nerves that innervate the adjacent muscles. Thus the diaphragmatic peritoneum is supplied centrally by the phrenic nerve (C4)—hence referred pain and hyperaesthesia from this area to the tip of the shoulder. The remainder of the parietal peritoneum is supplied segmentally by intercostal and lumbar nerves. In the pelvis the obturator nerve is the chief source of supply. The visceral peritoneum is innervated by afferent nerves which travel with the autonomic supply to the viscera. Pain from diseased viscera is due to ischaemia, muscle spasm and stretching of the visceral peritoneum, including mesenteric folds or involvement of the parietal peritoneum.

Several major structures lie on the posterior abdominal wall behind the peritoneum. These include the aorta and inferior vena cava with a number of their branches and tributaries; the cisterna chyli, lymph nodes and vessels; nerves (mostly branches of the lumbar plexus) including the sympathetic trunks; the kidneys, ureters, pancreas, ascending and descending colon and most of the duodenum and suprarenal glands. All these can be said to lie in the retroperitoneal space, though the term is often used to apply only to the area of the posterior abdominal wall behind the peritoneum that is not occupied by the major viscera and great vessels, e.g. over parts of psoas and other muscles. Haemorrhage and infection may develop in it and blood and pus may be confined to the retroperitoneal space.

Just before the sixth week of embryonic life the alimentary canal is a simple tube passing through to the hind end, its whole length supported by a dorsal mesentery attached in the midline in front of the aorta (Fig. 5.17). Three gut arteries leave the aorta and pass ventrally to supply the tube. The most cranial passes in the dorsal mesogastrium to supply the foregut, the next passes through the dorsal mesentery to supply the midgut and the last passes through the dorsal mesocolon to supply the hindgut. They are the coeliac, the superior mesenteric and the inferior mesenteric arteries respectively, and they continue to supply the derivatives of these parts of the alimentary canal in the adult.

The foregut possesses, in addition, a ventral mesogastrium (derived from the septum transversum, see p. 24) attached in the midline to the undersurface of the diaphragm and the anterior abdominal wall down to the umbilicus (Fig. 5.17). Its caudal free edge is crescentic and carries the left umbilical vein.

The derivatives of the foregut (liver and pancreas) and also the spleen are supplied with blood by the artery of the foregut, the coeliac artery. The liver develops as an outgrowth from the foregut at its junction with the midgut. A tube grows ventrally into the ventral mesogastrium, bifurcates, and cells proliferate from the blind end of the two divisions to form the two lobes of the liver, which are thus enclosed between the two layers of the ventral mesogastrium. The pancreas develops as two outgrowths, one into the ventral and one into the dorsal mesogastrium. These two parts subsequently fuse and exchange ducts by anastomosis, but the double origin of the pancreas is imprinted on the adult by the persistence of two pancreatic ducts, the main and the accessory. The spleen develops by proliferation of cells in the left leaf of the dorsal mesogastrium. It is not a derivative of the foregut itself. The coeliac artery supplies the stomach and these three organs in the adult, where it is properly called the coeliac trunk.

Lymph from the alimentary canal drains by lymph vessels that run with the arteries and end ultimately in lymph nodes that lie in front of the aorta at the roots of the three gut arteries. Lymph from the mucous membrane of the alimentary canal passes through several filters. In the mucous membrane itself, from tonsils to anal margin, are lymphoid follicles. In the mesentery, at its gut margin, are lymph nodes (the ‘para-’ group, e.g. paracolic). In the mesentery between its gut margin and root are further intermediate nodes. The preaortic nodes, inferior mesenteric, superior mesenteric, and coeliac, are interconnected by lymph vessels. All the lymph thus ultimately reaches the coeliac nodes, whence it passes to the cisterna chyli (see p. 247). This simple lymphatic arrangement persists in the adult.

By the end of the sixth week the liver has enlarged greatly and the gut has elongated, both to such an extent that the more leisurely growing abdominal walls cannot accommodate them. A loop of gut extrudes into the umbilical cord as the physiological hernia (Fig. 5.17). The loop remains in the umbilical cord for a full month. At the end of the tenth week the abdominal walls have grown enough to accommodate the abdominal contents and the hernia is reduced.

The herniated loop of gut is that supplied by the superior mesenteric artery and it is defined as the midgut. It is destined to produce all the small intestine from the distal part of the duodenum (i.e. distal to the entry of the bile duct) and the proximal part of the colon, almost as far as the left colic flexure. The apex of the loop is at the attachment of the vitellointestinal duct (see p. 24), the site of the ileal (Meckel’s) diverticulum. The main trunk of the superior mesenteric artery is directed to the apex of the loop. Many branches run from it to the proximal limb of the loop, extending from the ventral pancreatic bud to Meckel’s diverticulum. They persist as the jejunal and ileal branches. Only three branches run to the distal limb of the loop; all three persist in the adult as the ileocolic, right colic and middle colic arteries. Their directions are altered considerably after the reduction of the physiological hernia and the rotation of the gut.

As the loop of midgut in the physiological hernia returns to the abdominal cavity it rotates so that the distal limb goes up on the left and the proximal limb goes down on the right, i.e. to the observer looking at the front of the abdomen, in an anticlockwise direction (Fig. 5.18). The distal loop, developing into colon, thus comes to lie anterior to the commencement of the proximal loop. This part of the proximal loop becomes, after some rotation, plastered to the posterior abdominal wall as the duodenum, and the mesentery of the transverse colon thus comes to lie across it (Fig. 5.19). The last part of the midgut to be reincluded within the abdominal cavity is the caecum, which lies first near the midline, high up. It grows then to the right, turns downwards at the right colic flexure and stops elongating at the right iliac fossa. It leaves a trail of large intestine to indicate its migration and drags the attached lower end of the ileum with it.

Rotation of the midgut loop occurs around the axis of the superior mesenteric artery, so in the adult the branches to the proximal loop (jejunal and ileal arteries) come off its left side while the three branches to the distal loop (colic arteries) leave its right side (Figs 5.19 and 5.23).

The simple dorsal mesentery of the midgut containing the superior mesenteric artery is, of course, twisted and distorted during the return of the rotated loop of midgut and the subsequent migration of the caecum. Its attachment to the proximal loop causes it to pass across the posterior abdominal wall from the commencement of the loop (duodenum) to the ileocaecal junction (Fig. 5.14). The dorsal mesentery of the distal loop of the midgut hinges like a door across from the midline to the right. It comes into contact with the parietal peritoneum in the right paravertebral gutter, with which it fuses, the colic vessels lying immediately deep to it and in front of everything else on the posterior abdominal wall. The dorsal mesentery of the most distal part of the distal loop, pulled across transversely, does not fuse completely with the parietal peritoneum and persists, with the middle colic artery between its layers, as the transverse mesocolon (Fig. 5.19).

As the midgut loop returns to the abdominal cavity, the hindgut swings on its dorsal mesocolon like a door across to the left (Fig. 5.20), and the mesocolon fuses with the parietal peritoneum of the left paravertebral gutter; a white line (of Toldt) marks the site of fusion along the left side of the descending colon. Hence the left colic vessels lie in front of everything else on the posterior abdominal wall. At the pelvic brim fusion of the layers is not complete and a part of the dorsal mesocolon of the hindgut remains free as the sigmoid mesocolon of the adult.

The liver grows apace, and soon outstrips the ventral mesogastrium in which it lies. It grows caudally into the free edge of the ventral mesogastrium, which it pushes down until the left umbilical vein (ligamentum teres in the adult) notches its inferior border and is enclosed in a deep groove on its undersurface. This encroachment of the liver over the crescentic free margin of the ventral mesogastrium divides the latter into two separate parts, namely the falciform ligament between liver and anterior abdominal wall and the lesser omentum (gastrohepatic omentum) between liver and stomach (Fig. 5.17). The lesser omentum is attached to the liver along a fissure that runs backwards behind the fissure for the ligamentum teres (Fig. 5.33), and lodges the fibrous remnant of the ductus venosus (ligamentum venosum).

Coincident with the growth of the liver, the foregut rotates. The liver originally was ventral to the foregut, in the ventral mesogastrium, and both lie in the midline. As the liver grows it swings to the right, taking the ventral mesogastrium with it. The stomach swings across to the left and in doing so rotates (Fig. 5.21). It has already elongated and broadened, with its dorsal border becoming convex, and its ventral border concave. The distal end of the foregut, destined to become the duodenum (i.e. proximal to the entry of the bile duct), does not dilate in this manner, and its dorsal mesentery shortens. The duodenal part of the gut elongates into a loop which swings to the right and becomes plastered to the posterior abdominal wall (like the ascending and descending parts of the colon). At the same time its walls grow asymmetrically so that the ventral bile duct and pancreatic duct are carried around to open on the medial wall (Fig. 5.40) in line with the duct of the dorsal diverticulum. The duodenum is now fixed in position; so, too, is the oesophagus at the diaphragm. Between these two fixed points as an axis, the dorsal convexity of the stomach rotates to the left. The dorsal convexity becomes the greater curvature, and the original left side of the stomach now faces anteriorly. The concave ventral border becomes the lesser curvature, attached by the lesser omentum (originally the ventral mesogastrium) to the liver, and to the diaphragm between liver and oesophagus. The original right surface of the stomach now lies behind, and faces the peritoneum of the posterior abdominal wall. The space behind the stomach and the lesser omentum is the lesser sac, and the free edge of the lesser omentum lies over the opening from the greater sac into this space. The rotation of the stomach accounts for the left and right vagal nerve trunks becoming anterior and posterior, respectively, in relation to the stomach.

Meanwhile changes have occurred in the disposition of the dorsal mesogastrium. As the dorsal border of the stomach swings to the left the dorsal mesogastrium hinges to the left from this attachment and adheres to the parietal peritoneum as far left as the front of the left kidney. From the front of the left kidney, and from the diaphragm above it, the two layers of the dorsal mesogastrium pass to the oesophagus and the upper part of the greater curvature of the stomach. They form part of the greater omentum, and constitute the left boundary of the lesser sac behind the stomach. The spleen projects from the left leaf into the greater sac, and it divides this part of the greater omentum into gastrosplenic and splenorenal ligaments (Fig. 5.21).

The more caudal part of the dorsal mesogastrium is attached to the lower part of the greater curvature of the stomach, the inferior border of the pylorus and the first 2cm of the duodenum. Its dorsal attachment, in the midline, hinges to the left and becomes plastered to, and fused with, the peritoneum on the posterior abdominal wall over the pancreas. Below this it balloons down like an apron from the greater curvature, over the transverse mesocolon and transverse colon, and returns to the posterior abdominal wall (Fig. 5.13). The deeper part of the double layer fuses with the upper leaf of the transverse mesocolon and with the transverse colon itself, forming the inferior limit of the lesser sac. The superficial part hangs down from the greater curvature to the transverse colon, to which it adheres, forming the gastrocolic omentum, a part of the greater omentum, which is the anterior wall of the lower part of the lesser sac.

From the transverse colon the dependent dorsal mesogastrium hangs down over the front of the coils of small intestine as a fat-containing apron, the main greater omentum.

This is the artery of the foregut, and its three branches—the left gastric, splenic and common hepatic arteries—supply not only the gut from the lower part of the oesophagus down to the opening of the bile duct into the duodenum, but also the foregut derivatives (the liver and pancreas) and the spleen. It arises from the front of the abdominal aorta between the crura of the diaphragm a little below the median arcuate ligament, at the level of the body of T12 vertebra. It is usually a short wide trunk, flanked by the coeliac group of preaortic lymph nodes. The coeliac ganglia lie one on each side and they send to the artery sympathetic nerves which are carried along all its branches.

At the upper border of the pancreas the trunk divides into its three branches behind the peritoneum of the posterior wall of the lesser sac (Fig. 5.22).

The left gastric artery runs upwards across the left crus towards the oesophageal opening in the diaphragm. It gives off oesphageal branches and turns anteroinferiorly, raising a small fold of peritoneum, the left gastropancreatic fold. It then runs to the right in the lesser omentum along the lesser curvature and supplies the stomach.

The splenic artery passes to the left. It is characteristically very tortuous; the crests of its waves appear above the pancreas, and the troughs lie hidden behind its upper border. It runs across the left crus and left psoas to the hilum of the left kidney, where it turns forward in the splenorenal ligament to the hilum of the spleen. Apart from the spleen it is the main supply to the pancreas. Before breaking up into its terminal splenic branches it gives off about 6 short gastric arteries which run in the gastrosplenic ligament, and the left gastroepiploic artery which runs to the right in the greater omentum, a little distance away from the greater curvature, from where it gives branches to the stomach and the omentum. From the middle part of its course the splenic artery may give off a posterior gastric artery to the stomach, which raises a fold of parietal peritoneum, the gastrosplenic ligament, as it arches forwards towards the stomach.

The common hepatic artery passes over the upper border of the pancreas, downwards and to the right behind the peritoneum of the posterior abdominal wall as far as the first part of the duodenum. It then turns forward, raising a small fold of peritoneum, the right gastropancreatic fold, and curves upwards between the two layers of the lesser omentum as the hepatic artery. Here it meets the bile duct and lies on its left side, both in front of the portal vein surrounded by the peritoneum at the free edge of the lesser omentum. On reaching the porta hepatis, the hepatic artery divides into right and left branches to supply the right and left halves of the liver. These branches and the associated aberrant or accessory hepatic arteries are described on page 262.

The common hepatic usually gives off the right gastric and gastroduodenal arteries.

The right gastric artery leaves the common hepatic as it turns forwards into the lesser omentum. It runs to the left along the lesser curvature and anastomoses with the left gastric artery.

The gastroduodenal artery passes down behind the first part of the duodenum, where it may be eroded by a duodenal ulcer. At the lower border of the duodenum it divides into two. The right gastroepiploic artery passes forward between the first part of the duodenum and the pancreas, and turns to the left between the two leaves of the greater omentum. It runs close to the greater curvature of the stomach and anastomoses with the left gastroepiploic artery.

The other branch of the gastroduodenal artery is the superior pancreaticoduodenal artery. It divides into a smaller anterior and a larger posterior branch, which may arise directly from the gastroduodenal artery; they anastomose with similar branches of the inferior pancreaticoduodenal branch of the superior mesenteric artery. The pancreaticoduodenal arteries supply the duodenum, head of the pancreas and bile duct. The entrance of the bile duct marks the junction of foregut and midgut, and is the meeting place of the arterial distributions of their respective arteries, coeliac and superior mesenteric.

One or two small supraduodenal arteries may arise from the common hepatic artery or its branches.

This is the artery of the midgut and supplies the gut from the entrance of the bile duct to a level just short of the splenic flexure of the colon. The artery arises from the front of the aorta a centimetre below the coeliac trunk, at the level of the lower border of L1 vertebra. It is directed steeply downwards behind the splenic vein and the body of the pancreas, with the superior mesenteric vein on its right side. It lies anterior to the left renal vein, the uncinate process of the pancreas and the third part of the duodenum, in that order from above downwards (Fig. 5.26). With its vein it enters the upper end of the mesentery of the small intestine and passes down to the right along the root of the mesentery (Fig. 5.23). Pressure of the superior mesenteric artery on the left renal vein may produce left-sided varicocele, and pressure on the duodenum may give symptoms of chronic duodenal ileus, particularly when the angle between the artery and the aorta is smaller than usual.

The inferior pancreaticoduodenal artery is its first branch, arising from the posterior surface; it may come off the first jejunal branch. It divides into anterior and posterior branches which run in the curve between the duodenum and the head of the pancreas, supply both, and anastomose with the terminal branches of the superior pancreaticoduodenal artery.

The jejunal and ileal branches arise from the left of the main trunk and pass down between the two layers of the mesentery. The pattern of anastomosing arcades from which vessels enter the gut wall is described with the jejunum and ileum (see p. 254).

The ileocolic artery (Fig. 5.23) arises from the right side of the superior mesenteric trunk low down in the base of the mesentery. It descends to the right iliac fossa and divides into superior and inferior branches. The superior branch runs up along the left side of the ascending colon to anastomose with the right colic artery. The inferior branch runs to the ileocolic junction, and gives off anterior and posterior caecal arteries, an appendicular artery, and an ileal branch which ascends to the left on the ileum to anastomose with the terminal branch of the superior mesenteric artery.

The right colic artery (Fig. 5.23) arises from the right side of the superior mesenteric artery, or in common with the ileocolic artery. It runs to the right across the right psoas muscle, gonadal vessels, ureter and genitofemoral nerve, and quadratus lumborum, just behind the peritoneal floor of the right infracolic compartment. It divides near the left side of the ascending colon into two branches. The descending branch runs down to anastomose with the superior branch of the ileocolic artery. The ascending branch runs up across the inferior pole of the right kidney to the hepatic flexure where it anastomoses with a branch of the middle colic artery.

The middle colic artery (Fig. 5.23) arises from the right side of the superior mesenteric artery, as the artery emerges at the lower border of the neck of the pancreas, and descends between the two leaves of the transverse mesocolon. It lies to the right of the midline and at the intestinal border of the transverse mesocolon it divides into right and left branches which run along the margin of the transverse colon. The right branch anastomoses with the ascending branch of the right colic artery. The left branch supplies the transverse colon almost to the splenic flexure (the distal part of the midgut) where it anastomoses with a branch of the left colic artery. As the middle colic lies to the right of the midline it leaves a large avascular window to its left in the transverse mesocolon (Fig. 5.23). This window is the site of election for surgical access to the lesser sac and the posterior wall of the stomach.

This is the artery of the hindgut; its area of supply extends as far as the upper third of the anal canal, i.e. to the level of the pectinate (dentate) line (see p. 315). The inferior mesenteric artery arises from the front of the aorta behind the inferior border of the third part of the duodenum, opposite L3 vertebra, at the level of the umbilicus, 3 or 4cm above the aortic bifurcation. It is smaller than the superior mesenteric artery. It runs obliquely down to the pelvic brim, immediately beneath the peritoneal floor of the left infracolic compartment. It gives off the left colic and sigmoid arteries. It crosses the pelvic brim at the bifurcation of the left common iliac vessels over the sacroiliac joint, at which point it converges towards the ureter, with the inferior mesenteric vein lying between them, at the apex of the Λ-shaped attachment of the sigmoid mesocolon. Its branches cross to the left in front of the ureter and the other structures in the posterior abdominal wall of the left infracolic compartment. Beyond the pelvic brim it continues in the root of the sigmoid mesocolon as the superior rectal artery (see p. 294).

The left colic artery leaves the trunk and passes upwards and to the left behind the peritoneum. After a short course it divides into an ascending and a descending branch. The ascending branch continues laterally and upwards, crossing the left psoas muscle, gonadal vessels, ureter and genitofemoral nerve, and quadratus lumborum. It is crossed anteriorly by the inferior mesenteric vein. The descending branch passes laterally and downwards. The branches of these two arteries anastomose with each other as well as (above) with the left branch of the middle colic artery and (below) with the highest sigmoid artery, thus contributing to an anastomotic channel along the inner margin of the colon (Fig. 5.24 and p. 258).

The sigmoid arteries are two to four branches which pass between the layers of the sigmoid mesocolon, in which they form anastomosing loops. The last sigmoid branch anastomoses with the first branch of the superior rectal artery.

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.

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.

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.

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 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.

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.

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.

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 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.

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

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.

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

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.

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.

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.

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.