The stomach

The stomach is roughly J‐shaped, although its size and shape vary considerably. It tends to be high and transverse in the obese short subject and to be elongated in the asthenic individual; even in the same person, its shape depends on whether it is full or empty, on the position of the body and on the phase of respiration. The stomach has two surfaces – the anterior and posterior; two curvatures – the greater and lesser; and two orifices – the cardia and pylorus (Fig. 49).

The stomach projects to the left, above the level of the cardiac orifice (or cardia), to form the dome‐like gastric fundus. Along the lesser curve is a distinct notch, the incisura angularis. Between the cardiac orifice and the incisura is the body of the stomach, while the area between the incisura and the pylorus is the pyloric antrum. The junction of the pylorus with the duodenum is marked by a constriction externally and also by a constant vein that crosses it, the vein of Mayo.

It is the body of the stomach that bears the HCl‐secreting parietal cells. The antrum secretes the enzyme gastrin and its secretion is alkaline.

The thickened pyloric sphincter is easily felt and surrounds the lumen of the pyloric canal. The pyloric sphincter is an anatomical structure as well as a physiological mechanism. The cardia, on the other hand, although competent (gastric contents do not flow out of your mouth if you stand on your head), is not demarcated by a distinct anatomical sphincter. The exact nature of the cardiac sphincter action is still not fully understood, but the following mechanisms have been suggested, each supported by some experimental and clinical evidence.

1. Mucosal folds at the oesophagogastric junction act as a valve.
   
2. The acute angle of entry of the oesophagus into the stomach produces a valve‐like effect.
   
3. The circular muscle of the lower oesophagus is a physiological, as distinct from an anatomical, sphincter.
   
4. The arrangement of the muscle fibres of the stomach around the cardia either acts as a sphincter or else maintains the acute angle of entry of the oesophagus into the stomach.
   
5. The right crus of the diaphragm acts as a ‘pinch‐cock’ to the lower oesophagus as it pierces this muscle.
   
6. The positive intra‐abdominal pressure compresses the walls of the short segment of the intra‐abdominal oesophagus.

Relations (Fig. 50)

• Anteriorly – the abdominal wall, the left costal margin, the diaphragm and the left lobe of the liver.
  
• Posteriorly – the lesser sac, which separates the stomach from the pancreas, transverse mesocolon, left kidney, left suprarenal, the spleen and the splenic artery.
  
• Superiorly – the left dome of the diaphragm.

The lesser omentum is attached along the lesser curvature of the stomach; the greater omentum along the greater curvature. These omenta contain the vascular and lymphatic supply of the stomach.

The arterial supply (Fig. 51) to the stomach is extremely rich and comprises:

• the left gastric artery – from the coeliac axis;
  
• the right gastric artery – from the hepatic artery;
  
• the right gastro‐epiploic artery – from the gastroduodenal branch of the hepatic artery;
  
• the left gastro‐epiploic artery – from the splenic artery;
  
• the short gastric arteries – from the splenic artery.

The corresponding veins drain into the portal system.

The lymphatic drainage of the stomach accompanies its blood vessels. The stomach can be divided into three drainage zones (Fig. 52).

• Area I – the superior two‐thirds of the stomach drain along the left and right gastric vessels to the aortic nodes.
  
• Area II – the right two‐thirds of the inferior one‐third of the stomach drain along the right gastro‐epiploic vessels to the subpyloric nodes and thence to the aortic nodes.
  
• Area III – the left one‐third of the greater curvature of the stomach drains along the short gastric and splenic vessels lying in the gastrosplenic and splenorenal ligaments, then, via the suprapancreatic nodes, to the aortic group.

This extensive lymphatic drainage and the technical impossibility of its complete removal is one of the serious problems in dealing with stomach cancer. Involvement of the nodes along the splenic vessels can be dealt with by removing the spleen, the gastrosplenic and splenorenal ligaments and the body and tail of the pancreas. Lymph nodes among the gastroepiploic vessels are removed by excising the greater omentum. However, involvement of the nodes around the aorta and the head of the pancreas may render the growth incurable.

The vagal supply to the stomach (Fig. 53)

The anterior and posterior vagi enter the abdomen through the oesophageal hiatus. The anterior nerve lies close to the stomach wall but the posterior, and larger, nerve is at a little distance from it. The anterior vagus supplies branches to the cardia and lesser curve of the stomach and also a large hepatic branch. The posterior vagus gives branches to both the anterior and posterior aspects of the body of the stomach but the bulk of the nerve forms the coeliac branch. This runs along the left gastric artery to the coeliac ganglion for distribution to the intestine, as far as the mid‐transverse colon, and the pancreas.

The exact means by which the vagal fibres reach the stomach is of considerable practical importance to the surgeon. The gastric divisions of both the anterior and posterior vagi reach the stomach at the cardia and descend along the lesser curvature between the anterior and posterior peritoneal attachments of the lesser omentum (the anterior and posterior nerves of Latarjet). The stomach is innervated by terminal branches from the anterior and posterior gastric nerves and it is, therefore, possible to divide those branches that supply the acid‐secreting body of the stomach and yet preserve the pyloric innervation (highly selective vagotomy).

The vagus constitutes the motor and secretory nerve supply for the stomach. When divided, in the operation of vagotomy, the neurogenic (reflex) gastric acid secretion is abolished but the stomach is, at the same time, rendered atonic so that it empties only with difficulty; because of this, total vagotomy must always be accompanied by some sort of drainage procedure, either a pyloroplasty (to enlarge the pyloric exit and render the pyloric sphincter incompetent) or a gastrojejunostomy (to drain the stomach into the proximal small intestine). Drainage can be avoided if the nerve of Latarjet is preserved, thus maintaining the innervation and function of the pyloric antrum (highly selective vagotomy).

The duodenum

The duodenum curves in a C around the head of the pancreas and is 25 cm (10 in) long. At its origin from the pylorus it is completely covered with peritoneum for approximately 2.5 cm (1 in), but then becomes a retroperitoneal organ, only partially covered by serous membrane.

Relations (Figs 55, 56)

For descriptive purposes, the duodenum is divided into four sections.

The first part (5 cm (2 in)) ascends from the gastroduodenal junction, overlapped by the liver and gall bladder. Immediately posterior to it lie the portal vein, common bile duct and gastroduodenal artery, which separate it from the inferior vena cava.

The second part (7.5 cm (3 in)) descends in a curve around the head of the pancreas. It is crossed by the transverse colon and lies on the right kidney and ureter. Halfway along, its posteromedial aspect enters the common opening of the bile duct and the main pancreatic duct (of Wirsung) onto an eminence called the duodenal papilla. This common opening is guarded by the sphincter of Oddi. The accessory pancreatic duct (of Santorini) opens into the duodenum a little above the papilla.

The third part (10 cm (4 in)) runs transversely to the left, crossing the inferior vena cava, the aorta and the third lumbar vertebra. It is itself crossed anteriorly by the root of the mesentery and the superior mesenteric vessels. Its upper border hugs the pancreatic head.

The fourth part (2.5 cm (1 in)) ascends upwards and to the left to end at the duodenojejunal junction. It is surprisingly easy for the surgeon to confuse this with the ileocaecal junction, a mistake which may be disastrous. He confirms the identity of the duodenal termination by the presence of the suspensory ligament of Treitz, which is a well‐marked peritoneal fold descending from the right crus of the diaphragm to the duodenal termination, and by visualizing the inferior mesenteric vein, which descends from behind the pancreas immediately to the left of the duodenojejunal junction.

Small intestine

The length of the small intestine varies from 3 to 10 m (10–33 feet) in different subjects; the average is some 6.5 m (24 feet). Resection of up to one‐third or even half of the small intestine is compatible with a perfectly normal life, and survival has been reported with only 45 cm (18 in) of small intestine preserved.

The mesentery of the small intestine has a 15 cm (6 in) origin from the posterior abdominal wall, which commences at the duodenojejunal junction to the left of the 2nd lumbar vertebra, and passes obliquely downwards to the right sacro‐iliac joint; it contains the superior mesenteric vessels, the lymph nodes draining the small gut and autonomic nerve fibres.

The upper half of the small intestine is termed the jejunum; the remainder is the ileum. There is no sharp distinction between the two and this division is a conventional one only. The bowel does, however, change its character from above downwards, the following points enabling the surgeon to determine the level of a loop of small intestine at operation.

1. The jejunum has a thicker wall as the circular folds of mucosa (valvulae conniventes) are larger and thicker more proximally.
   
2. The proximal small intestine is of greater diameter than the distal.
   
3. The jejunum tends to lie at the umbilical region; the ileum in the suprapubic region and pelvis.
   
4. The mesentery becomes thicker and more fat‐laden from above downwards.
   
5. The mesenteric vessels form only one or two arcades to the jejunum, with long and relatively infrequent terminal branches passing to the gut wall. The ileum is supplied by shorter and more numerous terminal vessels arising from complete series of three, four or even five arcades (Fig. 57).

Large intestine

The large intestine is subdivided, for descriptive purposes, into:

• caecum with the appendix vermiformis;
  
• ascending colon (12–20 cm (5–8 in));
  
• hepatic flexure;
  
• transverse colon (45 cm (18 in));
  
• splenic flexure;
  
• descending colon (22–30 cm (9–12 in));
  
• sigmoid colon (12–76 cm (5–30 in), average 38 cm (15 in));
  
• rectum (12 cm (5 in));
  
• anal canal (4 cm (1.5 in)).

The large bowel may vary considerably in length in different subjects; the average is approximately 1.5 m (5 feet).

The colon (but not the appendix, caecum or rectum) bears characteristic fat‐filled peritoneal tags called appendices epiploicae scattered over its surface. These are especially numerous in the sigmoid colon. Their function, if any, is obscure but they may undergo torsion, which is an unusual cause of acute abdominal pain.

The colon and caecum (but not the appendix or rectum) are marked by the taeniae coli. These are three flattened bands commencing at the base of the appendix and running the length of the large intestine to end at the rectosigmoid junction. They represent the great bulk of the longitudinal muscle of the large bowel; because the taeniae are approximately 30 cm (12 in) shorter than the gut to which they are attached, the colon becomes condensed into its typical sacculated shape. These sacculations may be seen in a plain radiograph of the abdomen when the large bowel is distended and appear as incomplete septa projecting into the gas shadow. The radiograph of distended small intestine, in contrast, characteristically has complete transverse lines across the bowel shadow owing to the transverse mucosal folds of the valvulae conniventes.

The appendix

The appendix arises from the posteromedial aspect of the caecum approximately 2.5 cm (1 in) below the ileocaecal valve; its length ranges from 1.25 cm (0.5 in) to 22 cm (9 in). In the fetus it is a direct outpouching of the caecum, but differential overgrowth of the lateral caecal wall results in its medial displacement.

The position of the appendix is extremely variable – more so than that of any other organ (Fig. 58). Most frequently (75% of cases) the appendix lies behind the caecum. The appendix is usually quite free in this position, although occasionally it lies beneath the peritoneal covering of the caecum. If the appendix is very long, it may actually extend behind the ascending colon and abut against the right kidney or the duodenum; in these cases, its distal portion lies extraperitoneally.

In approximately 20% of cases, the appendix lies just below the caecum or else hangs down into the pelvis. Less commonly, it passes in front of or behind the terminal ileum, or lies in front of the caecum or in the right paracolic gutter.

A long appendix has been known to ulcerate into the duodenum or perforate into the left paracolic gutter. It may well be said that ‘the appendix is the only organ in the body that has no anatomy’.

The mesentery of the appendix, containing the appendicular branch of the ileocolic artery, descends behind the ileum as a triangular fold (Fig. 59). Another peritoneal sheet, the ileocaecal fold, passes to the appendix or to the base of the caecum from the front of the ileum. The ileocaecal fold is termed the bloodless fold of Treves although, in fact, it often contains a vessel and, if cut, proves far from bloodless.

The rectum

The rectum is 12 cm (5 in) in length. It commences anterior to the third segment of the sacrum and ends at the level of the apex of the prostate or, in the female, at the level of the lower end of the intrapelvic vagina, where it pierces levator ani and leads into the anal canal.

The rectum is straight in lower mammals (hence its name) but is curved in man to fit into the sacral hollow. Moreover, it presents, externally, a series of three lateral inflexions, marked on the inside by the valves of Houston, projecting left, right and left from above downwards.

Relations (Figs 60, 61)

The main relations of the rectum are important. They must be kept in mind when carrying out a rectal examination. They provide the key to the local spread of rectal growths and they are important in surgical resection of the rectum.

Posteriorly lie the sacrum and coccyx and the middle sacral artery, which are separated from it by extraperitoneal connective tissue containing the rectal vessels and lymphatics. The lower sacral nerves, emerging from the anterior sacral foramina, may be involved by growth spreading posteriorly from the rectum, resulting in severe sciatic pain.

Anteriorly, the upper two‐thirds of the rectum are covered by peritoneum and relate to coils of small intestine which, in the female, lie in the cul‐de‐sac of the pouch of Douglas between the rectum and the uterus. In front of the lower one‐third lie the prostate, bladder base and seminal vesicles in the male, or the vagina in the female. A layer of fascia (Denonvilliers) separates the rectum from the anterior structures and forms the plane of dissection which must be sought when performing excision of the rectum.

Laterally, the rectum is supported by the levator ani.

The anal canal (Fig. 62)

The anal canal is 4 cm (1.5 in) long and is directed downwards and backwards from the rectum to end at the anal orifice. The mid‐anal canal represents the junction between the endoderm of the hindgut and the ectoderm of the cutaneous invagination termed the proctodaeum. Failure of breakdown of the separating membrane results in imperforate anus.

1. The lower half is lined by squamous epithelium and the upper half by columnar epithelium; the latter presents vertical columns of mucosa (the columns of Morgagni) connected at their distal extremities by valve‐like folds (the valves of Ball). A carcinoma of the upper anal canal is thus an adenocarcinoma, whereas that arising from the lower part is a squamous tumour.
   
2. The blood supply of the upper half of the anal canal is from the superior rectal artery, derived from the inferior mesenteric artery (see Fig. 64), and its corresponding superior rectal vein, which drains into the inferior mesenteric vein, which, in turn, enters the splenic vein and thus drains into the portal vein (see Fig. 65). In contrast, the blood supply of the lower anal canal and the surrounding perianal skin is from the inferior rectal vessels, derived from the internal pudendal and, ultimately, the internal iliac artery and vein. The two venous systems communicate and therefore form one of the anastomoses between the portal and systemic venous systems.
   
3. The lymphatics above this mucocutaneous junction drain along the superior rectal vessels to the lumbar nodes, whereas, below this line, drainage is to the inguinal nodes. A carcinoma of the rectum that invades the lower anal canal may thus metastasize to the groin nodes.
   
4. The nerve supply to the upper anal canal is via the autonomic plexuses; the lower part is supplied by the somatic inferior rectal nerve, a terminal branch of the pudendal nerve (see Fig. 97b). (The lower canal is therefore sensitive to the prick of a hypodermic needle, whereas injection of an internal haemorrhoid with sclerosant fluid, by passing a needle through the mucosa of the upper part of the canal, is painless.)

The anal sphincter

Forming the walls of the anal canal is a rather complicated muscle arrangement which constitutes a powerful sphincter mechanism (Fig. 62). This comprises:

• the internal anal sphincter, of involuntary muscle, which continues above with the circular muscle coat of the rectum;
  
• the external anal sphincter, of voluntary muscle, which surrounds the internal sphincter and which extends further downwards and curves medially to occupy a position below and slightly lateral to the lower rounded edge of the internal sphincter, close to the skin of the anal orifice. The lowermost, or subcutaneous, portion of the external sphincter is traversed by a fan‐shaped expansion of the longitudinal muscle fibres of the anal canal that continue above with the longitudinal muscle of the rectal wall. At its upper end the external sphincter fuses with the fibres of levator ani.

In carrying out a digital rectal examination, the ring of muscle on which the flexed finger rests just over 2.5 cm (1 in) from the anal margin is the anorectal ring. This represents the deep part of the external sphincter where this blends with the internal sphincter and levator ani, and demarcates the junction between the anal canal and the rectum.

The anal canal is related posteriorly to the fibrous tissue between it and the coccyx (the anococcygeal body), laterally to the ischio‐anal fossa on either side, containing fat, and anteriorly to the perineal body, which separates it from the bulb of the urethra in the male and the lower vagina in the female. Note that the ischiorectal fossa is now often referred to, more accurately, as the ischio‐anal fossa – it relates to the anal canal rather than the rectum.

Rectal examination

The following structures can be palpated by the finger passed per rectum in the normal patient:

1. both sexes – the anorectal ring (see previous section), coccyx and sacrum, ischio‐anal fossae, ischial spines;
   
2. male – prostate, rarely the healthy seminal vesicles;
   
3. female – perineal body, cervix, occasionally the ovaries.

Abnormalities which can be detected include:

1. within the lumen – faecal impaction, foreign bodies;
   
2. in the wall – rectal growths, strictures, granulomata, etc., but not haemorrhoids unless these are thrombosed;
   
3. outside the rectal wall – pelvic bony tumours, abnormalities of the prostate or seminal vesicle, distended bladder, uterine or ovarian enlargement, collections of fluid or neoplastic masses in the pouch of Douglas.

Do not be deceived by foreign objects placed in the vagina. The commonest are a tampon or a pessary.

During parturition, dilatation of the cervical os can be assessed by rectal examination since it can be felt quite easily through the rectal wall.

Arterial supply of the intestine

The alimentary tract develops from the fore‐, mid‐ and hindgut; the arterial supply to each is discrete, although anastomosing with its neighbour. The foregut comprises the stomach and duodenum as far as the entry of the bile duct and is supplied by branches of the coeliac axis, which arises from the aorta at the T12 vertebral level (Fig. 51). The midgut extends from mid‐duodenum to the distal transverse colon and is supplied by the superior mesenteric artery (Fig. 64) arising from the aorta at the level of L1. Its branches are:

1. the inferior pancreaticoduodenal artery;
   
2. jejunal and ileal branches – supplying the bulk of the small intestine;
   
3. the ileocolic artery – supplying the terminal ileum, caecum and the commencement of the ascending colon and giving off an appendicular branch to the appendix; the latter branch is the most commonly ligated intra‐abdominal artery;
   
4. the right colic artery – supplying the ascending colon;
   
5. the middle colic artery – supplying the transverse colon.

The hindgut receives its supply from the inferior mesenteric artery (Fig. 64), arising from the aorta at L3 and giving the following branches:

1. the left colic artery – supplying the descending colon;
   
2. the sigmoid branches – supplying the sigmoid;
   
3. the superior rectal artery – supplying the rectum.

The portal system of veins

The portal venous system drains blood to the liver from the abdominal part of the alimentary canal (excluding the anal canal), the spleen, the pancreas and the gall bladder and its ducts.

The distal tributaries of this system correspond to, and accompany, the branches of the coeliac and the superior and inferior mesenteric arteries enumerated previously; only proximally (Fig. 65) does the arrangement differ.

The inferior mesenteric vein ascends above the point of origin of its artery to enter the splenic vein behind the pancreas.

The superior mesenteric vein joins the splenic vein behind the neck of the pancreas in the transpyloric plane to form the portal vein, which ascends behind the first part of the duodenum into the anterior wall of the foramen of Winslow and thence to the porta hepatis. Here the portal vein divides into right and left branches and breaks up into capillaries running between the lobules of the liver. These capillaries drain into the radicles of the hepatic vein through which they empty into the inferior vena cava.

Lymphatic drainage of the intestine (Fig. 66)

The arrangement of lymph nodes is relatively uniform throughout the small and large intestine. Numerous small nodes lying near, or even on, the bowel wall drain to intermediately placed and rather larger nodes along the vessels in the mesentery or mesocolon and thence to clumps of nodes situated near the origins of the superior and inferior mesenteric arteries. From these, efferent vessels link up to drain into the cisterna chyli.

The lymphatic drainage field of each segment of bowel corresponds fairly accurately to its blood supply. High ligation of the vessels to the involved segment of bowel with removal of a wide surrounding segment of mesocolon will, therefore, remove the lymph nodes draining the area. Division of the middle colic vessels and a resection of a generous wedge of transverse mesocolon, for example, would be performed for a growth of transverse colon.

The structure of the alimentary canal

The alimentary canal is made up of mucosa demarcated by the muscularis mucosae from the submucosa, the muscle coat and the serosa – the last being absent where the gut is extraperitoneal.

The oesophageal mucosa and that of the lower anal canal is stratified squamous epithelium; elsewhere, it is columnar. At the cardio‐oesophageal junction this transition is quite sharp, although occasionally columnar epithelium may line the lower oesophagus.

The gastric mucosa bears simple crypt‐like glands projecting down to the muscularis mucosae. The pyloric antrum secretes an alkaline juice containing mucus and the hormone gastrin. The body of the stomach secretes pepsin and also HCl, the latter from the oxyntic cells lying sandwiched deeply between the surface cells. The stomach mucosa also produces intrinsic factor.

The mucosa of the duodenum and small intestine, as well as bearing crypt‐like glands, projects into the bowel lumen in villous processes which greatly increase its surface area. The duodenum is distinguished by its crypts extending deep through the muscularis mucosae and opening into an extensive system of acini in the submucosa termed Brunner’s glands.

The mucosa of the large intestine is lined almost entirely by mucus‐secreting goblet cells; there are no villi.

The muscle coat of the alimentary tract is made up of an inner circular layer and an outer longitudinal layer. In the upper two‐thirds of the oesophagus and at the anal margin this muscle is voluntary; elsewhere it is involuntary. The stomach wall is reinforced by an innermost oblique coat of muscle and the colon is characterized by the condensation of its longitudinal layer into three taeniae coli.

The autonomic nerve plexuses of Meissner and Auerbach lie, respectively, in the submucosal layer and between the circular and longitudinal muscle coats.

The development of the intestine and its congenital abnormalities (Fig. 67)

The primitive endodermal tube of the gut is divided into:

1. the foregut (supplied by the coeliac axis) extending as far as the entry of the bile duct into the duodenum;
   
2. the midgut (supplied by the superior mesenteric artery) continuing as far as the distal transverse colon;
   
3. the hindgut (supplied by the inferior mesenteric artery) extending thence to the ectodermal part of the anal canal.

At an early stage rapid proliferation of the gut wall obliterates its lumen and this is followed by subsequent recanalization.

The foregut becomes rotated with the development of the lesser sac so that the original right wall of the stomach comes to form its posterior surface and the left wall its anterior surface. The vagi rotate with the stomach and therefore lie anteriorly and posteriorly to it at the oesophageal hiatus.

This rotation swings the duodenum to the right and the mesentery of this organ then blends with the peritoneum of the posterior abdominal wall – this blending process is termed zygosis.

The midgut enlarges rapidly in the 5 week fetus, becomes too large to be contained within the abdomen and herniates into the umbilical cord. The apex of this herniated bowel is continuous with the vitello‐intestinal duct and the yolk sac, but this connection, even at this early stage of fetal life, is already reduced to a fibrous strand.

The axis of this herniated loop of gut is formed by the superior mesenteric artery, which demarcates a cephalic and a caudal limb. The cephalic element develops into the proximal small intestine; the caudal segment differentiates into the terminal 62 cm (2 feet) of ileum, the caecum and the colon as far as the junction of the middle and left thirds of the transverse colon.

A bud that develops on the caudal segment indicates the site of subsequent formation of the caecum; it may well be that this bud delays the return of the caudal limb in favour of the cephalic gut during the subsequent reduction of the herniated bowel.

At 10 weeks this return of the bowel into the abdominal cavity commences. The midgut loop first rotates anticlockwise through 90°.

The cephalic limb returns first, passing upwards and to the left into the space left available by the bulky liver. In doing so, this midgut passes behind the superior mesenteric artery (which thus comes to cross the third part of the duodenum) and also pushes the hindgut – the definitive distal colon – over to the left.

When the caudal limb returns, it lies in the only space remaining to it, superficial to, and above, the small intestine with the caecum lying immediately below the liver.

The caecum then descends into its definitive position in the right iliac fossa, dragging the colon with it. The transverse colon thus comes to lie in front of the superior mesenteric vessels and the small intestine.

Finally, the mesenteries of the ascending and descending parts of the colon blend with the posterior abdominal wall peritoneum by zygosis. This embryological fusion of peritoneal surfaces is of major surgical importance. Thus, in mobilizing the right or left colon, an incision is made along this avascular line of zygosis lateral to the bowel, allowing it to be mobilized with its mesocolon and blood supply. In a similar fashion, the duodenum, head of pancreas and termination of the common bile duct can be mobilized bloodlessly by incising the peritoneum along the right border of the duodenum – Kocher’s manoeuvre (see page 82). Further mobilization of the third part of the duodenum, together with its second part and the pancreas, allows exposure of the abdominal aorta as far proximally as the level of its crossing by the left renal vein (see Fig. 108).

Numerous anomalies may occur in the highly complex developmental process.

1. Atresia or stenosis of the bowel may result from failure of recanalization of the lumen. Another cause of this may be damage to the blood supply to the bowel within the fetal umbilical hernia with consequent ischaemic changes. Imperforate anus – see page 87.
   
2. Meckel’s diverticulum represents the remains of the embryonic vitello‐intestinal duct (communication between the primitive midgut and yolk sac) and is, therefore, always on the anti‐mesenteric border of the bowel. As an approximation to the truth it can be said to occur in 2% of subjects, twice as often in males as in females, to be situated at 62 cm (2 feet) from the ileocaecal junction and to be 5 cm (2 in) long. In fact, it may occur anywhere from 15 cm (6 in) to 3.5 m (12 feet) from the terminal ileum and may vary from a tiny stump to a 15 cm (6 in) long sac. Occasionally, the diverticulum ends in a whip‐like solid strand.
   

   As well as a diverticulum – the commonest form – this duct may persist as a fistula or band connecting the intestine to the umbilicus, as a cyst hanging from the anti‐mesenteric border of the ileum or as a ‘raspberry tumour’ at the umbilicus, formed by the red mucosa of a persistent umbilical extremity of the diverticulum pouting at the navel (Fig. 68).

   
   

   The mucosa lining the diverticulum may contain islands of peptic epithelium with oxyntic (acid secreting) cells. Peptic ulceration of adjacent intestinal epithelium may then occur with haemorrhage or perforation.

   
   
3. The caecum may fail to descend; the peritoneal fold which normally seals it in the right iliac fossa passes, instead, across the duodenum and causes a neonatal intestinal obstruction. The mesentery of the small intestine in such a case is left as a narrow pedicle, which allows volvulus of the whole small intestine to occur (volvulus neonatorum).
   
4. Occasionally, reversed rotation occurs, in which the transverse colon comes to lie behind the superior mesenteric vessels with the duodenum in front of them; this may again be accompanied by extrinsic duodenal obstruction due to a peritoneal fold.
   
5. Exomphalos is persistence of the midgut herniation at the umbilicus after birth.

The liver (Fig. 69)

This is the largest organ in the body. It is related by its domed upper surface to the diaphragm, which separates it from the pleura, lungs, pericardium and heart. Its postero‐inferior (or visceral) surface overlaps the abdominal oesophagus, the stomach and the duodenum, the hepatic flexure of the colon and the right kidney and suprarenal, besides carrying the gall bladder.

The liver is divided into a larger right and small left lobe, separated superiorly by the falciform ligament and postero‐inferiorly by an ‘H’‐shaped arrangement of fossae (Fig. 69b,c):

• anteriorly and to the right – the fossa for the gall bladder;
  
• posteriorly and to the right – the groove in which the inferior vena cava lies embedded;
  
• anteriorly and to the left – the fissure containing the ligamentum teres;
  
• posteriorly and to the left – the fissure for the ligamentum venosum.

The cross‐bar of the ‘H’ is the porta hepatis. Two subsidiary lobes are marked out on the visceral aspect of the liver between the limbs of this ‘H’ – the quadrate lobe in front and the caudate lobe behind.

The ligamentum teres is the obliterated remains of the left umbilical vein which, in utero, brings blood from the placenta back into the fetus. The ligamentum venosum is the fibrous remnant of the fetal ductus venosus which shunts oxygenated blood from this left umbilical vein to the inferior vena cava, short‐circuiting the liver. It is easy enough to realize, then, that the grooves for the ligamentum teres, ligamentum venosum and inferior vena cava, representing as they do the pathway of a fetal venous trunk, are continuous in the adult. See also ‘The fetal circulation’, page 42.

Lying in the porta hepatis (which is 5 cm (2 in) long) are:

1. the common hepatic duct – anteriorly and to the right;
   
2. the hepatic artery – anteriorly and to the left;
   
3. the portal vein – posteriorly.

As well as these, autonomic nerve fibres (sympathetic from the coeliac axis and parasympathetic from the vagus), lymphatic vessels and lymph nodes are found there.

Segmental anatomy

The gross anatomical division of the liver into a right and left lobe, demarcated by a line passing from the attachment of the falciform ligament on its anterior surface to the fissures for the ligamentum teres and ligamentum venosum on its posterior surface, is simply a gross anatomical descriptive term with no morphological significance. Studies of the distribution of the hepatic blood vessels and ducts have indicated that the true morphological and physiological division of the liver is into right and left lobes demarcated by a plane that passes through the fossa of the gall bladder and the fossa of the inferior vena cava. Although these two lobes are not differentiated by any visible line on the dome of the liver, each has its own arterial and portal venous blood supply and separate biliary drainage. This morphological division lies to the right of the gross anatomical plane and in this the quadrate lobe comes to be part of the left morphological lobe of the liver while the caudate lobe divides partly to the left and partly to the right lobe (Fig. 70).

The right and left morphological lobes of the liver can be further subdivided into a number of segments, four for each lobe (Fig. 70c). The student need not learn the details of these, but of course to the hepatic surgeon, carrying out a partial resection of the liver, knowledge of these segments, with their individual blood supply and biliary drainage, is of great importance.

At the hilum of the liver, the hepatic artery, portal vein and bile duct each divide into right and left branches and there is little or no anastomosis between the divisions on the two sides (Fig. 71). From the region of the porta hepatis, the branches pass laterally and spread upwards and downwards throughout the liver substance, defining the morphological left and right lobes.

The hepatic veins (Figs 70c, 72)

These veins are massive and their distribution is somewhat different from that of the portal, hepatic arterial and bile duct systems already described. There are three major hepatic veins, comprising a right, a central and a left. These pass upwards and backwards to drain into the inferior vena cava at the superior margin of the liver. Their terminations are somewhat variable but usually the central hepatic vein enters the left hepatic vein near its termination. In other specimens it may drain directly into the cava. In addition, small hepatic venous tributaries run directly backwards from the substance of the liver to enter the vena cava more distally to the main hepatic veins. Although these are not of great functional importance they obtrude upon the surgeon during the course of a right hepatic lobectomy.

The three principal hepatic veins have three zones of drainage corresponding roughly to the right, the middle and the left thirds of the liver. The plane defined by the falciform ligament corresponds to the boundary of the zones drained by the left and middle hepatic veins. Unfortunately for the surgeon, the middle hepatic vein lies just at the line of the principal plane of the liver between its right and left morphological lobes and it is this fact which complicates the operation of right or left hepatic resection (Fig. 72).

The biliary system (Fig. 73)

The right and left hepatic ducts fuse in the porta hepatis to form the common hepatic duct (4 cm (1.5 in)). This joins with the cystic duct (4 cm (1.5 in)), draining the gall bladder, to form the common bile duct (10 cm (4 in)). The common bile duct commences approximately 2.5 cm (1 in) above the duodenum, then passes behind it to open at a papilla on the medial aspect of the second part of the duodenum. In this course the common duct either lies in a groove in the posterior aspect of the head of the pancreas or is actually buried in its substance.

As a rule, the common duct termination joins that of the main pancreatic duct (of Wirsung) in a dilated common vestibule, the ampulla of Vater, whose opening in the duodenum is guarded by the sphincter of Oddi. Occasionally, the bile and pancreatic ducts open separately into the duodenum.

The common hepatic duct and the supraduodenal part of the common bile duct lie in the free edge of the lesser omentum, where they are related as follows (Fig. 47):

• bile duct – anterior to the right;
  
• hepatic artery – anterior to the left;
  
• portal vein – posterior;
  
• inferior vena cava – still more posterior, separated from the portal vein by the foramen of Winslow.

The gall bladder (Fig. 73)

The gall bladder normally holds approximately 50 ml of bile and acts as a bile concentrator and reservoir. It also secretes mucus from the goblet cells in its wall. It lies in a fossa separating the right and quadrate lobes of the liver and is related inferiorly to the duodenum and transverse colon. (An inflamed gall bladder may occasionally ulcerate into either of these structures.)

For descriptive purposes, the organ is divided into the fundus, body and neck, the last opening into the cystic duct. In dilated and pathological gall bladders there is frequently a pouch present on the ventral aspect just proximal to the neck termed Hartmann’s pouch in which gallstones may become lodged.

Blood supply (Fig. 74)

The gall bladder is supplied by the cystic artery (a branch usually of the right hepatic artery), which lies in the triangle made by the liver, the cystic duct and the common hepatic duct (Calot’s triangle). Other vessels derived from the hepatic artery pass to the gall bladder from its bed in the liver. Interestingly, only rarely is there an accompanying vein to the cystic artery. Small veins pass from the gall bladder through its bed directly into tributaries of the right portal vein within the liver.

Development

The gall bladder and ducts are subject to numerous anatomical variations which are best understood by considering their embryological development. A diverticulum grows out from the ventral wall of the duodenum which differentiates into the hepatic ducts and the liver (see Fig. 76). Another diverticulum from the side of the hepatic duct bud forms the gall bladder and cystic duct.

Some variations are shown in Fig. 75.