17 Alimentary system
There is only superficial fascia on the abdominal wall. This forms two layers in the lower abdomen, a superficial layer of fatty tissue (Camper’s fascia) and a deeper fibrous layer (Scarpa’s fascia). Scarpa’s fascia is attached to the deep fascia of the thigh about 2.5 cm below the inguinal ligament. It extends into the perineum as Colles’ fascia, which is attached to the perineal body, perineal membrane and laterally to the rami of the pubis and the ischium. Scarpa’s fascia also extends onto the penis and scrotum. These attachments are important when considering the effect of rupture of the bulbous urethra. Urine will track into the scrotum, perineum and penis and into the lower abdominal wall deep to the plane of Scarpa’s fascia. However, it does not track into the thigh, because of the attachment of Scarpa’s fascia to the deep fascia of the thigh. Likewise, an ectopic testis in the groin does not descend any lower into the thigh because of this attachment.
A knowledge of the anatomy of the muscles of the abdominal wall (Fig. 17.1) is a prerequisite to understanding the basis of abdominal incisions. The abdominal wall consists principally of three sheets of muscle which are fleshy laterally, and aponeurotic in front and behind. As the aponeuroses pass forward they ensheath the rectus abdominis muscle.
Source: Rogers A W, Textbook of anatomy; Churchill Livingstone, Edinburgh (1992).
This is a vertical muscle on either side of the midline. It arises from the fifth, sixth and seventh costal cartilages and is inserted into the pubic crest. The anterior aspect of the muscle bears three transverse tendinous intersections: one at the level of the xiphoid, one at the level of the umbilicus, and one-half way between these two points. They are adherent to the anterior rectus sheath but not to the posterior sheath.
This arises from the outer surface of the lower eight ribs and is inserted into the linea alba (which runs between the xiphoid and the pubis), the pubic crest and pubic tubercle, and into the anterior half of the iliac crest. Between the anterior superior iliac spine and pubic tubercle, its lower recurved aponeurotic border forms the inguinal ligament.
This arises from the lumbar fascia, the anterior two-thirds of the iliac crest, and the lateral two-thirds of the inguinal ligament. The majority of its fibres run upwards and medially (at right angles to those of external oblique) and are inserted into the lower six costal cartilages and the linea alba. The lower fibres are attached to the pubic crest by the conjoint tendon common to internal oblique and transversus abdominis.
This arises from the deep surface of the lower six costal cartilages (interdigitating with the diaphragm), the lumbar fascia, the anterior two-thirds of the iliac crest, and the lateral third of the inguinal ligament. It is inserted into the linea alba and into the pubic crest by the conjoint tendon.
The lower border of the posterior aponeurotic part of the rectus sheath is marked by a crescentic free margin, the arcuate line of Douglas. At this point, the inferior epigastric vessels enter the sheath, passing upwards to anastomose with the superior epigastric vessels. The rectus sheaths fuse in the midline to form the linea alba, which runs from the xiphisternum to the pubic symphysis.
This is made through the linea alba skirting the umbilicus. It is an excellent incision for both routine and rapid access to the peritoneal cavity, the linea alba being almost a bloodless line. Structures encountered include skin, subcutaneous fat, linea alba, extraperitoneal fat and peritoneum.
The subcostal incision is used most commonly on the right-hand side for open cholecystectomy. On the left side it is used for elective splenectomy. On both sides it may be used to expose the kidneys. The incision is carried out about 2.5 cm below and parallel to the costal margin extending laterally from the midline. Structures encountered include skin, subcutaneous fat, anterior rectus sheath which is opened in the line of the incison, the rectus muscle and the posterior rectus sheath with the adherent extraperitoneal fat and peritoneum. The ninth intercostal nerve is in danger in the lateral part of the wound. Damage to this may cause weakness and atrophy of the rectus, with predisposition to incisional hernia formation.
The incision is centred on McBurney’s point (two-thirds of the way along a line drawn from the umbilicus to the anterior superior iliac spine) and at right angles to this line. Structures encountered include skin, subcutaneous fat, Scarpa’s fascia (at the lower end of the incision), external oblique aponeurosis (which is incised in the line of its aponeurotic fibres), the internal oblique and transversus muscles (which are split in the line of their fibres). Finally, extraperitoneal fat and peritoneum are reached.
The use of this incision is declining. It is placed about 2.5 cm lateral to, and parallel to, the midline. The anterior rectus sheath is opened, the rectus displaced laterally, and the posterior sheath together with the peritoneum is incised. The anterior rectus sheath adheres to the muscle at the tendinous intersections, and the sheath requires to be dissected off at this point. Bleeding will be encountered in doing this, as the segmental vessels enter at these points. The rectus is not attached to the posterior sheath. The upper posterior rectus sheath is a thick, well-defined structure, but below a point half way between the umbilicus and pubic symphysis it is composed of transversalis fascia only and is relatively thin. The inferior epigastric vessels anastomosing with the superior epigastric vessels, may be seen posterior to the muscle and may require dividing in a lower paramedian incision.
An incision is made at the lateral border of rectus abdominis below the level of the umbilicus, and the rectus is displaced medially. It was once popular for appendicectomy, but the disadvantage is that if the wound is extended vertically it may damage the nerves entering the rectus sheath to supply the rectus muscle. The use of the pararectus incision is increasing for open insertion of a Tenckhoff catheter for continuous ambulatory peritoneal dialysis.
The inguinal canal (Fig. 17.3) is an oblique passage in the lower part of the abdominal wall which transmits the spermatic cord and ilio-inguinal nerve in the male, and the round ligament of the uterus and the ilio-inguinal nerve in the female. It is approximately 4 cm long and passes downwards and medially from the deep inguinal ring to the superficial inguinal ring lying above and parallel to the inguinal ligament.
Source: Rogers op. cit.
The deep inguinal ring is a defect in the transversalis fascia lying 1 cm above the midpoint of the inguinal ligament. It lies immediately lateral to the inferior epigastric vessels. The superficial inguinal ring is aV-shaped defect in the external oblique aponeurosis and lies above and medial to the pubic tubercle.
As it passes through the canal, the spermatic cord obtains three coverings: (i) the external spermatic fascia from the external oblique aponeurosis at the superficial inguinal ring; (ii) the cremasteric fascia from the internal oblique containing the cremaster muscle; (iii) the internal spermatic fascia from the transversalis fascia. The cord contains the testicular artery, the pampiniform plexus of veins, and the vas deferens. Other structures include the cremasteric artery, the artery to the vas, the nerve to cremaster, sympathetic nerve fibres and lymphatics. The ilio-inguinal nerve lies on the cord but is not part of it.
The femoral artery and femoral vein enter the femoral triangle deep to the inguinal ligament within a prolongation of fascia termed the femoral sheath. This is derived from the transversalis fascia anteriorly, and posteriorly from the fascia covering iliacus. The medial part of the femoral sheath is occupied by the femoral canal. The upper opening of the femoral canal is called the femoral ring and will just admit the tip of the little finger in the male. In the female the pelvis is wider and the canal, therefore, is larger, and femoral herniae are consequently more common in the female. The boundaries of the femoral ring are:
The femoral canal contains fat, some lymphatics and a lymph node (Cloquet’s node). The canal functions as a dead space for expansion of the femoral vein and secondly as a pathway for lymphatics from the lower limb to the external iliac nodes. The femoral ring is narrow, and the lacunar ligament forms a sharp medial border. Because of this, irreducibility and strangulation occur commonly with femoral hernias. Also, femoral hernias are likely to be of the Richter’s type.
An indirect hernia passes through the deep inguinal ring and along the inguinal canal and reaches the scrotum if it is very large. The hernial sac is covered by the layers of the cord. A direct inguinal hernia bulges directly through the posterior wall of the inguinal canal medial to the inferior epigastric artery. It bulges through Hesselbach’s triangle, bounded by the inferior epigastric artery laterally, the inguinal ligament inferiorly, and the lateral border of rectus abdominis medially. Distinction between the two types of hernia at operation relates to the relationship to the inferior epigastric vessels. An indirect sac lies laterally, a direct hernia medial to the vessels.
Prior to surgery an attempt may be made to distinguish between the two types of hernia and between a femoral and an inguinal hernia. If an inguinal hernia protrudes through the superficial ring, it can be felt above and medial to the pubic tubercle. A femoral hernia is felt below and lateral to the pubic tubercle. If a hernia descends into the scrotum it is almost always an indirect inguinal hernia. If an inguinal hernia is reducible then application of pressure by the finger over the deep inguinal ring should control the hernia when the patient coughs if it is an indirect inguinal hernia. However, if the hernia appears medial to the point of finger pressure then it is a direct hernia.
The peritoneum is the serous membrane of the abdominal cavity. It consists of a parietal layer lining the abdominal and pelvic walls, and a visceral layer which more or less covers the contained organs. In the male the peritoneal cavity is a closed sac, but in the female the free extremities of the uterine tubes open into the cavity, constituting a possible pathway of infection from the exterior. The peritoneal cavity is subdivided into a main cavity, the greater sac, and a small cavity, the lesser sac (omental bursa). The greater sac is further divided by the transverse colon into a supracolic and infracolic compartment. The connection between the greater and lesser sac is known as the epiploic foramen or the foramen of Winslow.
The attachments of the peritoneum are complicated. It is convenient to start at the umbilicus and work down. Below the level of the umbilicus, the parietal peritoneum is smooth apart from some folds (Fig. 17.4). These are the median umbilical fold onthe median umbilical ligament (which is due tothe obliterated urachus passing from the bladder to the umbilicus), the medial umbilical folds on the obliterated umbilical arteries, and the lateral umbilical folds which are further lateral and contain the inferior epigastric arteries. The peritoneum of the pelvis is continuous with that of the abdominal cavity. It completely encloses the sigmoid colon, forming the pelvic mesocolon. It is applied to the front and side of the upper third of the rectum and to the front only of the middle third of the rectum. It is then reflected in the male onto the base and upper part of the bladder, forming the rectovesical pouch. In the female the peritoneum is reflected from the side and front of the rectum, to the upper part of the posterior wall of the vagina and then over the posterior upper and anterior surface of the uterus to the bladder. Between the uterus and the rectum is the recto-uterine pouch (of Douglas). The peritoneum passes off the lateral margins of the uterus to the pelvic wall, forming the broad ligaments, the upper borders of which contain the uterine tubes. The free upper margins of the broad ligament lateral to the uterine tubes form the infundibulopelvic fold.
Returning to the umbilicus, the falciform ligament, the sickle-shaped fold of peritoneum, passes upwards and slightly to the right of the midline to the liver. It contains the ligamentum teres, i.e. the obliterated umbilical vein, in its free edge, and this passes into the groove between the quadrate lobe and left lobe of the liver. Traced superiorly the two layers of the falciform ligament diverge from each other, the right limb joins the upper layer of the coronary ligament while the left layer passes to the left to form the anterior layer of the left triangular ligament. Elsewhere on the anterior abdominal wall, above the umbilicus, the peritoneum sweeps upwards and over the inferior aspect of the diaphragm to be reflected onto the liver and onto the right margin of the abdominal oesophagus. Details of the peritoneal reflections of the liver are described in the section on the liver. After enclosing the liver the peritoneum descends from the porta hepatis as a double layer, i.e. the lesser omentum, and then this separates to enclose the stomach. It reforms again at the greater curve and then loops downwards, again turning upwards and attaching to the length of the transverse colon, forming the greater omentum (Fig. 17.5).
The lower leaf of the greater omentum then continues upwards, enclosing the transverse colon within the peritoneum, and then passes upwards and backwards as the transverse mesocolon, a double layer of peritoneum, to the posterior abdominal wall, where it attaches along the anterior aspect of the pancreas. At the base of the transverse mesocolon, this double layer of peritoneum divides once again, the upper leaf passing upwards over the posterior abdominal wall to reflect onto the liver, while the lower leaf passes over the lower part of the posterior abdominal wall to cover the pelvic viscera and to join with the peritoneum of the anterior abdominal wall. However, the peritoneum of the posterior abdominal wall is interrupted as it is reflected along the small bowel from the duodenal jejunal flexures to the ileocaecal junction, forming the mesentery of the small intestine. The lines of peritoneal reflection on the posterior abdominal wall are shown in Fig. 17.6.
The lesser sac (Fig. 17.7) is entered via the epiploic foramen or foramen of Winslow. The lesser sac is a potential space lying behind the lesser omentum and stomach and projecting downwards to the transverse mesocolon. Superiorly is the superior recess, whose anterior border is the caudate lobe of the liver. The left wall of the lesser sac is formed by the spleen and the gastrosplenic and lienorenal ligaments. To the right the sac opens into the main peritoneal cavity via the epiploic foramen.
There are a number of potential spaces below the diaphragm in relation to the liver which may become the site of abscess formation (a subphrenic abscess). Abscesses may arise from such lesions as perforated peptic ulcers, perforated appendicitis, or perforated diverticulitis. Only two of the spaces are in fact directly subphrenic, the other two being subhepatic. The right and left subphrenic spaces lie between the diaphragm and the liver and are separated from one another by the falciform ligament. The right subhepatic space (pouch of Rutherford Morrison) is bounded by the posterior abdominal wall behind and by the liver above. The gall bladder, duodenum and right kidney are immediate relations. The left subhepatic space is the lesser sac itself. It may distend with fluid as a result of a perforated posterior gastric ulcer or as a result of acute pancreatitis (pseudocyst of the pancreas). At the present time most subphrenic abscesses are drained percutaneously under ultrasound control. However, the occasional one still requires open surgery and may be accessed if they are posteriorly placed by an incision below or through the bed of the twelfth rib. If they are placed anteriorly they can be drained through an incision below and parallel to the costal margin.
(See Fig. 17.10.) The oesophagus passes downwards and slightly to the left. Anteriorly lie the trachea and thyroid gland. Posteriorly lie the lower cervical vertebrae and the prevertebral fascia; to the left lie the left common carotid artery, the left inferior thyroid artery, the left subclavian artery and the thoracic duct; to the right, the right common carotid artery. The recurrent laryngeal nerves lie on either side in the groove between the trachea and the oesophagus.
(See Fig. 17.11.) The oesophagus passes down through the superior and posterior mediastinum, passing initially to the right to reach the midline opposite T5. It then passes downwards, forwards, and to the left to reach the oesophageal opening in the diaphragm at T10. The two vagus nerves form a plexus on the surface of the oesophagus in the posterior mediastinum, the left nerve being anterior and the right posterior.
Source: Rogers op. cit.
Anteriorly lie the left common carotid artery, the trachea, the left main bronchus which constricts it, thepericardium separating it from the left atrium andthe diaphragm. Posteriorly lie the thoracic vertebrae, the thoracic duct, the hemiazygos vein, and below, the descending aorta.
On the left side lie the left subclavian artery, theaortic arch, the left vagus nerve and its recurrent laryngeal branch, the thoracic duct and the left pleura. On the right side lie the pleura and azygos vein.
The oesophagus passes through the oesophageal opening in the right crus of the diaphragm at the level of T10. It then lies in a groove on the posterior surface of the left lobe of the liver, with the left crus of the diaphragm behind. It is covered anteriorly and to the left with peritoneum. The anterior vagus nerve is closely applied to its surface behind its peritoneal covering. The posterior vagus nerve is at a little distance from the posterior surface of the oesophagus.
In the neck it is from the inferior thyroid arteries, in the thorax from branches of the aorta, and in the abdomen from the left gastric and inferior phrenic arteries. Venous drainage of the cervical part is to the inferior thyroid veins; of the thoracic part to the azygos veins; and the abdominal part partly to the azygos vein (systemic) and partly to the left gastric veins (portal).
The upper third of the oesophagus is supplied with parasympathetic fibres via the recurrent laryngeal nerve and sympathetic fibres from the middle cervical ganglion via the inferior thyroid artery. Below the root of the lung the vagi and sympathetic nerves contribute to the oesophageal plexus.
The oesophagus consists of: (i) a mucous membrane lined by stratified squamous epithelium; occasionally there is gastric mucosa in the lower part of the oesophagus; (ii) a submucosa containing mucous glands; (iii) a muscular layer consisting of inner circular and outer longitudinal muscle; in the upper third it is striated, producing rapid contraction and swallowing; in the lower two-thirds it is smooth, exhibiting peristalsis; (iv) an outer layer of loose areolar tissue.
In the lower oesophagus there is a site of portosystemic anastomosis between the azygos vein (systemic) and the left gastric vein (portal). The oesophageal varices may arise at this site in portal hypertension.
The stomach is approximately J-shaped, having two surfaces: the anterior and posterior. It has two curvatures – the greater and lesser curve – and two orifices: the cardia and the pylorus. Initially the stomach projects to the left, the dome-like gastric fundus projecting above the level of the cardia. In the erect living subject the vertical part of the J shape of the stomach represents the upper two-thirds of the stomach. The lesser curvature of the stomach is vertical in its upper two-thirds but then turns upwards and to the right, where it becomes the pyloric antrum. The junction of the body with the pyloric antrum is marked along the lesser curve by a distinct notch termed the incisura angularis. Between the cardia and pylorus lies the body of the stomach, leading to the pyloric antrum which is a narrow area of the stomach immediately before the pylorus. The left margin of the body of the stomach is the greater curvature. In the erect subject this may reach or lie below the umbilicus. It then passes upwards to the right as the lower margin of the pyloric antrum. To the lesser curvature of the stomach is attached the lesser omentum and to the greater curvature the greater omentum, which to the left is continuous with the gastrosplenic ligament. The thickened pyloric sphincter is easily palpable at surgery and surrounds the pyloric canal. The junction of the pylorus with the duodenum is marked by a constant prepyloric vein of Mayo which crosses it vertically at this level. Unlike the cardiac sphincter of the stomach the pyloric sphincter is well marked anatomically.
The veins are named according to the arteries. The venous drainage is into the portal system. The stomach has such a rich blood supply that any three of the four main arteries may be ligated without any compromise of the arterial blood supply to the stomach.
The arrangements of lymph nodes in relation to the stomach is shown in Fig. 17.13. The lymphatic drainage of the stomach accompanies its blood vessels. The area of the stomach supplied by the splenic artery drains via lymphatics accompanying that artery to the lymph nodes of the hilum of the spleen, then to those situated along the upper border of the pancreas and eventually to the coeliac nodes. The cardiac area of the stomach drains along the left gastric artery to reach the coeliac nodes. The remainder of the stomach drains as follows: via branches of the hepatic artery through nodes along the lesser curve to the coeliac nodes and along the right gastroepiploic vessels to the subpyloric nodes and then to the coeliac nodes. Retrograde spread may occur into the hepatic lymph nodes at the porta hepatis. Enlargements of these nodes may cause external compression of the bile ducts to produce obstruct-ive jaundice. The extensive and complex lymphatic drainage of the stomach creates problems in dealing with gastric cancer. Involvement of the nodes around the coeliac axis may render the growth incurable.
The clinically important nerve supply (Fig. 17.14) of the stomach is the vagus nerves. The anterior and posterior vagus nerves enter the abdomen through the oesophageal hiatus. The anterior vagus nerve liesclose to the wall of the oesophagus and upper part of the stomach, but the posterior nerve is at a little distance from it. The anterior vagus runs caudally and supplies the anterior surface and lesser curve of the stomach. Before it reaches the stomach, it gives off a hepatic branch which passes in the lesser omentum to the liver and gall bladder and the pyloric branch to the pyloric sphincter. The posterior vagus nerve gives off a coeliac branch which passes to the coeliac plexus before sending a gastric branch to the posterior surface of the stomach. The gastric divisions of both anterior and posterior vagi reach the stomach at the cardia and descend along the lesser curve between the anterior and posterior peritoneal attachments of the lesser omentum. These nerves are referred to as the anterior and posterior nerves of Latarjet.
The vagus nerves used to be divided in operations for peptic ulceration. However, with the advent of H2 receptor antagonists and proton pump inhibitors and the discovery of the role of H. pylori in the aetiology of peptic ulceration, these operations are performed less and less. However, it is necessary to understand the role of the vagus, as vagotomy is still required in surgery for bleeding peptic ulcer, and also a knowledge of the oesophageal hiatus and the relations of the vagus nerve is required so that these nerves are not inadvertently damaged in repair of hiatus hernia. The vagus nerve constitutes both the motor and secretory nerve supply for the stomach, i.e. it is responsible for motility and control of gastric secretions. When the nerve is divided in the operation of vagotomy, acid secretion is cut down in the stomach, but so is motility, so that the stomach empties through an intact pylorus only with difficulty. Because of this, total vagotomy (truncal vagotomy) must always be accompanied by some form of drainage procedure: either a pyloroplasty to destroy the pyloric sphincter or a gastrojejunostomy to bypass the pyloric sphincter. In the operation of highly selective vagotomy (proximal selective vagotomy) it is possible to avoid the drainage procedure, as the nerve of Latarjet remains intact and this maintains the innervation of the pyloric antrum and hence its propulsive activity.
The surface of the gastric mucosa is covered by columnar epithelial cells that secrete mucus and alkaline fluid that protects the epithelium from mechanical injury and from gastric acid. The surface of the mucosa is studded with gastric pits, each pit being the opening of a duct into which the gastric glands empty. The gastric mucosa can be divided into three areas. The cardiac gland area is the small segment located near the gastro-oesophageal junction. Histologically it contains principally mucus-secreting cells, although occasionally a few parietal (oxyntic) cells are present. The remainder of the stomach is divided into the acid-secreting region (oxyntic gland area) and the pyloric gland area. The oxyntic gland area is the portion containing the parietal (oxyntic cells) and the chief (zymogen) cells. The pyloric end area constitutes the distal 30% of the stomach and contains G cells that produce gastrin. In this region there are few oxyntic and peptic cells, mucus-secreting cells predominating. As in the rest of the gastrointestinal tract the muscular wall of the stomach is composed of an inner circular layer and an outer longitudinal layer. However, in addition there is an incomplete inner layer of obliquely situated fibres which is more prominent near the lesser curvature. Figure 17.15 shows the histological featuresof the mucosa in the oxyntic gland area. Each gastricpit drains between three and seven tubular gastric glands. The neck of the gland contains many mucus cells, oxyntic cells being most numerous in the midportion of the glands and chief cells predominating in the basal portion.
The duodenum is C-shaped. It curves around the head of the pancreas and is approximately 25 cm long. The first 2–3 cm of the first part of the duodenum is completely covered with peritoneum, but then the duodenum becomes retroperitoneal. The duodenum is divided into four parts.
This is approximately 5 cm long and it ascends from the pylorus, being directed superiorly, posteriorly and to the right. It has a complete investment of visceral peritoneum on its first 2–3 cm. Anteriorly lie the liver and gall bladder. Immediately posterior to it lie the portal vein, the common bile duct and gastroduodenal artery. Behind these is the IVC. The relationship of thegastroduodenal artery to the first part of the duodenum is important because erosion of posterior duodenal ulcers into the gastroduodenal artery will cause haematemesis and melaena.
This descends in a curve around the head of the pancreas. It is approximately 7.5 cm long. The bile ducts and main pancreatic ducts enter the second part of the duodenum together at the duodenal papilla on its posteromedial side. The point of entry marks the junction of the foregut and midgut. The accessory pancreatic duct (of Santorini) opens into the duodenum a little above the papilla. The second part of the duodenum is crossed by the transverse colon and lies anteriorly to the right kidney and ureter.
The third part of the duodenum is approximately 10 cm long and runs horizontally to the left. It crosses the IVC, the aorta and the third lumbar vertebra. It is crossed anteriorly by the root of the mesentery and the superior mesenteric vessels.
This is approximately 2.5 cm long and ascends vertically to end by turning abruptly anteriorly and to the left to continue as the jejunum. At the duodenal-jejunal flexure the small intestine leaves the posterior abdominal wall and acquires a mesentery. At surgery the duodenojejunal flexure may be identified by the presence of the suspensory ligament of Treitz. This is a peritoneal fold descending from the right crus of the diaphragm to the termination of the duodenum.
The superior pancreaticoduodenal artery, which arises from the gastroduodenal artery, anastomoses with the inferior pancreatico duodenal artery, which originates from the superior mesenteric artery. These two arteries both lie in the curve between the duodenum and the head of the pancreas, supplying both the duodenum and the head of the pancreas.
The length of the small intestine is variable, averaging some 6 m in length. The upper half of the small intestine is termed the jejunum, the remainder being termed the ileum, although the distinction between the two is not sharply defined. The jejunum and ileum lie in thefree edge of the mesentery. The mesentery of the small intestine is about 15 cm long and is attached across the posterior abdominal wall. It commences at theduodeno-jejunal juncture to the left of the second lumbar vertebrae and passes obliquely downwards to the right sacroiliac joint. From left to right the root of the mesentery crosses anterior to the following structures:
The large intestine extends from the ileocaecal junction to the anus. It is approximately 1.5 m in length on average. It consists of the caecum, ascending colon, transverse colon, descending colon, sigmoid colon and rectum. The caecum is a dilated blind-ended pouch situated in the right iliac fossa and is usually completely covered with peritoneum. The ileocaecal valve lies on the left side of the junction between the caecum and ascending colon. Tumours may grow to a large size in the caecum without causing any obstruction until they encroach on the ileocaecal junction. The appendix arises from the posteromedial aspect of the caecum about 2.5 cm below the ileocaecal valve. The taenia coli, three flattened bands of longitudinal muscle which pass from caecum to rectosigmoid, converge at the base of the appendix. The taenia are shorter than the length of the bowel hence the sacculated appearance of the large bowel.
The ascending colon extends from the caecum to the undersurface of the liver where, at the hepatic flexure, it turns left to become the transverse colon. It is covered on its anterior and lateral aspect by peritoneum. Posterior relations include iliacus, quadratus lumborum, and the perirenal fascia over the lateral aspect of the kidney.
The transverse colon passes to the left, where it becomes the descending colon at the splenic flexure. It is attached to the anterior border of the pancreas by the transverse mesocolon. Superiorly it is related to the liver, gall bladder, greater curvature of the stomach and the spleen. Inferiorly are coils of small intestine. Anteriorly lie the anterior layers of the greater omentum. Posteriorly lie the right kidney, second part of the duodenum, pancreas, small intestine and the left kidney.
The descending colon passes from the splenic flexure to the sigmoid colon. Peritoneum covers its anterior and lateral surfaces. Between the splenic flexure and the diaphragm is a fold of peritoneum, the phrenicocolic ligament. Posteriorly to the descending colon lies the left kidney, quadratus lumborum and iliacus. Anteriorly lie coils of small bowel.
The sigmoid colon commences at the pelvic brim and extends to the rectosigmoid junction. It has a mesentery which occasionally is extensive allowing the sigmoid colon to hang down into the pelvis. The root of the sigmoid colon crosses the external iliac vessels and left ureter. The sigmoid loop rests on the bladder in the male and is related to the uterus and the posterior fornix of the vagina in the female. Hence the development of vesicocolic and vaginocolic fistulae in diverticular disease of the sigmoid colon. The taenia coli extend from the base of the appendix to the rectosigmoid junction. There are no taenia on the appendix or rectum. The colon, but neither the caecum, the appendix, nor rectum, possesses fat-filled peritoneal tags scattered along its surface. These are called appendices epiploicae and are most numerous in the sigmoid colon.
The appendix is attached to the posteromedial aspect of the caecum below the ileocaecal valve. Its length varies considerably from one subject to another but usually is within the range 5–10 cm. It can be as small as 2.5 cm or as long as 25 cm. The position of the appendix is variable. In 75% of cases the appendix lies behind the caecum or colon, i.e. retrocaecal or retrocolic. In 20% of cases it hangs down into the pelvis, and in 5% of cases it is either pre-ileal or retro-ileal (Fig. 17.16). The appendix bears a mesentery containing the appendicular artery, which is a branch of the ileocolic artery. The mesentery of the appendix descends behind the ileum as a triangular fold containing the appendicular artery in its free border. The appendicular artery is functionally an end artery, and, therefore, in acute appendicitis, if it thromboses, there is a consequent rapid development of gangrene and perforation of the appendix.
The rectum is about 12 cm long, commencinganterior to the third segment of the sacrum and ending about 2.5 cm in front of the coccyx, where it bends sharply backwards to become the anal canal. It isextraperitoneal on its posterior aspect in its upper third and extraperitoneal on its posterior and lateral aspect in its middle third. The lower third is completely extraperitoneal lying below the pelvic peritoneum. The rectum is curved to follow the contour of the sacralhollow. There are three lateral inflexions projected to the left, right and left again from above downwards. Each inflexion is capped by a valve of Houston.
The relations of the rectum are important in the understanding of a digital rectal examination and also in the spread of rectal cancer. Anteriorly in the male lies the rectovesical pouch, the base of the bladder, seminal vesicles and the prostate. A layer of fascia (of Denonvilliers) lies in front of the rectum, separating it from the anterior structures, and this is the plane of dissection which must be sought during abdominoperineal excision of the rectum. In the female lies the recto-uterine pouch (of Douglas) and the posterior wall of the vagina. The upper two-thirds of the rectum is covered with peritoneum anteriorly and related to coils of small bowel and the sigmoid colon in the rectovesical or recto-uterine pouch. Posteriorly lie the sacrum, coccyx and middle sacral artery. The lower sacral nerves also lie posteriorly and may be invaded by rectal cancer spreading posteriorly and resulting in sciatic pain. Laterally, below the peritoneal reflection, lie the levator ani and coccygeus.
The arterial blood supply of the large intestine is shown in Fig. 17.17. The large intestine is supplied by both branches of the superior and inferior mesenteric artery. The branches of the superior mesenteric artery are as follows: