The peritoneum is a serous membrane which lines the abdominal cavity; it covers the anterior and posterior walls, the undersurface of the diaphragm and the walls and floor of the pelvic cavity. All this is the parietal peritoneum. In places it leaves the posterior abdominal wall or diaphragm or pelvic floor to form a partial or complete investment for viscera; this is the visceral peritoneum, which forms the serous covering for many viscera.
Peritoneum consists of a single layer of flattened cells, with phagocytic properties, overlying areolar tissue which varies in both thickness and density in different places. Over expansile parts this areolar tissue is loose and cellular (e.g. transversalis fascia on the anterior abdominal wall) while over non-expansile parts it is often thick (e.g. iliac fascia, psoas fascia, parietal pelvic fascia); but loose or dense, thin or thick, these variously named fasciae are part of the one continuous extraperitoneal connective tissue lying between the parietal peritoneum and the walls of the abdominal and pelvic cavities.
Various folds or reflexions of peritoneum connect viscera to the abdominal walls or to one another. Some of these are properly called folds, but others are called mesentery, omentum or ligament. The double fold supporting the small intestine is the mesentery; the mesenteries supporting the transverse colon, sigmoid colon and appendix are the transverse mesocolon, sigmoid mesocolon and mesoappendix. The lesser omentum connects the stomach to the liver, and the greater omentum hangs down from the lower border of the stomach. The various ligaments associated with the liver, stomach and spleen are simply peritoneal folds attached to them, and the broad ligament stretches out on either side of the uterus.
Peritoneal folds of the anterior abdominal wall
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.
Peritoneal cavity: greater and lesser sacs
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).
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Figure 5.13 Peritoneum of the lesser sac and greater omentum, as seen in diagrammatic sagittal sections looking towards the left: A The theoretical developmental condition, with the two layers of the dorsal mesogastrium doubling back to form a fold (the greater omentum) and to overlie the transverse mesocolon (containing the transverse colon). B The end result, with fusion of adjacent double peritoneal layers. |
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Figure 5.14 Attachments of the parietal peritoneum to the posterior abdominal wall. |
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Figure 5.49 Diagrammatic transverse section through the left upper abdomen, viewed from below (like a CT scan), showing the peritoneal relationships of the spleen and renal fascia. The short gastric vessels run in the gastrosplenic ligament. The splenic vessels and the tail of the pancreas are in the splenorenal ligament. The kidney is surrounded by its own capsule, the perinephric fat and the renal fascia. |
Greater omentum
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).
Lesser omentum
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.
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Figure 5.35 Lesser omentum and the epiploic foramen. A Part of the omentum has been cut away to display the bile duct and hepatic artery in front of the portal vein. Interrupted lines indicate gastroduodenal artery behind first part of duodenum. B Transverse section of the right free margin at the level of the epiploic foramen, viewed from below, showing the foramen bounded in front and behind by the two great veins. |
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).
Peritoneal compartments
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).
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Figure 5.15 Infracolic compartment of peritoneum. The greater omentum, transverse mesocolon and transverse colon have been lifted up and the small intestine pulled towards the left side of the peritoneal cavity, to show the root of the transverse mesocolon, root of the mesentery and the right infracolic compartment. |
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).
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Figure 5.16 Sagittal section through the right lobe of the liver (L), kidney (K) and transverse colon (C), showing peritoneal reflections and the formation of the hepatorenal pouch. |
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Figure 5.32 Liver and peritoneal reflections. A In the anterior view the oesophagus is pulled upwards from its normal position behind the left lobe to show the peritoneal attachments. The peritoneal edges seen here are attached to the diaphragm, apart from the lower edges of the falciform ligament which are connected to the anterior abdominal wall. B Posterior view (posterior and visceral surfaces). The lesser omentum, whose cut edges are seen in the fissure for the ligamentum venosum and which continues round the structures in the porta hepatis, is attached to the lesser curvature of the stomach. All other peritoneal edges seen here are attached to the diaphragm. The caudate process connects the caudate lobe to the right lobe. |
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.
Nerve supply
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.
Retroperitoneal space
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.
