The peritoneum is a serous membrane with parietal and visceral layers, which encloses a space, the peritoneal cavity. The parietal peritoneum lines the walls of the abdomen and pelvis, while the visceral layer covers many of the abdominal and pelvic organs. In the male, the peritoneal cavity is a closed sac, but in the female, it communicates with the interior of the uterus and vagina via a microscopic channel through each uterine tube. Normally, the peritoneal cavity contains only a few millilitres (mL) of serous fluid, but in certain diseases, serous fluid can accumulate (ascites), sometimes up to several litres.
The peritoneal cavity comprises the greater and lesser sacs. The greater sac is very extensive and can be traced from the diaphragm above into the pelvic cavity below. The omental bursa (lesser sac) is located in the upper part of the abdomen behind the stomach and communicates with the greater sac through a narrow opening, the omental (epiploic) foramen.
The parietal peritoneum is applied to the inner aspect of the abdominal and pelvic walls (Fig. 4.30) and continues superiorly across most of the undersurface of the diaphragm. The peritoneum lining the anterior abdominal wall is raised into several folds or ridges. Below the umbilicus, the median umbilical
ligament often raises a midline ridge (median umbilical fold), on each side of which the occluded part of the umbilical artery (medial umbilical ligament) may produce a further peritoneal fold (medial umbilical fold) (Fig. 4.31). Above the umbilicus, the round ligament of the liver (Fig. 4.30) is contained in a large fold of peritoneum, the falciform ligament, which attaches the liver to the anterior abdominal wall and the diaphragm (Fig. 4.57).
Posteriorly, the peritoneum covers several organs that lie on the muscles of the posterior abdominal wall (Fig. 4.32). These retroperitoneal organs include the ascending and descending parts of the colon, the kidneys, ureters and suprarenal glands, and most of the pancreas and duodenum. Also lying behind the peritoneum are the aorta and its branches and the inferior vena cava and its tributaries.
The parietal peritoneum of the abdominal wall is innervated by the lower thoracic and first lumbar nerves. Inflammation spreading from an organ such as the appendix to this peritoneum causes well-localized pain and tenderness and rigidity of the abdominal muscles. The lower thoracic nerves also innervate the peritoneum covering the periphery of the diaphragm. Inflammation of this peritoneum consequently gives rise to pain in the lower thoracic wall and abdominal wall. By contrast, the peritoneum on the central part of the diaphragm receives sensory branches from the phrenic nerves (C3, C4 & C5) and irritation here may produce pain referred to the region of the shoulder (the fourth cervical dermatome; Fig. 3.6).
Most of the abdominal organs have a covering of visceral peritoneum and are suspended within the abdominopelvic cavity by mesenteries. Although organs possessing mesenteries are often termed intraperitoneal, they do not lie within the peritoneal cavity but merely project into it. Mesenteries consist of double layers of peritoneum containing the vessels and nerves of the intraperitoneal organs. Typically, a mesentery attaches to the posterior abdominal wall, where its peritoneal layers are continuous with the parietal peritoneum. Examples include the mesentery of the small intestine (Fig. 4.35) and the transverse mesocolon.
The mesenteries of the stomach (the omenta) do not attach to the abdominal wall but to other organs. The lesser curvature of the stomach is connected to the liver by the lesser omentum, while the upper part of the greater curvature is attached to the spleen by the gastrosplenic ligament (Figs 4.37 & 4.38). The major portion of the greater curvature gives attachment to the greater omentum.
The greater omentum is an apron-like fold of peritoneum with a free lower border (Fig. 4.33). Hanging behind the anterior abdominal wall and in front of most of the small intestine, this omentum is usually a conspicuous feature when the peritoneal cavity is opened. Superiorly, it attaches to both the transverse colon (Fig. 4.34) and the greater curvature of the stomach, enclosing the inferior part of the omental bursa (see below). The free inferior border of the omentum ascends on the right as far as the first part of the duodenum, while on the left it merges with the gastrosplenic ligament. The position of the greater omentum is influenced by previous episodes of intra-abdominal disease because it tends to adhere to sites of inflammation such as the appendix or gall bladder.
The transverse mesocolon (Fig. 4.35) has a long horizontal root, attached across the posterior aspect of the abdomen, principally to the pancreas. This mesocolon slopes downwards and forwards into the greater sac, dividing it into supracolic and infracolic compartments. Along its lower margin, close to the anterior abdominal wall, runs the transverse colon.
Infracolic compartment of the greater sac
This compartment lies below and behind the transverse mesocolon and is usually covered anteriorly by the greater omentum. The infracolic compartment consists of right and left spaces separated by the mesentery of the small intestine (Fig. 4.35). The root of this mesentery begins to the left of the midline near the transverse mesocolon and slopes downwards into the right iliac fossa. The mesentery is extensively folded and is attached to the jejunum and ileum. The left infracolic space communicates directly with the cavity of the pelvis. By contrast, the right infracolic space is confined inferiorly by the attachment of the lower part of the mesentery.
Behind the peritoneum on either side of the infracolic compartment lie the ascending and descending parts of the colon. Lateral to these are grooves lined by peritoneum, the right and left paracolic gutters (Fig. 4.68).
Another mesentery, the sigmoid mesocolon, lies in the left lower part of the infracolic compartment. Its root is shaped like an inverted ‘V’, with its apex overlying the bifurcation of the left common iliac vessels and the left ureter. Behind the sigmoid mesocolon lies the intersigmoid recess, which ends blindly at the apex of the ‘V’ but is continuous inferiorly with the pelvic cavity.
Small folds of peritoneum may produce additional peritoneal recesses (or fossae) near the ascending duodenum (paraduodenal recesses) and the caecum (retrocae- cal and ileocaecal recesses). A loop of bowel can become trapped in a peritoneal recess, producing an internal hernia, which may lead to intestinal obstruction.
The autonomic nerves that supply the abdominal organs also innervate the visceral peritoneum surrounding the organs. Pain conveyed by these nerves tends to be deeply felt and poorly localized.
Supracolic compartment of the greater sac
The supracolic compartment lies above and in front of the transverse mesocolon (Fig. 4.36). Its superior part intervenes between the diaphragm and the liver and is divided by the falciform ligament into two subphrenic spaces. The compartment includes the deep recess between the right lobe of the liver and the right kidney (the hepatorenal recess) and extends across the midline below the left lobe of the liver and in front of the stomach. Infection within the abdomen or pelvis can spread through the peritoneal cavity and may accumulate near the liver, producing an abscess. Abscesses between the diaphragm and the liver are termed subphrenic and those below the liver subhepatic.
Omental bursa (lesser sac)
The omental bursa is the small part of the peritoneal cavity behind the stomach (Figs 4.36 & 4.38). It communicates with the greater sac through a narrow opening, known as the omental or epiploic foramen, which lies between the first part of the duodenum and the visceral surface of the liver (Fig. 4.37).
The omental bursa is isolated from the greater sac by the stomach and several peritoneal folds. One of these folds, the lesser omentum, connects the lesser curvature of the stomach to the posterior surface of the liver (Fig. 4.37). Two further folds, the gastrosplenic and splenorenal (lienorenal) ligaments, attach the spleen to the greater curvature of the stomach and the left kidney, respectively (Fig. 4.38).
The omental bursa extends upwards behind the stomach and the caudate lobe of the liver as far as the diaphragm. On the left, it continues to the hilum of the spleen, terminating between the gastrosplenic and splenorenal ligaments. Inferiorly, the omental bursa usually extends a short distance below the greater curvature of the stomach between the gastric and colic attachments of the greater omentum. To the right, it communicates through the epiploic foramen with the hepatorenal recess of the greater sac.