Describe the peritoneum with its folds, the lesser sac and the peritoneal spaces. Add a note on its nerve supply and applied anatomy.

Definition and Arrangement

• The cavity is a potential space with about 50 mL of serous fluid
• Closed in the male; in the female it communicates with the exterior through the uterine tubes, uterus and vagina — a route for ascending infection

Peritoneal Folds

• A four-layered apron hanging from the greater curvature over the intestines
• Functions — stores fat; and it is the "policeman of the abdomen", migrating to wall off areas of inflammation and perforation
• It limits peritonitis, and is why a perforated appendix in a child, whose omentum is short, causes more generalised peritonitis than in an adult

The Lesser Sac and Epiploic Foramen

Peritoneal Recesses and Spaces

• Subphrenic spaces — right and left, between the diaphragm and the liver
• Subhepatic space — including the hepatorenal pouch (of morison), the lowest part of the peritoneal cavity in the supine position
• Paracolic gutters — right and left, alongside the ascending and descending colon; channels for the spread of pus
• Rectovesical pouch in the male and rectouterine pouch (of douglas) in the female — the lowest part in the erect position
• Duodenal, caecal and intersigmoid recesses — potential sites of internal hernia

Nerve Supply

• Foregut pain → epigastrium; midgut → umbilicus; hindgut → hypogastrium

Development of the Peritoneal Cavity

• The rotation explains why the left vagus becomes anterior and the right posterior on the stomach
• It also explains the lesser sac, which is simply the space trapped behind the rotating stomach
• Secondary retroperitoneal organs — the duodenum, pancreas and ascending and descending colon — became fixed when their mesenteries fused with the posterior wall

Applied Aspects

• The shifting pain of appendicitis is the classical illustration — it begins as dull periumbilical pain (visceral, midgut), and shifts to the sharp right iliac fossa pain when the inflamed appendix irritates the parietal peritoneum
• Free fluid or blood collects in Morison's pouch in the supine patient — the first place examined in a fast scan after abdominal trauma
• Subphrenic abscess — causes shoulder tip pain, a sympathetic pleural effusion, and swinging fever
• Ascites is detected clinically by shifting dullness and a fluid thrill; paracentesis is performed lateral to the rectus sheath to avoid the inferior epigastric artery
• Peritoneal dialysis exploits the very large surface area and rich vascularity of the membrane
• Adhesions after surgery are the commonest cause of small bowel obstruction in the developed world; in India, tuberculosis and hernia remain important
• Abdominal tuberculosis characteristically produces peritoneal studding, ascites and matted bowel loops, and remains an important differential for an abdominal mass in India
• Peritoneal seeding of ovarian and gastric carcinoma follows the flow of peritoneal fluid, which is why deposits collect in the pouch of Douglas and in the right paracolic gutter
• Internal hernia through a paraduodenal or intersigmoid recess is rare but causes obstruction and is easily missed
• Pneumoperitoneum for laparoscopy uses carbon dioxide because it is rapidly absorbed; residual gas irritates the diaphragm and causes shoulder tip pain afterward
• The peritoneum absorbs rapidly from the subphrenic region, which is why toxins from generalised peritonitis reach the circulation quickly and the patient deteriorates fast
• Rectal examination reaches the pouch of Douglas, where pelvic collections and peritoneal deposits can be felt — the reason it remains part of the examination of an acute abdomen
• Peritoneal dialysis catheters are placed in the pelvis, the most dependent part, so that fluid drains completely