15.2 Abdomen: Abdominal Wall & Inguinal Region, Peritoneum, Viscera, Diaphragm & Posterior Abdominal Wall

Why this area matters

The TOS lists:

• the abdominal cavity and anterolateral abdominal wall (fascia, muscles, nerves, vessels, inguinal region);
• the peritoneum and peritoneal cavity;
• the abdominal viscera (esophagus, stomach and duodenum, small and large intestine, rectum, anus, spleen, pancreas, liver, biliary ducts, gallbladder, kidneys, ureters, adrenal glands);
• the thoracic diaphragm;
• the posterior abdominal wall.

Surgical abdomen, trauma and hernia items in Surgery rest on this anatomy.

Abdominal Wall Architecture, Rectus Sheath & Inguinal Canal Anatomy

Anterior Abdominal Wall Layers (Superficial to Deep)

1. Skin
2. Superficial fascia: Camper's fascia (fatty outer layer) and Scarpa's fascia (membranous inner layer; continuous with Colles' fascia in the perineum)
3. External oblique muscle and aponeurosis
4. Internal oblique muscle and aponeurosis
5. Transversus abdominis muscle and aponeurosis
6. Transversalis fascia
7. Extraperitoneal preperitoneal fat
8. Parietal peritoneum

The Rectus Sheath Above vs. Below the Arcuate Line

The arcuate line (linea semicircularis) is a horizontal crescentic demarcation located midway between the umbilicus and the pubic symphysis:

• Above the Arcuate Line:
  • Anterior layer of rectus sheath: External oblique aponeurosis + the anterior leaf of the split internal oblique aponeurosis.
  • Posterior layer of rectus sheath: Posterior leaf of the split internal oblique aponeurosis + transversus abdominis aponeurosis + transversalis fascia.
• Below the Arcuate Line:
  • Anterior layer of rectus sheath: All three aponeuroses (external oblique, internal oblique, and transversus abdominis) pass completely anterior to the rectus abdominis muscle.
  • Posterior layer of rectus sheath: Absent. The rectus abdominis muscle rests directly against the transversalis fascia and underlying parietal peritoneum.
  • Surgical consequence: The inferior epigastric vessels (branches of the external iliac) pierce the transversalis fascia at the arcuate line to enter the rectus abdominis. Sudden severe contraction (vigorous coughing, trauma) can shear these vessels beneath the absent posterior sheath, causing a massive rectus sheath hematoma.

The Inguinal Canal: Boundaries & Contents

The inguinal canal is an oblique $4\text{ cm}$ intramuscular passage running downward and medially parallel to the medial half of the inguinal ligament:

• Floor: Inguinal ligament of Poupart (curled inferior border of external oblique aponeurosis) and the lacunar ligament of Gimbernat medially.
• Roof: Arched musculoaponeurotic fibers of the internal oblique and transversus abdominis muscles.
• Anterior Wall: External oblique aponeurosis throughout (reinforced laterally by internal oblique fibers).
• Posterior Wall: Transversalis fascia throughout (reinforced medially by the conjoint tendon / falx inguinalis, formed by fusion of internal oblique and transversus abdominis aponeuroses).
• Deep (Internal) Inguinal Ring: An oval evagination of the transversalis fascia, positioned approximately $1.5\text{ cm}$ superior to the midpoint of the inguinal ligament, situated immediately LATERAL to the inferior epigastric vessels.
• Superficial (External) Inguinal Ring: A triangular defect in the external oblique aponeurosis, situated medially above the pubic tubercle.

Contents of the Spermatic Cord

The spermatic cord in males transmits structures to and from the testis, enveloped by three concentric fascial layers:

1. External spermatic fascia: Derived from the external oblique aponeurosis.
2. Cremasteric muscle and fascia: Derived from the internal oblique muscle and aponeurosis; innervated by the genital branch of the genitofemoral nerve ($L_1\text{--}L_2$, mediating the cremasteric reflex: stroking the superior medial thigh causes testicular elevation).
3. Internal spermatic fascia: Derived from the transversalis fascia. (Note: The transversus abdominis muscle does not contribute a fascial tunic to the spermatic cord).

• Internal Neurovascular Contents (The Rule of 3s):
  • 3 Arteries: Testicular artery (from abdominal aorta), Cremasteric artery (from inferior epigastric), Artery of the ductus deferens (from inferior vesical).
  • 3 Nerves: Genital branch of genitofemoral nerve, sympathetic autonomic plexus, visceral sensory fibers. (Note: The ilioinguinal nerve travels within the inguinal canal on the anterior surface of the spermatic cord, exiting the superficial ring, but is NOT encased within the internal fascial layers of the cord).
  • 3 Other structures: Ductus (vas) deferens, Pampiniform venous plexus (coalesces into testicular veins; varicocele predominantly affects the left side due to perpendicular insertion into the left renal vein), testicular lymphatics (drain to para-aortic / lumbar lymph nodes, whereas scrotal skin drains to superficial inguinal lymph nodes).

Comparative Surgical Anatomy: Groin Hernias

• Hesselbach's Inguinal Triangle Boundaries:
  • Inferior boundary: Inguinal ligament (Poupart's).
  • Lateral boundary: Inferior epigastric vessels.
  • Medial boundary: Lateral border of the rectus abdominis muscle.

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Peritoneal Cavity, Pringle Maneuver, Mesenteric Watersheds & Retroperitoneal Viscera

The Epiploic Foramen of Winslow & The Pringle Maneuver

The lesser sac (omental bursa) is an extensive peritoneal space situated posterior to the stomach and lesser omentum. It communicates with the greater peritoneal sac solely through a narrow slit-like opening: the Epiploic Foramen of Winslow:

• Anterior boundary: The hepatoduodenal ligament (free edge of the lesser omentum) containing the portal triad:
  • Common bile duct: Positioned anterolaterally.
  • Proper hepatic artery: Positioned anteromedially.
  • Hepatic portal vein: Positioned posteriorly.
• Posterior boundary: The Inferior Vena Cava (IVC) and the right crus of the diaphragm, covered by parietal peritoneum.
• Superior boundary: The caudate lobe of the liver.
• Inferior boundary: The first part of the duodenum and horizontal portal vein.

The Pringle Maneuver: In life-threatening hepatic trauma or elective liver resection, uncontrollable parenchymal bleeding is managed by compressing the hepatoduodenal ligament between the surgeon's thumb and index finger (or an atraumatic vascular clamp) through the foramen of Winslow. This occludes the proper hepatic artery and portal vein (total hepatic inflow occlusion). If massive hemorrhage persists despite a correctly applied Pringle clamp, the bleeding originates from the retrohepatic Inferior Vena Cava or major hepatic veins.

Mesenteric Vascular Watershed Zones

• Celiac Trunk ($T_{12}$): Perfuses the foregut (distal esophagus to mid-duodenum at ampulla of Vater).
• Superior Mesenteric Artery ($L_1$): Perfuses the midgut (ampulla of Vater to proximal two-thirds of transverse colon).
• Inferior Mesenteric Artery ($L_3$): Perfuses the hindgut (distal one-third of transverse colon to upper rectum).
• Colonic Arterial Anastomoses: Formed by the continuous Marginal Artery of Drummond and the central Arc of Riolan (meandering mesenteric artery) linking the middle colic (SMA) and left colic (IMA).
• Vulnerable Watershed Ischemic Zones:
  1. Griffith's Point (Splenic Flexure): The junction between terminal branches of the middle colic artery (SMA) and ascending branch of the left colic artery (IMA). This is the most vulnerable site in the entire gastrointestinal tract to ischemic colitis during systemic hypotension, cardiogenic shock, or aortic clamping.
  2. Sudeck's Point (Rectosigmoid Junction): The critical watershed between the lowest sigmoid branch of the IMA and the superior rectal artery / middle rectal collaterals from the internal iliac artery.

Retroperitoneal Viscera: SAD PUCKER

Organs positioned behind the parietal peritoneum are classified as primary or secondary retroperitoneal:

• S: Suprarenal (adrenal) glands (primary)
• A: Aorta and Inferior Vena Cava (primary)
• D: Duodenum (2nd, 3rd, and 4th parts; secondary; the 1st part is intraperitoneal)
• P: Pancreas (head, neck, body; secondary; the tail is intraperitoneal within the splenorenal ligament)
• U: Ureters (primary)
• C: Colon (ascending and descending colons; secondary; transverse and sigmoid colons are intraperitoneal)
• K: Kidneys (primary)
• E: Esophagus (distal abdominal segment; primary)
• R: Rectum (upper third is retroperitoneal, middle third is retro/subperitoneal, lower third is entirely extraperitoneal)

The thoracic diaphragm

Memory aid: "I 8 10 Eggs At 12".

• Nerve supply: motor from the phrenic nerve (C3, C4, C5). Sensory from the phrenic nerve centrally, which is why gallbladder or subphrenic irritation refers pain to the shoulder, and from the lower intercostal nerves peripherally.
• Hernias:
  • sliding hiatal hernia (the most common type; the gastroesophageal junction moves up);
  • paraesophageal hernia (the fundus herniates; risk of strangulation);
  • congenital Bochdalek hernia (posterolateral, usually on the left).

The abdominal viscera: key relations and blood supply

• Stomach:
  • Lesser curvature: left gastric artery (from the celiac trunk) and right gastric artery (from the hepatic artery).
  • Greater curvature: right gastroepiploic (from the gastroduodenal artery) and left gastroepiploic and short gastric arteries (from the splenic artery).
  • A posterior duodenal ulcer can erode the gastroduodenal artery and cause massive bleeding. A posterior gastric ulcer can erode the splenic artery.
• Duodenum: four parts, C-shaped around the pancreatic head. The second (descending) part receives the bile and pancreatic ducts at the major duodenal papilla (ampulla of Vater), which marks the boundary between foregut and midgut.
• Liver:
  • Divided into right and left functional lobes by the plane of the gallbladder and IVC (Cantlie line), and into eight Couinaud segments based on portal and hepatic venous branching, which guides resection.
  • The falciform ligament carries the ligamentum teres; the hepatoduodenal ligament carries the portal triad.
  • About 75% of hepatic blood comes from the portal vein and 25% from the hepatic artery.
• Biliary tree and gallbladder:
  • The right and left hepatic ducts form the common hepatic duct, which joins the cystic duct to form the common bile duct. This runs behind the pancreatic head and joins the pancreatic duct at the ampulla, guarded by the sphincter of Oddi.
  • The cystic artery usually arises from the right hepatic artery within Calot's triangle (cystic duct, common hepatic duct and liver edge).
  • The fundus lies at the tip of the ninth costal cartilage on the midclavicular line, which is the site of the Murphy sign.
• Pancreas: head, uncinate process, neck, body and tail; the tail lies in the splenorenal ligament near the splenic hilum. The splenic vein runs behind the body, and the SMA and SMV pass behind the neck and in front of the uncinate process.
• Spleen: lies under ribs 9 to 11 on the left. Rupture after trauma may cause referred left shoulder pain (Kehr sign).
• Kidneys, ureters and adrenals:
  • The kidneys lie at about T12 to L3; the right is lower because of the liver.
  • At the hilum, the order from front to back is renal vein, renal artery, renal pelvis.
  • The left renal vein crosses in front of the aorta, below the SMA, where it can be compressed (nutcracker syndrome). It receives the left gonadal and left adrenal veins, which explains left-sided varicocele with left renal tumors.
  • The ureter narrows at the ureteropelvic junction, where it crosses the iliac vessels at the pelvic brim, and at the ureterovesical junction. These three sites are where stones lodge.
  • The adrenals sit on the upper poles. The right adrenal vein drains directly into the IVC; the left drains into the left renal vein.

Posterior abdominal wall

• Muscles:
  • Psoas major: hip flexor. An inflamed retrocecal appendix lying on it causes the psoas sign, and a tuberculous psoas abscess can track down to the groin.
  • Quadratus lumborum and iliacus.
• Lumbar plexus (L1 to L4): iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous (compressed in meralgia paresthetica), femoral and obturator nerves.
• Abdominal aorta: enters at T12 and bifurcates at L4, at about the level of the iliac crests and the umbilicus. Its branches are:
  • unpaired anterior: celiac (T12), SMA (L1), IMA (L3);
  • paired: inferior phrenic, middle suprarenal, renal (L1 to L2), gonadal (L2);
  • lumbar arteries and the median sacral artery.
  • Abdominal aortic aneurysms usually arise below the renal arteries.
• Inferior vena cava: formed at L5 by the common iliac veins, and runs to the right of the aorta.

Portal venous system and portosystemic anastomoses

The portal vein forms behind the pancreatic neck from the SMV and splenic vein; the IMV usually drains into the splenic vein. In portal hypertension, blood is diverted into systemic veins at:

Referred pain from abdominal organs

• Foregut structures (stomach, duodenum, liver, gallbladder, pancreas) refer pain to the epigastrium.
• Midgut structures (small bowel, appendix, cecum) refer pain to the periumbilical region.
• Hindgut structures refer pain to the suprapubic region.

This explains why appendicitis begins with periumbilical pain. The pain moves to the right iliac fossa (McBurney point, one-third of the way from the anterior superior iliac spine to the umbilicus) once the parietal peritoneum is inflamed.