Posterior Abdominal Wall, Major Vessels and Nerves

Psoas Major

• Origin: Transverse processes, bodies, and intervertebral discs of T12-L5 vertebrae.
• Insertion: Lesser trochanter of the femur (joining iliacus to form iliopsoas).
• Innervation: Ventral rami of L1-L3 (direct lumbar plexus branches, formed within its substance).
• Action: Hip flexion; trunk flexion when the lower limb is fixed.
• Relations: The lumbar plexus forms within its substance; the ureter runs on its anterior surface; the genitofemoral nerve typically pierces or emerges from its anterior surface; a key retroperitoneal landmark against which many other structures are described (kidneys, ureters, gonadal vessels, colon).

Psoas Minor

• Origin: Bodies of T12-L1 vertebrae.
• Insertion: Pectineal line/iliopubic eminence, via a long, thin tendon.
• Innervation: Ventral ramus of L1.
• Action: Weak trunk flexion; of minor functional significance, and absent in a substantial proportion of individuals (variable, present in roughly 50% or fewer depending on population studied), a common anatomy exam distractor, similar to pyramidalis in the anterior wall.

Iliacus

• Origin: Iliac fossa (internal surface of the ilium).
• Insertion: Lesser trochanter of the femur (joining psoas major).
• Innervation: Femoral nerve (L2-L4).
• Action: Hip flexion (with psoas major, as iliopsoas).
• Relations: Forms the floor of the iliac fossa; femoral nerve emerges between iliacus and psoas major.

Quadratus Lumborum

• Origin: Iliac crest and iliolumbar ligament.
• Insertion: 12th rib and transverse processes of L1-L4 vertebrae.
• Innervation: Ventral rami of T12-L4 (subcostal nerve and upper lumbar plexus branches).
• Action: Lateral flexion of the trunk (same side); fixes the 12th rib during deep inspiration/forced expiration (stabilising the diaphragm's costal origin).
• Relations: Forms part of the posterior abdominal wall floor, related anteriorly to the kidney; the subcostal, iliohypogastric, and ilioinguinal nerves cross its anterior surface obliquely en route to the anterior abdominal wall.

Extent:

• Begins at the aortic hiatus of the diaphragm (T12), descends anterior to the vertebral bodies (slightly left of the midline), terminates by bifurcating into the two common iliac arteries at approximately L4 (the level of the iliac crests/intercristal plane, see Overview section).

Major branches (organised by pattern: unpaired visceral, paired visceral, paired parietal, terminal):

Unpaired (Visceral) Branches

• Coeliac trunk: arises at T12, immediately below the aortic hiatus, gives rise to the left gastric, splenic, and common hepatic arteries, supplying the foregut derivatives (stomach, proximal duodenum, liver, gallbladder, pancreas, spleen).
• Superior mesenteric artery (SMA): arises at L1, supplies the midgut derivatives (distal duodenum to proximal two-thirds of transverse colon).
• Inferior mesenteric artery (IMA): arises at L3, supplies the hindgut derivatives (distal one-third of transverse colon to upper rectum).

Paired (Visceral) Branches

• Inferior phrenic arteries: arise near the aortic hiatus (T12 level), supply the inferior diaphragm, and give rise to the superior suprarenal arteries.
• Middle suprarenal arteries: arise at approximately L1, direct branches to the suprarenal glands.
• Renal arteries: arise at approximately L1-L2, supply the kidneys (right renal artery longer, crossing behind the IVC).
• Gonadal arteries (testicular/ovarian): arise at approximately L2, just below the renal arteries, descending a long retroperitoneal course to the gonads (testis, via the inguinal canal within the spermatic cord; ovary, within the suspensory ligament of the ovary), reflecting the gonads' high embryological origin.

Paired (Parietal) Branches

• Lumbar arteries: typically four pairs, segmental branches supplying the posterior abdominal wall, analogous to the posterior intercostal arteries of the thorax.

Terminal Branches

• Common iliac arteries: the terminal bifurcation at L4, each further dividing into internal and external iliac arteries (supplying the pelvis/perineum and lower limb respectively).

Formation:

• Formed by the union of the two common iliac veins at approximately L5, slightly to the right of the midline (asymmetrically positioned throughout its course, reflecting its embryological development, unlike the more midline-positioned aorta).

Course:

• Ascends along the right side of the vertebral column (to the right of the aorta), passing posterior to the liver (within a groove/tunnel, receiving the hepatic veins directly here), then through the caval opening of the diaphragm at T8, to enter the right atrium.

Tributaries:

• Common iliac veins (formation), lumbar veins (segmental, variable connections with the ascending lumbar veins/azygos system), right gonadal vein (drains directly into the IVC, asymmetric, unlike the left, which drains via the left renal vein), renal veins (left renal vein longer, crossing anterior to the aorta), right suprarenal vein (drains directly into the IVC, unlike the left), inferior phrenic veins, and the hepatic veins (terminal tributaries, just below the diaphragm).
• Notably, the IVC does not receive direct venous drainage from the gut, spleen, or pancreas, this territory drains instead via the portal venous system to the liver first, only reaching the IVC secondarily via the hepatic veins after passing through hepatic sinusoids.

Relations:

• Right side of the aorta throughout its abdominal course; posterior relations include the vertebral bodies and right psoas major; crossed anteriorly by the right gonadal artery, the third part of the duodenum, the head of the pancreas, and the portal vein/bile duct (near the epiploic foramen, forming its posterior boundary).

Portal vein formation:

• Formed by the union of the superior mesenteric vein and the splenic vein, posterior to the neck of the pancreas: the inferior mesenteric vein typically joins the splenic vein just before this confluence (though anatomical variation exists, occasionally joining the SMV or the confluence angle directly).

Tributaries (summary):

• Superior mesenteric vein (midgut territory), splenic vein (spleen, pancreas, and part of the stomach via short gastric veins), inferior mesenteric vein (hindgut territory), left and right gastric veins (draining directly into the portal vein near the porta hepatis), cystic vein (variable, sometimes draining directly into the liver or the portal vein's right branch).
• The portal vein ascends within the hepatoduodenal ligament (with the hepatic artery proper and common bile duct, forming the portal triad) to the porta hepatis, dividing into right and left branches supplying the corresponding functional hepatic lobes.

Portosystemic anastomoses:

• Sites where the portal venous system (draining the gut, spleen, pancreas) communicates with the systemic venous system (draining to the IVC/SVC) via small, normally low-flow anastomotic channels, these become clinically significant collateral pathways, dilating substantially in portal hypertension, when the normal portal-to-hepatic pathway is obstructed and pressure backs up, forcing blood through these alternative routes.

Formation:

• Formed within the substance of psoas major, from the ventral rami of L1-L4 (with a variable contribution from T12, the subcostal nerve).

Iliohypogastric nerve (L1):

• Emerges from the lateral border of psoas major, runs anterior to quadratus lumborum, pierces the internal oblique/transversus abdominis to run between these layers, supplying them along with the skin over the suprapubic/hypogastric region and the lateral gluteal region (via a lateral cutaneous branch).

Ilioinguinal nerve (L1):

• Runs a course just below the iliohypogastric nerve, similarly piercing internal oblique, then traversing the inguinal canal (though entering it partway along its course rather than via the deep ring), exiting via the superficial ring to supply the skin of the upper medial thigh and anterior scrotum/labium majus.

Genitofemoral nerve (L1, L2):

• Pierces the anterior surface of psoas major, descends on its surface, dividing into genital branch (enters the inguinal canal via the deep ring, supplies the cremaster muscle and scrotal/labial skin, mediates the cremasteric reflex) and femoral branch (passes beneath the inguinal ligament with the femoral vessels, supplies a small area of skin over the femoral triangle).

Lateral femoral cutaneous nerve (L2, L3):

• Emerges from the lateral border of psoas major, crosses iliacus, passes close to the ASIS beneath/through the inguinal ligament, purely sensory, supplying the anterolateral thigh.

Femoral nerve (L2-L4):

• The largest branch of the lumbar plexus, emerges from the lateral border of psoas major, descends in the groove between psoas major and iliacus, passes beneath the inguinal ligament (lateral to the femoral sheath) to enter the thigh; full detail in Lower Limb content.

Obturator nerve (L2-L4):

• Descends within psoas major, emerges from its medial border, crosses the pelvic brim, exits via the obturator canal to reach the medial thigh; full detail in Lower Limb content.

• The lumbar sympathetic trunks continue directly from the thoracic sympathetic chains (passing posterior to the medial arcuate ligament), running along the anterolateral surface of the lumbar vertebral bodies, medial to psoas major, giving grey/white rami communicantes to the lumbar spinal nerves and contributing lumbar splanchnic nerves to the prevertebral plexuses.
• Thoracic splanchnic nerves (greater, lesser, least; see Thorax content) synapse predominantly in the coeliac, aorticorenal, and superior mesenteric prevertebral ganglia, providing sympathetic supply to the foregut and midgut.
• Lumbar splanchnic nerves contribute additional sympathetic fibres to the inferior mesenteric and hypogastric (superior/inferior) plexuses, providing sympathetic supply to the hindgut and pelvic viscera.

• Autonomic nerve networks surrounding the origins of the major unpaired aortic branches, formed by the convergence of sympathetic (splanchnic nerve) and parasympathetic (vagal, or pelvic splanchnic for the hindgut/pelvis) fibres, distributing along the corresponding arteries to reach their target organs.
• Coeliac plexus (around the coeliac trunk origin), the largest prevertebral plexus, receiving the greater and lesser splanchnic nerves and vagal fibres, distributing to foregut structures; a common target for coeliac plexus block/neurolysis in the management of severe, refractory upper abdominal pain (e.g., pancreatic cancer pain), given its accessibility and the concentration of foregut visceral afferent fibres passing through it.
• Superior mesenteric plexus (around the SMA origin), continuous with the coeliac plexus, distributing to midgut structures.
• Inferior mesenteric plexus (around the IMA origin), receives lumbar splanchnic nerves, distributing to hindgut structures.
• Superior and inferior hypogastric plexuses: continue inferiorly into the pelvis, providing autonomic supply to pelvic viscera (covered in dedicated pelvic anatomy content).