Anterior Abdominal Wall

Skin.

• Superficial fascia: Below the umbilicus, splits into two recognisable layers:
• Camper's fascia: superficial, fatty layer, continuous with the superficial fascia elsewhere in the body (including into the thigh and perineum).
• Scarpa's fascia: deeper, membranous layer, continuous with the deep perineal (Colles') fascia and the fascia lata of the thigh (fusing with it just below the inguinal ligament), this fusion line is clinically important, since it means extravasated fluid/urine deep to Scarpa's fascia (e.g., from a ruptured urethra) is contained within the abdominal wall/perineum and cannot spread into the thigh.
• Deep fascia: Thin layer investing the muscles, not usually a distinct surgical entity in the abdomen (unlike the limbs).
• Muscular layer: Three flat muscles (external oblique, internal oblique, transversus abdominis) laterally, plus rectus abdominis (and variably pyramidalis) in the midline.
• Transversalis fascia: Deep to the muscular layer, a thin fascial sheet continuous with the fascia lining the entire abdominal cavity; contributes to the deep inguinal ring and femoral sheath.
• Extraperitoneal fat and parietal peritoneum: innermost layers.

External Oblique

• Origin: Outer surfaces of ribs 5–12 (interdigitating with serratus anterior and latissimus dorsi).
• Insertion: Via a broad aponeurosis, to the linea alba (anterior rectus sheath contribution), pubic tubercle and crest, and iliac crest; the free inferior edge of the aponeurosis folds back on itself to form the inguinal ligament (ASIS to pubic tubercle).
• Innervation: Anterior rami of T7–T11 intercostal nerves, subcostal nerve (T12).
• Action: Bilaterally, flexes the trunk, compresses abdominal contents (raises intra-abdominal pressure); unilaterally, lateral flexion (same side) and contralateral rotation of the trunk.

Internal Oblique

• Origin: Thoracolumbar fascia, iliac crest, lateral two-thirds of the inguinal ligament.
• Insertion: Via an aponeurosis splitting around rectus abdominis (contributing to both anterior and posterior rectus sheath above the arcuate line), to ribs 10–12 and costal cartilages, linea alba; lower fibres arch over the spermatic cord/round ligament, contributing to the conjoint tendon (with transversus abdominis) and, in males, giving rise to the cremaster muscle.
• Innervation: Anterior rami of T7–T11 intercostal nerves, subcostal nerve (T12), and the iliohypogastric and ilioinguinal nerves (L1).
• Action: Bilaterally, flexes the trunk, compresses abdominal contents; unilaterally, lateral flexion (same side) and ipsilateral rotation of the trunk (note: opposite rotation direction to external oblique on the same side, since the two muscles' fibres run perpendicular to each other).

Transversus Abdominis

• Origin: Inner surfaces of costal cartilages 7–12 (interdigitating with the diaphragm's costal origin), thoracolumbar fascia, iliac crest, lateral third of the inguinal ligament.
• Insertion: Via an aponeurosis, to the linea alba (contributing to the posterior rectus sheath below the arcuate line, and the anterior sheath above it, alongside internal oblique's posterior lamina), pubic crest; lower fibres contribute to the conjoint tendon with internal oblique.
• Innervation: Anterior rami of T7–T11 intercostal nerves, subcostal nerve (T12), iliohypogastric and ilioinguinal nerves (L1).
• Action: Compresses abdominal contents (raises intra-abdominal pressure); the deepest and most purely "corset-like" of the three flat muscles, with minimal role in trunk flexion/rotation given its transverse fibre orientation.

Rectus Abdominis

• Origin: Pubic crest and pubic symphysis.
• Insertion: Xiphoid process and costal cartilages 5–7.
• Innervation: Anterior rami of T7–T11 intercostal nerves and the subcostal nerve (T12), these nerves pierce the posterior rectus sheath to enter and supply the muscle.
• Action: Flexes the trunk (lumbar spine); compresses abdominal contents; stabilises the pelvis.
• Distinguishing features: Paired, strap-like muscle running the full vertical length of the anterior abdominal wall, divided into segments by tendinous intersections (typically 3, at the level of the umbilicus, xiphoid, and midway between), these intersections are firmly adherent to the anterior rectus sheath only (not the posterior sheath), producing the visible "six-pack" segmentation and limiting the spread of a rectus sheath haematoma across intersections anteriorly, while allowing it to spread more freely along the posterior surface.

Pyramidalis

• Origin: Pubic crest and pubic symphysis (anterior to rectus abdominis's lower attachment).
• Insertion: Linea alba (variable height, tapers superiorly).
• Innervation: Subcostal nerve (T12).
• Action: Tenses the linea alba; a small, functionally minor muscle, absent in a proportion of individuals (variable, present in roughly 80% but with significant population variation), of minimal clinical or functional significance but a common anatomy exam distractor.

• A fibrous compartment enclosing rectus abdominis (and pyramidalis inferiorly), formed by the aponeuroses of the three flat muscles splitting and reuniting around the rectus muscle.
• Above the arcuate line:
• Anterior wall: External oblique aponeurosis + anterior lamina of internal oblique aponeurosis.
• Posterior wall: Posterior lamina of internal oblique aponeurosis + transversus abdominis aponeurosis.
• Below the arcuate line:
• Anterior wall: All three aponeuroses (external oblique, internal oblique, transversus abdominis) pass entirely anterior to rectus abdominis.
• Posterior wall: Only transversalis fascia remains posterior to the muscle (no aponeurotic contribution), the rectus muscle here lies in direct contact with transversalis fascia and the peritoneum, separated only by this thin fascial layer.

• The point (typically midway between the umbilicus and pubic symphysis, though the exact level varies) where all three flat muscle aponeuroses shift to pass entirely anterior to rectus abdominis, marking the transition.
• Clinically significant as a zone of relative weakness in the posterior rectus sheath, and as a landmark in extraperitoneal/laparoscopic surgical approaches (e.g., extraperitoneal hernia repair dissection planes are defined relative to this line).
• Below this line, the inferior epigastric vessels enter the rectus sheath (having arisen from the external iliac vessels), ascending on the deep/posterior surface of rectus abdominis to anastomose with the superior epigastric vessels (from the internal thoracic artery), an important anastomotic connection between the subclavian and external iliac arterial systems.

• A fibrous midline raphe formed by the fusion/decussation of the aponeuroses of the flat muscles from both sides, extending from the xiphoid process to the pubic symphysis.
• Relatively avascular and nerve-poor compared to the surrounding muscle bellies, making it a common site for midline surgical incisions (minimal bleeding, no muscle division required) and for umbilical/epigastric hernias (a natural point of relative weakness, particularly at the umbilicus itself).

• The curved lateral border of rectus abdominis, marking the line where the aponeuroses of the flat muscles fuse before contributing to the rectus sheath.
• Surface landmark; relevant to the Spigelian hernia, a rare hernia occurring through the aponeurosis along this line, typically at or below the arcuate line where the posterior wall is weaker.

• Scar remnant of the fetal umbilical cord attachment, located approximately at the L3/L4 vertebral level (variable with body habitus).
• Marks the junction of the embryological midgut's vascular supply reference point (vitelline duct remnant site) and is a common site for umbilical hernias, given the natural fascial defect present in fetal life that must close postnatally.
• Dermatome T10: a key surface landmark for spinal level localisation (see Thorax content), and the classic site of early, poorly localised periumbilical pain in appendicitis, reflecting the visceral (midgut) referred pain pattern before localisation to the right iliac fossa.

• Nerves: Anterior rami of the lower six thoracic intercostal nerves (T7–T11) and the subcostal nerve (T12) provide the primary segmental motor and sensory supply to the anterolateral abdominal wall muscles and overlying skin, running between the internal oblique and transversus abdominis layers (the same neurovascular plane as the intercostal nerves in the thoracic wall, a direct continuation); the iliohypogastric and ilioinguinal nerves (L1, from the lumbar plexus) supply the lowest part of the wall, near the inguinal region.
• Arteries: Superior epigastric artery (terminal branch of the internal thoracic artery, supplying the upper rectus sheath contents) anastomoses with the inferior epigastric artery (branch of the external iliac artery, supplying the lower rectus sheath contents) within the rectus sheath, a clinically important collateral pathway between the subclavian and external iliac arterial systems. Additional supply from the lower posterior intercostal arteries, subcostal artery, and the deep circumflex iliac artery (branch of the external iliac artery) laterally.
• Veins: Generally accompany the arteries; superficial venous drainage includes the thoraco-epigastric veins, connecting the superficial epigastric vein (draining to the femoral vein/great saphenous system) and the lateral thoracic vein (draining to the axillary vein), another important collateral pathway, this time between the SVC (via axillary) and IVC (via femoral) systems, relevant to portal hypertension collateral flow patterns via the paraumbilical veins as well (see later sections).

Surgical incisions:

• Midline (linea alba) incision: avascular, no muscle division, rapid access, but weaker healing (relatively poor blood supply of the linea alba itself) with a somewhat higher incisional hernia risk than muscle-splitting incisions.
• Paramedian incision: through the rectus sheath, retracting (rather than cutting) rectus abdominis; stronger closure but less commonly used now given longer operative time.
• Pfannenstiel incision: transverse, low suprapubic, through the anterior rectus sheath with retraction of the recti (common in gynaecological/obstetric surgery, including caesarean section), good cosmesis and strength, lower incisional hernia risk.
• McBurney's (gridiron) incision: oblique, at McBurney's point (one-third of the way from the ASIS to the umbilicus), splits (rather than cuts) the external oblique, internal oblique, and transversus abdominis muscle fibres in the direction of their fibres, classic appendicectomy approach, minimising muscle damage.
• Kocher's (subcostal) incision: below and parallel to the right costal margin, classic approach for open cholecystectomy.

Rectus sheath haematoma:

• Bleeding into the rectus sheath, typically from rupture of the inferior epigastric artery/vein or a branch, or direct rectus muscle tear (trauma, sudden forceful contraction, anticoagulation).
• Above the arcuate line: Haematoma is typically confined by the strong posterior sheath and limited by the tendinous intersections anteriorly, tending to remain localised.
• Below the arcuate line: Lacking a strong posterior aponeurotic sheath (only transversalis fascia present), a haematoma here can spread more extensively, tracking into the pelvis/extraperitoneal space, and can mimic an acute abdomen clinically.

Abdominal wall hernias:

• Umbilical hernia: through the natural umbilical fascial defect; common in infants (often resolves spontaneously) and in adults with raised intra-abdominal pressure (obesity, pregnancy, ascites).
• Epigastric hernia: through the linea alba, above the umbilicus, where small defects allow herniation of extraperitoneal fat (and occasionally bowel).
• Incisional hernia: through a previous surgical incision site, related to inadequate healing/wound infection/increased intra-abdominal pressure post-operatively; more common with midline incisions than muscle-splitting or transverse incisions, given the comparatively poorer blood supply and tensile strength of the linea alba.
• Spigelian hernia: through the linea semilunaris, typically at or below the arcuate line (relatively rare but classically tested).