2.4 The Anterior Abdominal Wall

Layers of the Anterior Abdominal Wall

The anterior abdominal wall is built from superficial to deep by a predictable sequence of layers. Knowing this sequence is essential for surgical approach, hernia repair, and abdominal puncture.

Superficial Layers

1. Skin — thin and mobile over the underlying muscles, except at the umbilicus where it is tethered.
2. Superficial fascia — divided into two layers:
   • Camper's fascia — the outer, fatty layer continuous with the superficial fat of the rest of the body; thickness varies with nutritional status.
   • Scarpa's fascia — the deeper, membranous layer of dense collagenous tissue; it continues into the perineum as Colles' fascia and into the thigh just below the inguinal ligament, where it fuses with the deep fascia (fascia lata). This continuity explains why extravasated urine from a membranous urethral injury spreads under Scarpa's fascia into the lower abdominal wall but is blocked from the thigh.

The Three Flat Muscles

1. External oblique — the most superficial of the flat muscles. Its fibers run inferomedially ("hands in pockets"). Its aponeurosis contributes to the anterior rectus sheath and forms the inguinal ligament (the rolled-under inferior edge of the external oblique aponeurosis, running from the ASIS to the pubic tubercle).
2. Internal oblique — deep to the external oblique; fibers run superomedially (perpendicular to external oblique). Its aponeurosis splits to contribute to both the anterior and posterior rectus sheath above the arcuate line.
3. Transversus abdominis — the deepest flat muscle; fibers run transversely. Its aponeurosis contributes to the posterior rectus sheath above the arcuate line and the anterior sheath below it.

Together the three flat muscles compress the abdomen, assist forced expiration, and produce trunk flexion and rotation.

The Rectus Abdominis and Rectus Sheath

1. Rectus abdominis — paired strap muscles running vertically on either side of the midline from the pubic symphysis/crest to the costal cartilages of ribs 5–7. They are interrupted by three or four transverse tendinous intersections that adhere to the anterior sheath, producing the "six-pack" appearance in lean individuals.

The rectus sheath encloses the rectus abdominis and is formed by the aponeuroses of the three flat muscles:

• Above the arcuate line — the internal oblique aponeurosis splits: half passes anterior to the rectus (joining the external oblique aponeurosis to form the anterior sheath), half passes posterior (joining the transversus abdominis aponeurosis to form the posterior sheath).
• At and below the arcuate line (located midway between the umbilicus and pubis) — all three aponeuroses pass anterior to the rectus. The posterior sheath is deficient; only transversalis fascia lies posterior to the rectus. This is why inferior epigastric vessels enter the rectus sheath from below at this level.

The linea alba is the midline fibrous raphe where the aponeuroses of the two sides meet, extending from the xiphoid process to the pubic symphysis. It is avascular and is the preferred incision for midline laparotomy because it is bloodless and preserves nerve supply to the recti.

Deep Layers

1. Transversalis fascia — a thin fibrous lining of the entire abdominal wall, continuous with the fascia of the diaphragm and pelvis. It forms the posterior wall of the inguinal canal.
2. Extraperitoneal fat — variable adipose layer.
3. Parietal peritoneum — the serous lining of the abdominal cavity.

Innervation and Vascular Supply

The abdominal wall is innervated segmentally by the lower intercostal nerves (T7–T11), the subcostal nerve (T12), and branches of the lumbar plexus (iliohypogastric and ilioinguinal nerves, L1). The nerves run between the internal oblique and transversus abdominis in the neurovascular plane. This is why a lateral abdominal-wall incision risks denervation of the rectus — it interrupts the nerves before they enter the rectus sheath.

Arterial supply comes from the superior epigastric (continuation of the internal thoracic, entering the upper rectus sheath), inferior epigastric (from the external iliac, entering below the arcuate line), the lower intercostal and subcostal arteries laterally, and the deep circumflex iliac artery. The superior and inferior epigastric arteries anastomose within the rectus, providing a collateral route between the subclavian and external iliac systems.

The Inguinal Canal

The inguinal canal is an oblique passage (~4 cm) through the lower abdominal wall, parallel to and just above the inguinal ligament. It transmits the spermatic cord in males and the round ligament of the uterus in females, plus the ilioinguinal nerve in both sexes.

The canal has four boundaries:

• Anterior wall — external oblique aponeurosis (reinforced laterally by internal oblique fibers).
• Posterior wall — transversalis fascia (reinforced medially by the conjoint tendon, the fused tendon of internal oblique and transversus abdominis).
• Roof — internal oblique and transversus abdominis fibers arching over the canal.
• Floor — the inguinal ligament (and lacunar ligament medially).

The canal has two openings: the deep (internal) inguinal ring — an opening in the transversalis fascia lateral to the inferior epigastric vessels, marking the lateral boundary of the Hesselbach triangle; and the superficial (external) inguinal ring — a triangular opening in the external oblique aponeurosis just superolateral to the pubic tubercle.

Inguinal Hernias

The PA-CAT samples the two inguinal hernia types through their anatomical relationship to the inferior epigastric vessels:

• Indirect inguinal hernia — protrudes through the deep inguinal ring, lateral to the inferior epigastric vessels. It follows the full length of the canal and can protrude through the superficial ring into the scrotum. It is congenital, resulting from persistence of the processus vaginalis, and can occur at any age (the classic hernia of infants and young men).
• Direct inguinal hernia — protrudes through the Hesselbach triangle (bounded laterally by the inferior epigastric vessels, medially by the rectus abdominis, inferiorly by the inguinal ligament), medial to the inferior epigastric vessels. It pushes directly forward through the posterior wall and rarely enters the scrotum. It is acquired and common in older men with elevated intra-abdominal pressure.

Other Abdominal Wall Hernias

• Umbilical hernia — through the umbilical ring; common in infants (often closes spontaneously) and in multiparous or obese adults.
• Epigastric hernia — through the linea alba above the umbilicus; protrusion of extraperitoneal fat or a small peritoneal sac.
• Incisional hernia — through a healed surgical incision, due to wound dehiscence or weak healing.
• Femoral hernia — below the inguinal ligament through the femoral canal, medial to the femoral vein; more common in women, with a high risk of strangulation because the femoral ring is narrow and rigid.

This regional anatomy is high-yield on the PA-CAT because it links structure (wall layers and rings) directly to a common clinical presentation (groin mass) and to the surgical distinction between hernia types.