Anterior abdominal wall

Introduction

This section will explore the layers of fascia and muscles involved in forming the anterior abdominal wall and the rectus sheath

Fascia of the abdominal wall

Superficial fascia of the abdominal wall

Above the level of the umbilicus:

• a superficial fatty layer: a layer containing a variable amount of fat

Below the level of the umbilicus:

• Superficial fatty layer (Camper’s fascia)
• This is a poorly defined layer of fat under the skin. It has little structural integrity
• In males, Camper's fascia extends onto the penis and scrotum in the form of Dartos' Fascia.

Deep fibrous layer (Scarpa’s fascia).

• This is a well defined fibrous layer of tissue deeper to Camper's fascia
• The fibrous nature of the layer gives it good tensile strength
• It is attached to the deep fascia of thigh 2.5 cm below the inguinal ligament
• This stops urine reaching the thigh when bulbous urethra ruptures
• This stops an undescended testis from descending into the thigh
• Extends into perineum as Colles’ fascia.
• Colles’ fascia is attached to the perineal body, perineal membrane and laterally to the rami of the pubis and ischium.

Other fascia in the anterior abdominal wall:

• Investing fascia
• Layers of fascia that cover the external aspect of the anterior abdominal wall muscles.(external oblique, internal oblique and transversus abdominis)
• Endoabdominal fascia:
• a membranous sheet of fascia internal to all muscle groups.
• Can be named after the portion of muscle it associates with
• For example, the fascia lining the internal surface of the transversus abdominis muscle is called the transversalis fascia.
• Parietal Peritoneum:

Anterior abdominal wall muscles:

There are 4 muscles in the anterior abdominal wall. They are paired (4 on the right and 4 on the left)

Muscle Name	Notes
vertical Strap like muscle
Rectus Abdominis	Strap-like muscle extending inferiorly down the anterior abdominal wall, just off the midline. Forms the classic “6 Packs appearance”.
Flat Muscles
External Oblique	The most superficial of the flat muscles. Inferiorly forms a thick band spanning from the anterior superior iliac crest to the pubic tubercle.
Internal Oblique	A flat muscle deep to the external oblique and superficial to the transversus abdominis.
Transversus Abdominis	Deepest of the flat muscles.

Rectus abdominis muscle:

• Long strap-like muscle, extending the length of the abdominal wall.
• Enclosed in the rectus sheath

Origin	5th, 6th, 7th costal cartilages
Insertion	Pubic crest
Innervation	Lower six thoracic nerves (T6-T12)

• The rectus abdominis has tendinous intersections along its course.
  • The long muscle fibres are interrupted by these tendinous intersections allowing for increased contraction strength.
  • The intersections attach to the anterior rectus sheath.
    • This forms the appearance known as the "six pack".
  • The number of intersections varies by genetics:
    • Classically there are 3 intersections:
      1. Level of xyphoid
      2. Level of umbilicus
      3. Halfway between the 2 points

External oblique muscle:

• The most superficial sheet muscle.
• Its fibres pass inferomedially.
• The inferior margin is thickened forming the inguinal ligament.
  • This spans between the anterior superior iliac spine (ASIS) and pubic tubercle.

Origin	External surfaces of 5th–12th ribs
Insertion	Linea alba, pubic tubercle, ASIS and anterior half of iliac crest
Innervation	Anterior rami of lower 6 thoracic nerves and subcostal nerve

Internal oblique muscle:

• It is a broad sheet-like muscle located deep to the external oblique and superficial to the transversus abdominis.
• Fibres are arranged in a fan-like fashion.
  • The superior fibres are perpendicular to fibres of external oblique.
  • The inferior fibres are parallel to the external oblique (hand in pocket appearance).

Origin	Thoracolumbar fascia posteriorly. Anterior 2/3 of the iliac crest laterally.
Insertion	Linea alba, inferior borders of lower 2 ribs, and pubis
Innervation	Lower sixth thoracic nerves and iliohypogastric and ilioinguinal nerves.

Transversus abdominis

The innermost of the 3 sheet muscles. Its fibres run horizontally.

Origin	Internal surfaces of the 7th to 12th ribs Thoracolumbar fascia posteriorly Iliac crest
Insertion	Linea alba forming an aponeurosis with the internal oblique Pubic crest
Innervation	Lower sixth thoracic nerves and iliohypogastric and ilioinguinal nerves.

Function of the anterior abdominal wall muscles:

• The muscles act to compress and support the abdominal viscera
• The muscles work together to flex and rotate the trunk and stabilise the pelvis.

Sensory supply of the anterior abdominal wall

Cutaneous innervation of the trunk:

Each nerve supplying the abdominal wall originates from the spinal cord and exits through the intervertebral foramen as a thoracoabdominal nerve (T7-T11), subcostal nerve (T12), or a branch from the lumbar plexus (L1).

Pathway and Branching:

Dermatomes of the anterior abdominal wall:

• The sensory supply (skin and parietal peritoneum) is carried by thoracic nerves:
  • T10 anterior cutaneous branch: Supplies sensation around the umbilicus.
  • T7-T9 anterior cutaneous branches: Cover areas above the umbilicus.
  • T11 and T12 anterior cutaneous branches: Cover areas below the umbilicus.
  • Iliohypogastric and Ilioinguinal Nerves (L1):
    • Iliohypogastric nerve: Supplies sensation to the skin above the pubic region
    • Ilioinguinal nerve: Supplies sensation to the groin area and inner thigh

Coverage Summary

• Posterior Cutaneous Branch: Supplies the back and paraspinal region.
• Lateral Cutaneous Branch: Supplies the side and lateral parts of the abdominal wall.
• Anterior Cutaneous Branch: Supplies the front of the abdominal wall along the midline.

Midline Rectus Sheath:

• The 3 sheet muscles form a thick aponeurosis medially, enclosing the rectus abdominis muscle.
• The lateral boundary where the rectus abdominis meets the aponeuroses is the linea semilunaris.
• Medially, the aponeuroses join to form the linea alba.
• The linea alba has minimal blood supply, allowing rapid access to the abdomen (useful in surgical emergencies), but also slower healing.

Arcuate line:

• The halfway point between umbilicus and pubic symphysis.
• It is where the inferior epigastric vessels enter the rectus sheath.
• Below the arcuate line, all aponeuroses run anterior to the rectus abdominis, thinning the posterior wall and allowing vessels to pierce more easily.

Muscle aponeurosis	Relation above the arcuate line	Relation below the arcuate line
External oblique	Anterior to rectus abdominis	Anterior to rectus abdominis
Internal oblique	Splits into anterior and posterior layers around rectus abdominis	All aponeurosis anterior to rectus abdominis
Transversus abdominis	Posterior to rectus abdominis	Anterior to rectus abdominis

Because the aponeurosis is anterior below the arcuate line, the posterior sheath is thinner, creating a potential area of weakness where a Spigelian hernia can occur.

Contents of the rectus sheath:

1. Rectus muscle
2. Anastomosing superior and inferior epigastric vessels
3. Lymphatics
4. Nerves supplying muscle and overlying skin

Arteries of the anterior abdominal wall

Internal thoracic artery

• Branch of subclavian artery
• Divides into 2 main branches
  1. Musculophrenic (supplies muscles and diaphragm)
     • Runs along the costal margin
     • Branches into intercostal arteries
  2. Superior epigastric artery
     • Runs in the rectus sheath deep to rectus abdominis
     • Anastomoses distally with the inferior epigastric

Inferior epigastric artery

• Branch of the external iliac artery
• Runs superiorly, entering rectus sheath at the level of the arcuate line
• Runs deep to rectus abdominis in rectus sheath
• Anastomoses superiorly with the superior epigastric artery

Superficial epigastric artery

• Branch of the femoral artery
  • Runs in superficial fascia towards umbilicus

Venous drainage of the anterior abdominal wall

Venous drainage largely follows the arteries. A key difference is the thoracoepigastric vein.

Thoracoepigastric vein

This vein provides an indirect anastomosis between the IVC and SVC.

It is a branch of the superficial epigastric vein (branch of the femoral vein).

It anastomoses superiorly with the axillary vein which drains into the SVC, bypassing the IVC.

If the IVC is blocked, blood from the lower limb can still return to the heart via this route.

Route of bypass

Layers of the anterior abdominal wall

The layers of the abdominal wall differ in different parts of the abdominal wall.

A cut down through layers in the midline will reveal layers vastly different to a cut lateral to the rectus abdominis muscles.

Layers of the abdominal wall lateral to rectus abdominis

layers of the abdominal wall at midline

Layers of the abdominal wall just lateral to midline (a cut through the rectus abdominis)

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