Dermatomes of the Abdominal Wall

Dermatomes of the Anterolateral Abdominal Wall

• A dermatome is an area of skin supplied mainly by afferent nerve fibers from the dorsal root of a spinal nerve.
• The anterolateral abdominal wall is divided into dermatomes:
  • T7: Over the xiphoid process
  • T10: Umbilicus
  • L1: Lies just above the inguinal ligament and the symphysis pubis

Skin Blood Supply and Venous Drainage

• The skin near the midline is supplied by branches of the superior and inferior epigastric arteries.
• The skin of the flanks is supplied by branches of the intercostal, lumbar, and deep circumflex iliac arteries.
• The skin in the inguinal region is supplied by the superficial epigastric, superficial circumflex iliac, and superficial external pudendal arteries, which are branches of the femoral artery.
• Venous drainage from the upper abdominal wall passes mainly into the axillary vein via the lateral thoracic vein.
• The lower abdominal wall drains into the femoral vein via the superficial epigastric and great saphenous veins.

Superficial Fascia

• Divided into two layers:
  • Superficial fatty layer (Camper's fascia)
  • Deep membranous layer (Scarpa's fascia)
• The superficial fatty layer is continuous with the superficial fat over the body, and may be extremely thick (8 cm or more) in obese individuals.
• The deep membranous layer is thin and fades out laterally and superiorly, where it becomes continuous with the superficial fascia of the back and thorax, respectively.

Deep Fascia

• A thin layer of connective tissue covering the muscles.
• Lies immediately deep to the membranous layer of superficial fascia.

Muscles of the Anterior Abdominal Wall

• Three broad thin sheets of muscle:
  • External oblique
  • Internal oblique
  • Transversus abdominis
• Additionally, there are two muscles:
  • Rectus abdominis (on either side of the midline anteriorly)
  • Pyramidalis (may be present in the lower part of the rectus sheath)

External Oblique Muscle

• Origin: lower eight ribs
• Insertion: xiphoid process, linea alba, iliac crest, pubic crest, and tubercle
• Action: support and compress abdominal contents, assist in flexion and rotation of the trunk, assist in forced expiration, micturition, defecation, and vomiting
• Innervation: lower six thoracic nerves, iliohypogastric and ilioinguinal nerves (L1)

Internal Oblique Muscle

• Origin: iliac crest and lateral 2/3 of inguinal ligament
• Insertion: lower three ribs, xiphoid process, linea alba, and symphysis pubis
• Action: same as external oblique
• Innervation: same as external oblique

Transversus Abdominis Muscle

• Origin: lower six costal cartilages, iliac crest, and lateral 2/3 of inguinal ligament
• Insertion: xiphoid process, linea alba, and symphysis pubis
• Action: compress abdominal contents
• Innervation: same as external oblique

Conjoint Tendon

• Formed by the lowest tendinous fibers (aponeurosis) of the internal oblique and transversus abdominis muscles
• Attaches medially to the linea alba, with a lateral free border
• Has an essential role in protecting a weak area in the abdominal wall, where a weakening of the conjoint tendon may lead to a direct inguinal hernia

Rectus Abdominis Muscle

• Origin: symphysis pubis and pubic crest
• Insertion: 5th, 6th, and 7th costal cartilages, and xiphoid process
• Action: compress, flex vertebral column, and act as an accessory muscle of expiration
• Innervation: lower six thoracic nerves

Rectus Sheath

• The composition of the walls of the rectus sheath changes at different levels
• The anterior wall is formed by the aponeurosis of the external oblique above the costal margin
• The posterior wall is formed by the thoracic wall above the costal margin
• Between the costal margin and the arcuate line, the aponeurosis of the internal oblique splits to enclose the rectus muscle
• Below the arcuate line, the aponeuroses of all three muscles (external oblique, internal oblique, and transversus abdominis) form the anterior wall, and the posterior wall is absent

Linea Alba

• A fibrous band that separates the two rectus sheaths from each other
• Extends from the xiphoid process to the symphysis pubis
• Formed by the fusion of the aponeuroses of the lateral muscles of the two sides

Transversalis Fascia

• A thin layer of fascia that lines the transversus abdominis muscle
• Named according to the structure it overlies (e.g., diaphragmatic fascia, psoas fascia, iliac fascia)

Extra-peritoneal Fat

• A thin layer of connective tissue that contains a variable amount of fat
• Lies between the fascia transversalis and the parietal peritoneum

Parietal Peritoneum

• A thin serous membrane
• Lines the walls of the abdomen
• Continuous below with the parietal peritoneum lining the pelvis

Anterolateral Abdominal Wall Arteries

• Superior epigastric artery (from internal thoracic artery)
• Inferior epigastric artery (from external iliac artery, from common iliac artery)
• Deep circumflex iliac artery (from external iliac artery)
• Lower two posterior intercostal arteries (from descending thoracic aorta)
• Four lumbar arteries (from abdominal aorta)

Anterolateral Abdominal Wall Veins

• Superficial veins network radiates out from the umbilicus
• Above the umbilicus: drains into the axillary vein via the lateral thoracic vein
• Below the umbilicus: drains into the femoral vein via the superficial epigastric and great saphenous veins
• Paraumbilical veins form an important portal-systemic venous anastomosis