Abdomen: Boundaries, Topography, and Regions

Abdomen Boundaries

• It's the region exists between the diaphragm and the pelvis
• Roof: Diaphragm
• Posterior: 5 Lumbar vertebrae + Mm. Of the posterior abdominal wall
• Inferior: Has no boundary, continuous with the pelvic cavity, at the Superior Pelvic aperture
• Anterior and lateral: Anterior Abdominal Wall, Muscles

Topography of the Abdomen

• It's divided by imaginary lines into 9 regions
• These lines run through structures

Transverse Planes

• Transpyloric plane: located at the tip of the 9th costal cartilages/pylorus of stomach , L1 vertebra level
• Subcostal plane: On the tip of the 10th costal cartilages, L3 vertebra
• Transtubercular plane: On the tubercles if iliac crests, i.e. L5 vertebra level
• Interspinous plane: anterior superior iliac spines, promontory of sacrum

Vertical Planes

• Mid-clavicular plane: runs mid-point of clavicle to the mid-point of inguinal ligament
• Semilunar line: lateral border of rectus abdominis muscle

Abdomen Regions

• Can be split into 9 regions
• Region 1: umbilical (around the umbilicus)
• Region 2: epigastric
• Region 3: hypogastric
• Regions 4 and 5: L hypochondriac and R hypochondriac
• Regions 6 and 7: L inguinal and R inguinal
• Regions 8 and 9: L lumbar and R lumbar

Cutaneous Nerves in the Anterior Abdominal Wall

• The Lower 5 intercostal nerves + subcostal nerve + L1 spinal nerve provide cutaneous innervation
• Includes the ilioinguinal and iliohypogastric nerves
• All anterior rami of spinal nerves
• Umbilical region gets its innervation from T10

Layers of the Anterior Abdominal Wall (superficial to deep)

• Skin
• Superficial fascia (two layers)
  • superficial fatty layer: Camper’s fascia
  • deep membranous layer: Scarpa’s fascia
• Has no deep fascia
• External oblique muscle
• Internal oblique muscle
• Transversus abdominis muscle
• Layer just lateral to midline is the rectus abdominis muscle
• Transversalis fascia
• Extraperitoneal tissue layer
• Peritoneum is the membrane covering some organs of the abdominal cavity

External Abdominal Oblique Muscle

• Most superficially located muscle of the abdominal wall
• Origin: 5-12 ribs
• Insertion: Linea alba and the inguinal ligament
• Parts of inguinal ligament include the reflected part, lacunar ligament and pectineal Ligament
• Innervation: lower 5 intercostal nerves + the subcostal n.+ L1
• Features the Superficial inguinal ring (opening in the aponeurosis)
• Includes lateral crus, medial crus, and intercrural fibers

Inguinal Ligament

• Runs from anterior sup. iliac spine to pubic symphysis
• Extends medially as the lacunar ligament
• Extends to pectineal line as pectineal ligament

Internal Abdominal Oblique Muscle

• Middle layer muscle of the abdominal wall
• Its fibers run at a 90 degree angle to the external oblique muscle fibers
• Origin: thoracolumbar fascia, iliac crest and inguinal ligament
• Insertion: linea alba + conjoint tendon (common tendon with the transversus abdominis muscle) + Pubic crest+pecten pubis
• Innervation: lower 5 intercostal nerves + subcostal nerve + L1 spinal nerve
• Is the same innervation as the external oblique

Transversus Abdominis Muscle

• Deepest muscle in the abdominal wall
• Origin lies on the inferior 6 costal cartilages, the thoracolumbar fascia and inguinal ligament
• Insertion: conjoint tendon + linea alba
• Innervation: lower 5 intercostal nerves + subcostal nerve + L1. spinal nerve
• Same as external oblique

Conjoint Tendon

• Known as the Falx Inguinalis
• Formed by the common tendon of internal oblique+ transversus abdominis muscles

Transversalis Fascia

• Located posterior (deep) to the Transversus Abdominis muscle
• Features a deep inguinal ring

Functions of Anterior Abdominal Muscles

• Support + protection + movements of the trunk
• The external oblique turns the trunk to the opposite side
• The internal oblique turns the trunk to the same side
• All abdominal muscles contract during events that increase intra-abdominal pressure
• Includes coughing, sneezing, vomiting or parturition (during birth of a child)

Rectus Abdominis Muscle

• Located: on both sides of midline, specifically on the linear alba
• Origin: Xiphoid process (sternum)
• Insertion: symphysis pubis
• Innervation: lower 5 intercostal nerves.+ subcostal nerve
  • Is different from the previous 3 abdominal wall muscles
• Function: flexes the trunk
• Has tendinous intersections numbering at 3-4
• Enveloped by a sheath: Rectus Sheath

Rectus Sheath

• 4-5cm below umbilicus are recognizable structures = Arcuate Line (Semicircular Line)
• Features described ABOVE are the Line:
  • Anterior layer: the external oblique apon. + anterior lamina of internal oblique’s aponeurosis
  • Posterior layer: Posterior lamina of internal oblique apon. + transversus abdominis apon
• Features described BELOW are the Line:
  • Anterior layer : the external oblique apon.+internal oblique apon.+transversus abdominis aponeurosis
  • Posterior layer: Transversalis fascia only

Rectus Sheath Contents

• It encloses the rectus abdominis muscle+ superior epigastric artery+ inferior epigastric artery+ lower 5 intercostal nerves + subcostal nerve

Anterior Abdominal Wall Arteries

• Musculophrenic
• Superior epigastric
• Inferior epigastric
• Deep circumflex iliac
• Superficial circ.Iliac

Inguinal Canal

• It's surgically important due to being the a common site for inguinal hernias
• It is an obliquely located tubelike structures
• 3-4cm long
• Features two openings:
  • Superficial inguinal ring: is medial and formed from the external oblique apon.
  • Deep inguinal ring has a transverse fascia that's located laterally

Inguinal Canal Walls

• Anterior wall: skin + superficial fascia+ external oblique (medially) +internal oblique (laterally)
• Posterior Wall: Reflected inguinal ligament + conjoint tendon + transversalis fascia
• Inferior Wall: inguinal ligament.+ lacunar ligament.
• Superior Wall: inferior margins of internal oblique and transversus abdominis muscle

Structures Passing Through the Inguinal Canal

• Spermatic cord in males
• Round ligament of uterus in females
• Ilioinguinal nerve
• Genital branch of genitofemoral nerve

Inguinal Hernia Classifications

• Indirect Inguinal hernia: It is a piece of organ that passes through the deep inguinal ring
  • Then courses in inguinal canal and passes and through the superficial inguinal ring
  • Finally protruding outwards
• Direct inguinal hernia: a piece of organ pushes directly out
  • Abdominal wall passes through the supeficial inguinal ring
  • Protrudes outwards
• Reference for differentiating direct and indirect inguinal hernias is the inferior epigastric artery.
  • Indirect inguinal hernia: if lateral to the inferior epigastric artery.
  • Direct inguinal hernia: if it's Medial to the inferior epigastric artery.

Folds of the Anterior Abdominal Wall Peritoneum

• A view from inside of the anterior abdominal wall shows with some folds of parietal peritoneum:
  • The median umbilical fold, which has the urachus
  • Medial umbilical fold, which has the umbilical artery
  • Lateral umbilical fold, which has the inferior epigastric vessels