Anterior abdominal wall and inguinal region anatomy

• This resource covers the anatomy of the anterior abdominal wall and the inguinal region.

Abdomen Definition and Boundaries

• Region located between the diaphragm and the pelvis.
• The roof is the diaphragm.
• The posterior boundary consists of the 5 lumbar vertebrae, as well as the muscles of the posterior abdominal wall.
• Inferiorly, there is no distinct boundary as it is continuous with the pelvic cavity, specifically through the superior pelvic aperture.
• Anteriorly and laterally, it is bordered by the anterior abdominal wall, including its muscles.

Abdominal Cavity Boundaries

• Superiorly, it is bordered by the diaphragm.
• Posteriorly, it is bordered by the lumbar vertebrae and the muscles of the posterior abdominal wall.
• Inferiorly, there is no border; it makes up a continuous structure along with the pelvic cavity, and the superior pelvic aperture.
• The anterior and lateral borders consist of the anterior abdominal wall and its muscles.

Topography of Abdomen (Planes)

• Imaginary lines divide the abdomen into 9 regions for descriptive purposes.
• These lines pass through specific anatomical structures.

Transverse Planes:

• Transpyloric plane passes through the tip of the 9th costal cartilages and the pylorus of the stomach at the L1 vertebral level.
• Subcostal plane passes through the tip of the 10th costal cartilages at the L3 vertebral level.
• Transtubercular plane passes through the tubercles of the iliac crests at the L5 vertebral level.
• Interspinous plane passes through the anterior superior iliac spines and the promontory of the sacrum.

Vertical Planes:

• Mid-clavicular plane runs from the midpoint of the clavicle to the midpoint of the inguinal ligament.
• Semilunar line runs along the lateral border of the rectus abdominis muscle.

Abdomen Regions

• The abdomen is divided into 9 regions: umbilical, epigastric, hypogastric, left and right hypochondriac, left and right inguinal, and left and right lumbar.
• The umbilical region is located around the umbilicus.

Skin Innervation of Anterior Abdominal Wall

• Lower 5 intercostal nerves, the subcostal nerve, and the L1 spinal nerve (ilioinguinal and iliohypogastric nerves) provide cutaneous innervation.
• All nerves are anterior rami of spinal nerves.
• Umbilical region receives innervation from T10.

Layers of the Anterior Abdominal Wall

• Skin
• Superficial fascia (two layers):
  • Superficial fatty layer (Camper's fascia).
  • Deep membranous layer (Scarpa's fascia).
• There is no deep fascia.
• External oblique muscle
• Internal oblique muscle
• Transversus abdominis muscle, just lateral to the rectus abdominis muscle.
• Transversalis fascia
• Extraperitoneal tissue layer (extraperitoneal fascia)
• Peritoneum (membrane covering some organs of the abdominal cavity)

Muscles of the Anterior Abdominal Wall

• Three muscles are located mainly laterally with aponeuroses that unite at the midline (linea alba).
• External oblique muscle (most superficial)
• Internal oblique muscle
• Transversus abdominis muscle
• Rectus abdominis muscle runs on both sides of the midline (linea alba)
• Pyramidalis muscle runs inferiorly

External Abdominal Oblique Muscle

• Most superficially located in the abdomen.
• Originates from the 5th to 12th ribs.
• Inserts on the linea alba and inguinal ligament.
• The parts of the inguinal ligament consist of reflected part, lacunar ligament, and pectineal Lig.
• Innervated by the lower 5 intercostal nerves and the subcostal nerve (L1).
• Superficial inguinal ring is an opening in the aponeurosis of the external Oblique Muscle.
• It has lateral crus, medial crus, and intercrural fibers.

Inguinal Ligament

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

Superficial Inguinal Ring

• Lat. crus
• Medial crus
• Intercrural fibers

Internal Abdominal Oblique Muscle

• Middle layer muscle, with fibers running at 90 degrees to the external oblique.
• Originates from the thoracolumbar fascia, iliac crest, and inguinal ligament.
• Inserts on the linea alba, conjoint tendon (with transversus abdominis), pubic crest, and pecten pubis.
• Innervated by the lower 5 intercostal nn + subcostal n + L1, similar to the external oblique.

Conjoint Tendon (Falx Inguinalis)

• The shared tendon of the internal oblique and transversus abdominis muscles.

Transversus Abdominis Muscle

• Deepest abdominal muscle.
• Originates from the inferior 6 costal cartilages, thoracolumbar fascia, and inguinal ligament.
• Inserts on the conjoint tendon and linea alba.
• Innervated by the lower 5 intercostal nerves and subcostal nerve L1, same as the external oblique

Transversalis Fascia

• Located posterior (deep) to the transversus abdominis muscle.
• Contains the deep inguinal ring.

Functions of Anterior Abdominal Muscles

• Provide support, protection, and movements of the trunk.
• External oblique: turns the trunk to the opposite side.
• Internal oblique: turns the trunk to the same side.
• Muscles contract during coughing, sneezing, vomiting, and parturition (birth), increasing intra-abdominal pressure.

Rectus Abdominis Muscle

• Located on both sides of the midline (linea alba).
• Originates from the xiphoid process (sternum).
• Inserts on the symphysis pubis.
• Innervated by lower 5 intercostal nerves and the subcostal nerve.
• Flexes the trunk.
• Contains tendinous intersections (3-4 in number).
• Enveloped by the rectus sheath.

Rectus Sheath

• 4-5cm below the umbilicus at the arcuate line (semicircular line)
• Above the arcuate line:
  • Anterior layer: external oblique aponeurosis + anterior lamina of internal oblique's aponeurosis.
  • Posterior layer: posterior lamina of internal oblique aponeurosis + transversus abdominis aponeurosis.
• Below this line:
  • Anterior layer: external oblique aponeurosis + internal oblique aponeurosis + transversus abdominis aponeurosis.
  • Posterior layer: only transversalis fascia.

Rectus Sheath Contents

• Rectus abdominis muscle
• Superior epigastric artery
• Inferior epigastric artery
• Lower 5 intercostal nerves
• Subcostal nerve

Arteries of Anterior Abdominal Wall

• Musculophrenic
• Superior epigastric
• Inferior epigastric
• Deep circumflex iliac
• Superficial circumflex iliac

Inguinal Canal

• Surgically important because it is the site of inguinal hernias.
• Obliquely located, tubelike structure 3-4cm in length.
• It has two openings: superficial inguinal ring (external oblique aponeurosis) located medially, and deep inguinal ring (transversalis fascia) located laterally.

Walls of the Inguinal Canal:

• Anterior is skin, superficial fascia, external oblique (medially), and internal oblique (laterally).
• Posterior is reflected inguinal ligament, conjoint tendon, and transversalis fascia.
• Inferior consists of the inguinal and lacunar ligaments.

Structures passing through the Inguinal Canal:

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

Inguinal Hernia

• Indirect inguinal hernia involves an organ passing through the deep inguinal ring, coursing through the inguinal canal, and protruding outwards through the superficial inguinal ring.
• Direct inguinal hernia involves an organ pushing directly anteriorly, passing through the abdominal wall via the superficial inguinal ring, and protruding outwards.
• Reference point is the inferior epigastric artery:
  • If the hernia is lateral to the artery, it is an indirect inguinal hernia.
  • If the hernia is medial to the artery, it is a direct inguinal hernia.