The Anatomy of the Abdominal Wall
The abdominal wall is mainly made up of muscles and the tissues that support them. They combine with the spinal column to give the midsection its structure. The abdominal muscles cover the front and sides of the abdomen (known clinically as the anterolateral area). They support it, help keep you upright, and assist with breathing. They also allow you to bend and twist the trunk.
The anterolateral abdominal muscles are divided into four pairs:
External oblique muscles: These abs originate on the 5th through the 12th ribs and insert on the linea alba and pubic tubercle. They flex and rotate the trunk and support the abdominal organs.
The linea alba is a band of fibrous tissue that runs along the midline of the abdominopelvic wall, from the xiphoid process down to the pubic symphysis.
Internal oblique muscles: These muscles originate on the thoracolumbar fascia, iliac crest, and connective tissue under the inguinal ligament. They insert onto the 10th through 12th ribs, the linea alba, and the pubic bone. They flex and rotate the trunk and support the abdominal organs along with the external obliques.
Transversus abdominis muscles: These abdominal muscles originate on the 7th through 12th costal cartilages, thoracolumbar fascia, iliac crest, and connective tissue under the inguinal ligament. They insert onto the linea alba and pubic bone. The transversus abdominis supports the organs of the abdomen.
Rectus abdominis muscles: These muscles originate at the pubic symphysis and pubic crest and insert onto the xiphoid process and the 5th through 7th costal cartilages. They flex the trunk and control the tilt of the pelvis. The rectus abdominis are covered by the rectus sheath, which is formed by the aponeuroses (broad tendinous structure) of the abdominal muscles.
Three abdominal muscles are posterior:
Psoas major muscles: These muscles originate at the transverse processes (lateral projections), bodies, and intervertebral discs of the 12th thoracic vertebra and the five lumbar vertebrae. They insert onto the lesser trochanter (bony protrusion) of the femur. The psoas major muscles are innervated by the lumbar plexus (L1–L3), and they flex the thighs.
Iliacus muscles: These muscles originate on the iliac fossa, sacral ala, and the anterior sacroiliac ligaments. They insert onto the lesser trochanter and shaft of the femur and to the tendon of the psoas major. They’re innervated by the femoral nerve (L2–L4) and work with the psoas major to flex the thighs.
Quadratus lumborum: These muscles originate on the medial (toward the midline of the body) parts of the 12th ribs and transverse processes of the lumbar vertebrae. They insert onto the iliolumbar ligament (between the ilium and the L5 vertebra) and the iliac crest. The quadratus lumborum muscles are innervated by branches from T12 through L4 nerves, and they extend and laterally flex the vertebral column.
A number of nerves run to the muscles and skin of the abdomen:
Thoracoabdominal nerves: Five pairs of thoracoabdominal nerves continue from the 7th through 11th intercostal nerves. They run between the layers of abdominal muscles to innervate the muscles of the anterolateral abdominal wall. Anterior and lateral and cutaneous branches provide nerve supply to the skin.
Subcostal nerves: These nerves stem from the anterior rami of the 12th thoracic spinal. They run just inferior to the 12th ribs and down to below the umbilicus (the navel). Subcostal nerves innervate the abdominal wall muscles and the skin (via cutaneous branches) between the iliac crests and the umbilicus.
Iliohypogastric nerves: The iliohypogastric nerves stem from the anterior rami of the 1st lumbar spinal nerves and form branches that run below the subcostal nerves to the lower part of the abdominal wall. They innervate the skin over the iliac crests, upper iliac (inguinal) regions, and hypogastric (pubic) regions (the area below the navel).
Ilioinguinal nerves: These nerves stem from the anterior rami of the 1st lumbar spinal nerves. They run between the layers of abdominal muscle and down to the inguinal canal. They innervate the scrotal skin in men and labia majora in women, the area over the pubic bone, and the medial portions of the thigh. They also innervate the internal oblique and transversus abdominis muscles.
Lateral cutaneous nerves of the thigh: These nerves run from the 2nd and 3rd lumbar spinal nerves inferiorly on the iliacus muscles to the thighs. They supply the skin on the anterolateral parts of the thighs.
Femoral nerves: These nerves come from the 2nd through 4th lumbar spinal nerves and run along the lateral part of the psoas major muscles to innervate the iliacus muscle before descending into the thigh to supply extensors of the knee.
Obturator nerves: These nerves come from the 2nd through 4th lumbar spinal nerves and run from the medial part of the psoas major muscle through the pelvis and into the thighs, where they supply the adductor muscles.
Lumbosacral trunk: These nerves come from the 4th and 5th lumbar nerve roots. They pass over the sacrum and descend into the pelvis to help form the sacral plexus.
A glistening membrane called the peritoneum serves as the lining for the innermost part of the abdominal cavity. It has two layers:
Parietal peritoneum: Lines the internal surface of the abdominopelvic wall
Visceral peritoneum: Covers the abdominal organs