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The diaphragm (principal respiratory muscle) and the psoas major, iliacus, and quadratus lumborum muscles (trunk and lower limb muscles) form the posterior abdominal wall. The ventral rami from the subcostal nerve and lumbar spinal nerves provide somatic innervation to the abdominal wall and lower limb muscles and skin.


The muscles of the posterior abdominal wall are as follows (Figure 11-1):

  • Diaphragm. A dome-shaped muscle that separates the abdominal and thoracic cavities.

    • Origin. The internal surface of the xiphoid process, ribcage, and lumbar vertebrae via the left and right crura.

    • Insertion. An aponeurosis called the central tendon (of the diaphragm); the right dome of the diaphragm is slightly higher than the left because of the liver.

    • Innervation. Each half of the diaphragm is innervated by the right or left phrenic nerve (C3–C5).

    • Action. Primary muscle of respiration; additionally, the diaphragm increases intra-abdominal pressure for defecation, urination, vomiting, and childbirth.

    • Apertures. The diaphragm has the following openings:

      • Caval hiatus (T8). Transmits the IVC.

      • Esophageal hiatus (T10). Transmits the esophagus, vagus nerves, and left gastric vessels.

      • Aortic hiatus (T12). Transmits the aorta, thoracic duct, azygos and hemiazygos veins, and sympathetic trunk.

  • Quadratus lumborum muscle

    • Attachments. Iliac crest, lumbar transverse processes, and the 12th rib.

    • Actions. Laterally flexes the vertebral column.

    • Innervation. Branches from T12-L4 ventral rami.

  • Psoas major muscle

    • Attachments. Courses from L1–L5 vertebrae, deep to the inguinal ligament to the lesser trochanter of the femur.

    • Actions. Flexes the hip joint.

    • Innervation. Branches from the L2 ventral ramus.

  • Iliacus muscle

    • Attachments. Iliac fossa and lesser trochanter of the femur. Between its attachments, the iliacus muscle courses deep to the inguinal ligament and joins with the psoas major muscle to attach to the lesser trochanter of the femur. The combination of these two muscles in the thigh is often referred to as the iliopsoas muscle.

    • Actions. Flexes the hip joint.

    • Innervation. Femoral nerve (L2–L4).

Figure 11-1:

Muscles and nerves of the posterior abdominal wall.


The somatic nerves of the posterior abdominal wall are the ventral rami of the subcostal and lumbar spinal nerves. These nerves for the most part course between the internal oblique and transverse abdominis muscles (Figure 11-1).

  • Subcostal nerve (T12). Arises from the T12 ventral ramus.

    • Motor. Segmentally supplies abdominal body wall muscles (external oblique, internal oblique, transverse abdominis, and rectus abdominis).

    • Sensory. Anterolateral region of the T12 dermatome.

  • Iliohypogastric nerve (L1). Arises from the L1 ventral ramus.

    • Motor. Internal oblique and transverse abdominis muscles.

    • Sensory. Skin in the hypogastric region.

  • Ilioinguinal nerve (L1). Arises from the L1 ventral ramus; after coursing through the body wall, enters the inguinal canal laterally and then exits the superficial inguinal ring to ...

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