Chapter 11

### Big Picture

The diaphragm forms the superior and much of the posterior border of the posterior abdominal wall. In addition, the psoas major, iliacus, and quadratus lumborum muscles form the posterior abdominal wall. These muscles function in respiration (diaphragm) as well as trunk and lower limb motion.

### Muscles and Fascia

The muscles that form much of the structure of the posterosuperior abdominal wall are as follows (Figure 11-1):

• Diaphragm. A dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. The origin of the muscles of the diaphragm is along the internal circumference of the ribcage, sternum, and lumbar vertebrae. This portion of muscle consists of the esophageal hiatus, at the T10 vertebral level, and the aortic hiatus, at the T12 vertebral level. The muscle fibers are directed to the center of the diaphragm, to the central tendon. The central tendon consists of collagen tissue and the venal caval hiatus, which is at the T8 vertebral level. When the muscle is stimulated to contract (phrenic nerve, C1–C4), the muscle fibers shorten, causing the central tendon to move inferiorly and flatten. This action results in inspiration. The structural components of the diaphragm are as follows:
• Right crus. Forms part of the aortic hiatus. The right crus also loops around the esophagus to form the esophageal hiatus and contributes to the suspensory ligament of the duodenum (ligament of Treitz).
• Left crus. Forms part of the aortic hiatus.
• Quadratus lumborum muscle. Attaches to the iliac crest, lumbar transverse processes, and the 12th rib. The quadratus lumborum muscle is the bed on which the kidneys lie. The muscle laterally flexes the vertebral column and stabilizes the 12th rib during breathing. The subcostal and lumbar intercostal nerves provide innervation.
• Psoas major muscle. Attaches to the lumbar vertebrae superiorly and to the lesser trochanter of the femur inferiorly. Between these attachments, the psoas major muscle courses deep to the inguinal ligament and lateral to the femoral nerve. The psoas major muscle flexes the hip joint (when the vertebrae are stabilized) and the lumbar vertebrae. The psoas major muscle is innervated by the L1–L3 spinal nerves.

###### Figure 11-1

Muscles of the posterior abdominal wall.

The appendix is in close relationship with the parietal peritoneum, including that covering the right psoas muscle. When the appendix is inflamed, the inflammation irritates the parietal peritoneum. The parietal peritoneum is innervated by somatic nerves, including pain fibers, and thus inflammation of the parietal peritoneum results in a “shooting” abdominal pain. To diagnose acute appendicitis, the physician will push on McBurney's point when the patient is in a supine position. If the patient suffers from acute appendicitis, the pain reflex will flex the hip joint.

• Psoas minor muscle. Attaches to the L1 vertebra and pubic ...

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