Where does the lumbar plexus form in relation to the psoas major?

Where does the lumbar plexus form in relation to the psoas major?

• A. Posterior to the psoas major.
• B. Within the psoas major as the ventral rami traverse the muscle.
• C. Superior to the psoas major.
• D. In the posterior abdominal wall, away from the psoas major.

Answer: (B)

The lumbar plexus forms within the psoas major as the ventral rami pass through the muscle. The ventral rami from L1 to L4 course laterally across the psoas major and braid together inside the muscle to create the plexus, from which its branches emerge to innervate the abdominal wall and lower limb. So the correct location is inside the psoas major, not behind it, not above it, and not somewhere distant in the posterior wall.

The lumbar plexus forms within the psoas major as the ventral rami pass through the muscle. The ventral rami from L1 to L4 course laterally across the psoas major and braid together inside the muscle to create the plexus, from which its branches emerge to innervate the abdominal wall and lower limb. So the correct location is inside the psoas major, not behind it, not above it, and not somewhere distant in the posterior wall.