Laterally, it blends with the anterior layer at the lateral border of quadratus lumborum.
In the name-typical form "quadratus" the forewing has a yellowish white spot before the second median.
In the forearm, this action is performed by pronator quadratus and pronator teres muscle.
The ilioinguinal nerve closely follows the iliohypogastric nerve on the quadratus lumborum, but then passes below it to run at the level of the iliac crest.
It forms an arch over the quadratus lumborum muscle.
At the upper border of the pronator quadratus muscle it pierces the interosseous membrane and reaches the back of the forearm, where it anastomoses with the dorsal interosseous artery.
Laterally, it blends with the middle layer at the lateral border of the quadratus lumborum.
The lumbar vessels travel through the erector spinae muscles or between the erector spinae muscle and quadratus lumborum muscle.
