The medial arcuate ligament (also medial lumbocostal arch and internal arcuate ligament) is a tendinous fascia that arches over the psoas major muscle as it passes posterior the diaphragm.

The purpose of the medial arcuate ligament is to attach the diaphragm to the spine (lumbar vertebra L1 - L2) The medial arcuate ligament is an arch in the fascia covering the upper part of the psoas major.

The sympathetic chain enters the abdomen by passing deep into this ligament of the diaphragm.

This is in contrast to the parasympathetic vagus nerve which passes through the esophageal hiatus.

This article incorporates text in the public domain from page 404 of the 20th edition of Gray's Anatomy (1918)