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The posterior aorta coming from the heart passes backward and gives rise to the internal iliac arteries, and of these the umbilical arteries are branches.

When the aneurism occurs in the posterior aorta no diagnostic symptoms are appreciable; when it occurs in the internal iliac arteries an examination per rectum will reveal it.

The anterior part of the plexus lies in front of the internal iliac artery, between the lumbar transverse processes and the psoas minor.