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Medical Definition of Ischiadic plexus
1. Formed by the fourth and fifth lumbar (lumbosacral trunk) and first, second, and third sacral nerves; it lies on the inner surface of the posterior wall of the pelvis usually embedded in the piriformis muscle; its nerves supply the lower limbs, its major product being the sciatic nerve. Synonym: plexus sacralis, ischiadic plexus, sciatic plexus. (05 Mar 2000)