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During the placement of an epidural catheter for labor analgesia, local anesthetic and an opioid analgesic agent are injected into the epidural space. These medications exert their actions by diffusing across the dura mater into the subarachnoid space and act on the spinal nerve roots, spinal cord and paravertebral nerves. From the outside to inside, the path through layers of tissue and ligaments that are necessary to be crossed to correctly place an epidural progress is: skin, subcutaneous tissue, supraspinous ligaments, interspinous ligament, ligamentum flavum, epidural space, dura mater, subarachnoid space.