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The outside of the Dura Mater (a protective layer of the spinal cord and brain) of the spinal cord.
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Why is the epidural space absent in the brain?
In the spine, the epidural space (also known as "extradural space" or "peridural space") is the outermost part of the spinal canal. It is the space within the canal (formed by the surrounding vertebrae) lying outside the dura mater (which encloses the arachnoid mater, subarachnoid space, the cerebrospinal fluid, and the spinal cord). In humans the epidural space contains lymphatics, spinal nerve roots, loose fatty tissue, small arteries, and a network of large, thin-walled blood vessels called the epidural venous plexus.The upper limit of the epidural space is the foramen magnum, which is the point where the spine meets the base of the skull. The lower limit is at the tip of the sacrum, at the sacrococcygeal membrane.In the head, the dura is continuous with the periosteum, the tough fibrous lining of the inside of the skull. This means that, in the head, the epidural space is known as a potential space, which means that normally it does not exist. In rare circumstances, a torn artery (e.g. the middle meningeal artery) may cause bleeding which is sufficient to separate both the dura and periosteum from the bone; this is an epidural hematoma.The space between the dura and the arachnoid (in both head and spine), the subdural space, is also a potential space. Bleeding may also occur here.reference: wikipedia
Layers of the spinal cord?
The spinal cord, like the brain, is surrounded by the three meninges. The dura mater extends from the foramen magnum to the sacrum and coccyx. The dura is attached to the foramen magnum and the periosteium covering the uppemost cervical vertebrae and their ligaments. Through the remainder of the vertebral canal, the dura is not attached to the vertebrae, being separated by the epidural (or peridural or extradural) space, which contains fat and the internal vertebral venous plexus. In caudal analgesia, an anesthetic solution injected into the sacral hiatus diffuses upward into the epidural space. This may be used in surgical procedures relating to pelvic and perineal regions. Extensions of dura (dural sheaths) surround the nerve roots and spinal ganglia, and continue into the connective tissue coverings (epineurium) of the spinal nerves. The arachnoid invests the spinal cord loosely. Continuous with the cerebral arachnoid above, it traverses the foramen magnum and descends to about the S2 vertebral level. The subarachnoid space, which contains cerebrospinal fluid, is a wide interval between the arachnoid and pia. Because the spinal cord ends at about the level of the L2 vertebra, whereas the subarachnoid space continues to S2, access can be gained to the C.S.F. by inserting a needle between the vertebral lamina below the end of the cord, a procedure termed lumbar puncture. By this means, the pressure of C.S.F. can be measured, the fluid can be analyzed, a spinal anesthetic can be introduced, or fluid can be replaced by a contrast medium for radiography (myelography). The pia mater invests the spinal cord closely, ensheathes the anterior spinal artery (as the linea splendens), and enters the anterior median fissure. Laterally, the pia forms a discontinuous longitudinal septum, the denticulate ligament, which sends about 21 tooth-like processes laterally to fuse with the arachnoid and dura on each side. The ligament is a surgical landmark in that it is attached to the spinal cord about midway between the attachments of dorsal and ventral roots. http://www.dartmouth.edu/~humananatomy/part_7/chapter_41.html
What is spinal anesthesia?
Spinal Block: A form of anesthesia where medication is administered into the spinal fluid to lessen labor pain or provide anesthesia for a Cesarean delivery. Epidural Block: Sometimes just referred to as "an epidural." A form of anesthesia where medication is administered through a catheter placed in the patient's back that lessens labor pain (analgesic) or provides pain relief for a Cesarean delivery (anesthetic).
