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The medulla spinalis and its membranes
In the spine, the epidural space (from Ancient Greek ἐπί, "on, upon" + dura mater also known as "epidural cavity", "extradural space" or "peridural space") is an anatomic space that 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 connective tissue, fatty tissue, small arteries, and a network of internal vertebral venous plexuses.
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 epidural space is known as a potential space. In rare circumstances, a torn artery (e.g. the middle meningeal artery) may cause bleeding which is sufficient to create epidural space; 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.
In other mammals
In other mammals, the relationship between the spinal canal and its contents is similar to that in humans, although many species possess a tail into which the epidural space is prolonged.