|The medulla spinalis and its membranes|
|Gray's||subject #193 875|
- For a discussion about the anesthetic procedure, see Epidural.
In the spine, the epidural space (from Ancient Greek ἐπί, "on, upon" + dura mater, 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.
In humans 
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, 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 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.
A unique property of the epidural venous plexus is that the veins are prevented from collapsing due to external pressure because the bony spinal canal prevents that pressure being transmitted. This means that for many diving mammals, e.g. whales, when diving a large fraction of venous return to the heart takes place via the epidural space, as veins such as the vena cava may be substantially compressed by the pressure at depth.
See also