- For a discussion about the anesthetic procedure, see Epidural.
In the spine, the epidural space is the space outside the tough membrane called the dura mater (sometimes called the "dura"), and within the spinal canal, which is formed by the surrounding vertebrae. Adherent to the inside of the dura is a much thinner and more fragile membrane, the arachnoid mater. Enclosed within the arachnoid is the subarachnoid space, which contains the cerebrospinal fluid, and the spinal cord.
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.
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.