Pair of oily-contrast myelograms of the lumbar spine showing a protruding disc between the 4th & 5th lumbar vertebrae. The spinal canal is the vertical white band: in the image at left the posterior slipped disc" is represented by the pinch in this band; in the lateral view at right it appears as a small dark patch to the left of the cord. Myelography is an X-ray technique used to examine the neural tissue within the spinal canal, through the introduction of a radio- opaque contrast medium into the sub-arachnoid space, the fluid-filled region that surrounds the spinal cord. The use of oily contrast (myodil) involves a risk of arachnoiditis."