Slipped disc. Coloured water-contrast myelogram of the human spine showing a slipped disc at lower centre. A slipped disc (here between the 5th lumbar vertebra and the sacrum) places pressure on the spinal cord, causing pain. The pressure on the cord is highlighted by a decrease in the width of the contrast medium (grey/white, down centre). 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. A hypodermic needle injecting the medium can also be seen (yellow, centre left).