Section of the spinal cord of a monkey at the entrance of the posterior root of a spinal nerve. Most of the root is composed of coarse medullated fibres, but there is seen on its lower aspect a fasciculus of quite fine medullated fibres (A). These enter a well-defined tract of fine fibres at the surface of the cord close to the tip of the posterior horn of grey matter. The tract is the postero-lateral tract, or tract of Lissauer (B). There is good evidence that the fine fibres of the posterior root and the tract of Lissauer convey predominantly impulses related to the sensation of pain. The photograph is at a magnification of X 350.

Original card