mesencephalic tegmentum

n.
  1. A part of the mesencephalon consisting of white fibers running lengthwise through gray matter from the substantia nigra to the level of the cerebral aqueduct.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.