White matter is tissue found in the brain. It contains nerve fibers. Many of these nerve fibers (axons) are surrounded by a type of fat called myelin. The myelin gives the white matter it's color. Myelin acts as an insulator. It plays an important role in the speed of nerve signaling.
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David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, David R. Eltz, Stephanie Slon, and Nissi Wang.