hair cast

n.
  1. A small, nodular accretion of epithelial cells and keratinous debris, appearing along the shaft of a hair and resulting from failure of the internal root sheath to disintegrate.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.