circumventricular organ

[sûr′kəm-vĕn-trĭkyə-lər]
n.
  1. Any of the structures in or near the base of the brain that differ from normal brain tissue in having capillaries that lack the usual blood-brain barrier and thus are not isolated from certain compounds in the blood.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.