At several locations around the ventricles are the circumventricular organs (CVO’s), which have permeable fenestrated capillaries, with exception to the subcommisural organ. The surface area of the BBB is approximately 5000-fold greater than that of the CVO’s (Crone, 1971). The circumventricular organs are midline structures bordering the 3rd and 4th ventricles. These barrier-deficient areas are recognized as important sites for communicating with the CSF and between the brain and peripheral organs via blood-borne products. CVO's include the pineal gland, median eminence, neurohypophysis, subfornical organ, area postrema, subcommissural organ, organum vasculosum of the lamina terminalis, and the choroid plexus. The intermediate and neural lobes of the pituitary are sometimes included (Davson and Segal, 1996).