Cerebrospinal Fluid

Fluid that is located in the brain ventriclen and around the spinal cord in the spine and can switch between these areas. In order for drugs to get into this fluid (and thus into the brain or CNS), they must be able to cross the blood-brain barrier.

https://de.wikipedia.org/wiki/Liquor_cerebrospinalis _{(Wikipedia CC-by-sa-3.0)}