The whole brain tissue is enveloped by a lipid layer called blood-brain barrier (BBB). The endothelial cells of brain are connected together by tight junctions, which form a barrier between the vasculature and brain. If we remove this barrier, molecules can directly enter the brain, which causes brain damage. The function of BBB is to restrict the entry of various substances and selectively allow the passage of nutrients into the brain and thus provides protection against toxins.
The BBB also restrict the entry of drugs into the brain, which is the main obstacle for the drugs that need to act on brain.
Thus, the blanks can be filled with blood-brain barrier; brain.