"[C]onsiderable attention has been paid to the 'blood-brain barrier'. ...[I]n 1909 ...it was realized that certain substances, such as dyes, did not reach the brain. ...[T]hey arrived swiftly within every other tissue but not the brain or spinal cord. ...There was apparent restriction both to foreign substances (sucrose, insulin, penicillin) and natural ones (urea, sodium, potassium, creatinine). By comparison with muscular tissue, where there is equilibrium between blood and tissue for the injected substance within seconds or minutes, such a balance in the brain may take hours."