Animal experiments have confirmed the necessity of delivering hormones into the ventricle: the powerful hormone endorphin does not change behavior if it is given intravenously; vasopressin will improve memory in animals only if it given into the ventricles; insulin given through the bloodstream does not control apetite but it is the best hormone for such control if given into the brain; and bombesin... only stops stomach ulceration if given into the brain.
The Fabric of Mind (1985)