Cerebrospinal fluid (CSF) is formed by capillary knots called the , which hangs inside the of the brain. Ordinarily, CSF flows from the lateral ventricles into the ventricle and then through the into the fourth ventricle. Some of the fluid continues down the of the spinal cord, but most of it circulates into the by passing through three tiny openings in the walls of the . As a rule, CSF is formed and drained back into the venous blood at the same rate. If its drainage is blocked, a condition called occurs, which results in increased pressure on the brain.