File:Anatomic risk zones for colloid cysts of the third ventricle.png

The third ventricle was divided into three zones along the sagittal axis. Zone I—from lamina terminalis to a vertical line drawn from the mammillary body and tangential to the massa intermedia; zone II—from the posterior boundary of zone I to the rostral apex of the midbrain tegmentum at the inlet of the cerebral aqueduct; zone III—from the posterior boundary of zone II to the posterior limit of the third ventricle. Zones I and III are referred to collectively as “risk zones.” Lesions in zone II can be large and are rarely symptomatic.