The ventricular system of the brain consists of two lateral ventricles and the midline 3^rd and 4^th ventricles connected by the cerebral aqueduct. CSF, largely secreted by the choroid plexuses of the ventricles, fills these brain cavities and the subarachnoid space of the brain and spinal cord.

The lateral ventricles, the 1^st and 2^nd ventricles, are the largest cavities of the ventricular system and occupy large areas of the cerebral hemispheres. Each lateral ventricle opens through an interventricular foramen into the 3^rd ventricle. The 3^rd ventricle, a slit-like cavity between the right and the left halves of the diencephalon, is continuous posteroinferiorly with the cerebral aqueduct, a narrow channel in the midbrain connecting the 3^rd and 4^th ventricles.

The pyramid-shaped 4^th ventricle in the posterior part of the pons and medulla extends inferoposteriorly. Inferiorly, it tapers to a narrow channel that continues into the cervical region of the spinal cord as the central canal. CSF drains into the subarachnoid space from the 4^th ventricle through a single median aperture and paired lateral apertures. These apertures are the only means by which CSF enters the subarachnoid space. If they are blocked, CSF accumulates and the ventricles distend, producing compression of the substance of the cerebral hemispheres.