Deep within the white matter of the cerebral hemispheres are cavities filled with cerebrospinal fluid that form the ventricular system. These cavities include a pair of C-shaped lateral ventricles with anterior, inferior, and posterior “horns” protruding into the frontal, temporal, and occipital lobes, respectively. Most of the cerebrospinal fluid is produced in the ventricles, and about 70 percent of it is secreted by the choroid plexus, a collection of blood vessels in the walls of the lateral ventricles. The fluid drains via interventricular foramina, or openings, into a slitlike third ventricle, which, situated along the midline of the brain, separates the symmetrical halves of the thalamus and hypothalamus. From there the fluid passes through the cerebral aqueduct in the midbrain and into the fourth ventricle in the hindbrain. Openings in the fourth ventricle permit cerebrospinal fluid to enter subarachnoid spaces surrounding both the brain and the spinal cord.

structures of the human brainSagittal section of the human brain, showing structures of the cerebellum, brainstem, and cerebral ventricles.