The interior of the brain contains a series of cavities (Fig. 20.1). The cerebrum contains a median cavity, the third ventricle; and two lateral ventricles, one in each hemisphere. Each lateral ventricle opens into the third ventricle through an interventricular foramen.
The third ventricle is continuous, caudally, with the cerebral aqueduct which traverses the midbrain and opens into the fourth ventricle.
The fourth ventricle is situated dorsal to the pons and medulla, and ventral to the cerebellum. It communicates, inferiorly, with the central canal which traverses the lower part of the medulla and the spinal cord. The entire ventricular system is lined by an epithelial layer called the ependyma.