The cerebral ventricles are a series of interconnected, fluid-filled spaces that lie in the core of the forebrain and brainstem (Figure 1.17). The presence of ventricular spaces in the various subdivisions of the brain reflects the fact that the ventricles are the adult derivatives of the open space or lumen of the embryonic neural tube (see Chapter 22). Although they have no unique function, the ventricular spaces present in sections through the brain provide another useful guide to location. The largest of these spaces are the lateral ventricles (one within each of the cerebral hemispheres). These particular ventricles are best seen in frontal sections, where their ventral surface is usually defined by the basal ganglia, their dorsal surface by the corpus callosum, and their medial surface by the septum pellucidum, a membranous tissue sheet that forms part of the midline sagittal surface of the cerebral hemispheres. The third ventricle forms a narrow midline space between the right and left thalamus, and communicates with the lateral ventricles through a small opening at the anterior end of the third ventricle (called the interventricular foramen). The third ventricle is continuous caudally with the cerebral aqueduct, which runs though the midbrain. At its caudal end, the aqueduct opens into the fourth ventricle, a larger space in the dorsal pons and medulla. The fourth ventricle narrows caudally to form the central canal of the spinal cord. The ventricles are filled with cerebrospinal fluid, and the lateral, third, and fourth ventricles are the site of the choroid plexus, which produces this fluid. The cerebrospinal fluid percolates through the ventricular system and flows into the subarachnoid space through perforations in the thin covering of the fourth ventricle; it is eventually absorbed by specialized structures called arachnoid villi or granulations (see Figure 1.18), and returned to the venous circulation.