The ventricular system is derived from the inner lumen of the developing neural tube. As the brain continues to grow, the caverns and canals of the ventricular system adapt to the shape of the cerebral hemispheres, diencephalon, pons, medulla, and cerebellum, which form the surrounding walls (eFig. 9.13). Inferior and lateral to the corpus callosum are two large, fluid-filled cavities that represent the beginning of the ventricular system. These most rostral cavities are the two C-shaped lateral ventricles, located within the cerebral hemispheres (eFig. 9.14). As the lateral ventricles extend through all of the lobes of the cerebral hemispheres, they are divided into five named parts. In the frontal lobe is the anterior (frontal) horn, which transitions into the body within the frontal and parietal lobes (eFig. 9.15). Projecting into the occipital lobe is the posterior (occipital) horn (eFig. 9.15). A final horn extends inferiorly and anteriorly as the inferior (temporal) horn in the temporal lobe (eFig. 9.15). Near the splenium of the corpus callosum, the body, posterior, and inferior horns come together at the atrium/trigone of the lateral ventricles (eFig. 9.15). Lining most of the ventricles is the choroid plexus (eFig. 9.16), a series of modified ependymal cells responsible for producing 0.5 L of cerebrospinal fluid (CSF) a day in adults.