The path to and from the cerebral cortex is achieved through various white matter pathways coursing through the spinal cord, brainstem, and cerebral hemispheres. Beneath the gray matter of the cortical surface is an expansion of white matter referred to as the corona radiata. This white matter pathway condenses to form the internal capsule, a V-shaped structure when viewed in horizontal sections that contains axons traversing to and from various cortical and deep nuclear structures (eFig. 9.11). The internal capsule is divided into three parts based on connections to different parts of the cortex and underlying structures. The most anterior portion of this white matter pathway is the anterior limb, which is bounded medially by the head of the caudate and laterally by the globus pallidus and putamen. The anterior limb transitions into the genu (Latin for “knee”) at the level of the interventricular foramen (of Monro) and completes its course as the posterior limb, situated lateral to the thalamus and medial to the globus pallidus and putamen. In addition to this more vertical stream of axonal connections is the horizontally running corpus callosum. The corpus callosum (eFig. 9.12) is formed by myelinated axons horizontally linking the two cerebral hemispheres to one another, and it is divided into a rostrum, genu, body, and splenium (eFig. 9.12).
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.
From the lateral ventricles, CSF flows through the interventricular foramen (of Monro) to the slitlike third ventricle, which is surrounded by the thalamus and hypothalamus (eFig. 9.15). The third ventricle communicates with the fourth ventricle via the cerebral aqueduct (aqueduct of Sylvius), which courses through the midbrain (eFig. 9.15). Surrounded by the pons and medulla anteriorly and the cerebellum posteriorly, the fourth ventricle sends CSF out of the ventricular system and into the subarachnoid space via the lateral foramina of Luschka and midline foramen of Magendie (eFig. 9.15).