Mid-sagittal view of the brain and ventricular system. The medial surface of the left cerebral hemisphere is in view. It is separated from the right hemisphere by the midline falx cerebri (largely removed) occupying the interhemispheric fissure. The lower free edge of this dural fold contains the inferior sagittal sinus, which enters the straight sinus at the intersection of the falx cerebri and tentorium cerebelli. Here, the straight sinus also receives the great cerebral vein (of Galen) which, in turn, receives blood from deep structures via the internal cerebral vein. The straight sinus empties posteriorly into the confluence of sinuses. The left and right cerebral hemispheres are connected by the corpus callosum, the largest commissural structure in the brain. It consists of four parts: rostrum, genu, body, and splenium. The anterior horns of the lateral ventricles are separated by a thin, transparent membrane, the septum pellucidum. This sheet extends from the corpus callosum to the fornix. The midline third ventricle can be seen, including the massa intermedia, a small, variably present connection of glial tissue between opposing surfaces of the two thalami. The third ventricle communicates via the cerebral aqueduct with the fourth ventricle, which is bounded by the posterior surfaces of the pons and medulla and the cerebellum. From the fourth ventricle cerebrospinal fluid exits into the cisterna magna via paired lateral apertures of Luschka (not labeled) and the median aperture (foramen of Magendie) located in the inferior aspect of the roof of the fourth ventricle. Posterior to the midbrain tectum (consisting of the paired superior and inferior colliculi, the corpora quadrigemina) is the quadrigeminal cistern, a dilation of the subarachnoid space between the splenium of the corpus callosum and superior surface of the cerebellum. The pineal gland and great cerebral vein are contained within it. The midbrain tegmentum lies anterior to the cerebral aqueduct. The basilar artery courses at or near the midline of the basilar pons. Posterior to the basis pons is the pontine tegmentum. The basilar artery terminates by giving off the posterior cerebral arteries within the interpeduncular cistern. The lamina terminalis stretches upward to fill the interval between the optic chiasm and the rostrum of the corpus callosum. Anterior to this is the lamina terminalis cistern containing the anterior cerebral artery (ACA). Its A2 or postcommunicating segment extends anterior to the lamina and rostrum of the corpus callosum. The cut surface of the anterior communicating artery is also in view connecting the contralateral ACA. In addition, orbital branches of the ACA can be seen coursing on the orbital surface of the frontal lobe. Below the lamina terminalis is the stalk of the pituitary gland with the gland resting in the sella turcica just posterosuperior to the sphenoid sinus. The mammillary body is located on the inferior surface of the hypothalamus. (Image courtesy of AL Rhoton, Jr.)