Midsagittal perspective of deep brain structures. Septal veins along the septum pellucidum form the center of this dissection as they drain to the internal cerebral veins, then join the basal veins and precentral cerebellar vein to form the great cerebral vein of Galen just inferior to the splenium of the corpus callosum. The thalamus forms the lateral wall of the third ventricle, and the hypothalamus is present at the floor, with the anterior commissure, lamina terminalis, and pituitary infundibulum and its recess anteriorly. Anterior circulation arteries including the middle cerebral artery and paired anterior cerebral arteries are visible here as they wrap around the corpus callosum medial to the cingulate gyrus to supply the frontal lobes. Thalamic perforating arteries arise from the P1 segment of the PCA to supply the thalamus. A cross-section of the left midbrain demonstrates the substantia nigra (pars compacta posteriorly, pars reticulata anteriorly) just anterior to the red nucleus within the tegmentum of the midbrain. (Image courtesy of AL Rhoton, Jr.)