Lateral view of the left choroidal fissure and thalamus. A series of perpendicular cuts demonstrate the diencephalo-mesencephalic junction. Note the red nucleus, substantia nigra and superior colliculus of the midbrain, but also the medial and lateral geniculate nuclei of the thalamus (diencephalon). The first and most superior C-shaped structure seen here is the major commissural bundle, the corpus callosum (CC). The septum pellucidum is a midline partition that separates the lateral ventricles and connects to the fornix and the corpus callosum. The second C-shaped structure and major hippocampal output is the fornix (Latin for "arch"). It arcs around the thalamus and descends posterior to the anterior commissure to terminate mainly in the mammillary bodies of the hypothalamus. The choroidal fissure (CF) is a c-shaped cleft between the thalami and the fornices on each side. The CF is made of a thin layer of pia and ependyma. Embryologically, the choroid plexus (also C-shaped) "sprouted" into the lateral ventricle through the choroid fissure. The anterior choroidal artery runs in the ambient cistern and enters the temporal horn at the inferior choroidal point. This point is located anterior to the lateral geniculate nucleus and posterior to the crus cerebri medially and uncus laterally. The medial temporal lobe can be seen as a "ladder" with the following "steps" going from medial to lateral: the parahippocampal gyrus, the dentate gyrus, the fimbria, and the hippocampus. The anterior parahippocampal gyrus forms a "hook" known as the uncus. The uncus has an anterior and posterior surface and a group of small gyri. The apex of the uncus projects medially and is in close proximity to the oculomotor nerve and midbrain. Posteriorly there are small gyri overlying the infolded head of hippocampus (uncinate gyrus, band of Giacomini, and intralimbic gyrus). The semianular sulcus separates the ambiens gyrus superiorly from the semilunar gyrus. The uncal recess separates the hippocampus from the amygdala. (Image courtesy of AL Rhoton, Jr.)