A, The uncus, a medial projection at the anterior end of the parahippocampal gyrus, has an anterior and posterior segment. The sylvian vallecula is the site where the middle cerebral artery exits the carotid cistern to enter the sylvian cistern. The collateral sulcus extends along the lateral margin of the parahippocampal gyrus, and the rhinal sulcus extends along the lateral edge of the uncus. The rhinal and collateral sulci are frequently not continuous, although they are in this case. B, Enlarged view. The posterior segment of the uncus is divided by the uncal notch into an upper and lower part. The lower part is formed by the parahippocampal gyrus and the upper part is formed predominantly by the hippocampal head. The dentate gyrus, at its anterior margin, blends into the upper part of the posterior segment. C, Medial view of the uncus directed across the sella and tentorial edge. The carotid artery and middle cerebral artery face the anterior segment of the uncus. The posterior cerebral artery courses along the medial side of the posterior segment. The anterior choroidal artery ascends as it passes backward across the anterior segment to reach the upper part of the posterior segment. The anterior choroidal artery enters, and the inferior ventricular vein exits, the choroidal fissure by passing through the inferior choroidal point located just behind the head of the hippocampus and the posterior uncal segment. The inferior ventricular vein drains the roof of the temporal horn and empties in the basal vein. D, The medial part of the parahippocampal gyrus and the lower part of the posterior uncal segment have been removed to expose the dentate gyrus and the choroidal fissure. The beaded dentate gyrus blends into the posterior edge of the upper part of the posterior uncal segment and the medial side of the hippocampal head. E, Additional parahippocampal gyrus has been removed while preserving the dentate gyrus to expose the choroid plexus in the temporal horn. The amygdala, partially enclosed in the anterior segment, forms the anterior wall of the temporal horn. F, The part of the parahippocampal gyrus, lateral to the dentate gyrus, has been removed to expose the roof of the temporal horn, which is formed by the tapetum, the thin layer of fibers from the corpus callosum that separate the optic radiation from the wall of the temporal horn. The dentate gyrus and fimbria have been preserved. The choroid plexus is attached along the choroidal fissure located between the fimbria and lower margin of the thalamus. The inferior choroidal point at the lower end of the choroidal fissure is located behind the posterior uncal segment and the hippocampal head. The dentate gyrus and fimbria extend along the lateral edge of the lateral geniculate body. (Images courtesy of AL Rhoton, Jr.)