Central nervous system disorders

II/1.4: The brain’s basal surface

On the basis of the frontal lobe is the straight gyrus along the longitudinal cerebral fissure in the midline, and the orbit(ofront)al gyri laterally from it. Laterally, the straight gyrus is separated from the above by the olfactory sulcus. On brains that were conserved by formaldehyde in situ and taken out later, an impression caused by the orbital roof is also seen.

At the lateral end of the temporal lobe the inferior temporal gyrus turns to the basis, and further back it is called lateral occipitotemporal gyrus. The two are partly separated by a varying groove which is often missing. More medially, the medial occipitotemporal gyrus is seen, and – similarly to the former gyrus – it reaches the basis of the occipital lobe without a sharp border. The two convolutions are separated by the occipitotemporal sulcus. Medially from the medial occipitotemporal gyrus is a longitudinal convolution called parahippocampal gyrus. They are separated by the collateral sulcus. This also has a varying appearance and is often intermitted by additional convolution parts, mainly on the frontal part. This groove creates the swelling called collateral eminence in the lateral cerebral ventricle’s lower (temporal) horn. The parahippocampal gyrus reaches from the basal to the medial surface of the lobe (this is often called ’mesial’ surface in practice), and its anterior part which has a hook shaped curve is called the uncus of the parahippocamppal gyrus.

As we saw before, the occipital lobe doesn’t separate sharply from the temporal lobe, and the temporal gyri continue here with no definitive border. Only an arbitrary border can be made by connecting the parietooccipital sulcus on the medial surface and preoccipital notch on the lower part of the convexity. The small lingual gyrus is seen medially from the posterior end of the occipitotemporal gyrus. In front of, and medially from the lingual gyrus, the posterior part of the parahippocampal gyrus and the collateral sulcus extends a little bit and therefore they form the collateral trigone in the lateral ventricle’s occipital horn.