The calcarine fissure begins a quarter of an inch below the posterior end of the corpus callosum and runs backward and slightly upward to the lower end of the occipito-parietal sulcus; and then curves downward to a point near the occipital pole where it ends bifid (Figs. 31 and 34). It is thus divided by the sulcus occipito-parietalis into an anterior calcarine and a pos terior calcarine fissure. These three furrows are continuous with one another superficially in the human brain; but buried annec tant gyri actually separate them from each other: the gyrus cunei separates the occipito-parietal sulcus from the calcarine fissure and the anterior calcarine fissure is separated from the posterior calcarine by the gyrus cuneo-lingualis (Cunningham.) The anterior calcarine fissure indents the medial wall of the posterior horn of the lateral ventricle, producing the calcar avis.
Hippocampal Fissure (Figs. and 34).—A crescentic fissure, convex downward, begins under the splenium of the corpus callosum in continuity with the callosal sulcus and winds forward beneath the thalamus to within an inch of the temporal pole where it is closed by the uncus. It is the hippocampal fissure, an embryonic cleft between the hemisphere vesicle and the mesencephalon. In the floor of this fissure lie the dentate fascia, and the fimbria hippocampi and crus fornicis. Anterior to the crus and fimbria there is a deep crescentic cleft in the hemisphere, called the chorioidal fissure; behind them runs the fimbriodentate sulcus between the fimbria and the fascia dentata. The fascia dentata is separated from the hippocampal gyrus by a very superficial groove, the sulcus hippocampi (formerly called the hippocampal fissure), which is parallel with the ventricular eminence, called the hippocampus, but does not produce it. Elliot Smith has shown that the hippocampus is not an indentation of the hemisphere but a thickening of it.
The hippocampal fissure contains a considerable extension of the subarachnoid space. As it winds around the mid-brain it is named the cisterna ambiens mesencephali.
The chorioidal fissure (Figs. 34 and 45) describes about two-thirds of a circumference along the concavity of the fornix. It extends from near the foramen interventriculare backward over the thalamus; and then downward and forward along the floor of the hippocampal fissure. The chorioidal fissure is a
complete one, involving the whole hemisphere wall. A single layer of epithelium derived from the roof-plate separates it from the lateral ventricle. The pia mater, dipping into it, forms the chorioid plexus of that ventricle. The fissure is peculiar in the fact that between the inter-brain and the fornix there is a transverse slit by means of which it is continuous with the same fissure on the opposite side. In this antero superior part, which is in direct continuity with the transverse fissure of the cerebrum, is the border of the chorioid tela of the third ventricle.
Collateral Fissure.—The collateral is a long fissure (Figs. 31 and 34). It reaches from near the occipital almost to the temporal pole. It is situated below and parallel with the cal carine and hippocampal fissures, being separated from the former by the lingual gyrus and from the latter by the hippo campal gyrus. The gyrus fusiformis lies below and lateral to this fissure. Anterior to the collateral fissure, there is a small sulcus between the gyrus hippocampi and the temporal pole, called the ectorhinal sulcus, which represents a very important fissure (f. rhinalis) in osmatic animals. The collateral fissure is occasionally interrupted by two annectant gyri and divided into a temporal, an occipital and an intermediate part. This signifies a persistence of its embryonic condition. The inter mediate portion, sometimes assisted by the anterior part, indents the ventricular wall and produces the eminentia col lateralis in the inferior horn of the lateral ventricle.
The inferior temporal sulcus is usually a series of indenta tions rather than a continuous sulcus (Figs. 31 and 34). It is about equal in extent to the collateral fissure from which it is separated by the fusiform gyrus. It is parallel with the infero lateral border of the cerebral hemisphere. As the inferior tem poral gyrus, which forms this border reaches over onto the tentorial area a variable distance, even in the two sides of the same brain, the position of the inferior temporal sulcus is not constant; but it is usually one-quarter or one-half inch medial to the border.