The plexus chorioideus ventriculi lateralis passes forward, becoming more deeply placed, toward the Anterior cornu. Here is found the Y-shaped foramen interventricu lare Monroi, which connects the two lateral ventricles with each other and with the third ventricle. Behind, the choroid plexus continues outward and downward into the inferior cornu.
The hind horn, the cornu posterius, forms a narrowing cleft of variable length, with convex lateral and concave medial arched walls. The lateral superior wall is formed by the fibre-radiation of the corpus callosum ; the remaining boundaries are contributed by the medullary portion of the occipital lobe. On the medial wall, usually two longi tudinal ridges project into the ventricle. The upper and less constant ridge is the bulbus cornu posterius, and is due to the laterally arching callosal fibres—the forceps posterior, which here curve around the deeply incising fissura parieto-occipitalis. The lower and constant ridge is the calcar avis and owes its existence to the deep penetra tion of the fissura calcarina.
The lower horn, the cornu inferius, curves downward and far forward in the temporal lobe, to end blindly before reaching the tip. The roof is formed laterally by the callosal radiation known as the tafietum, medially by the cauda corporis striati and the stria terminalis. The floor exhibits the eminentia collateralis, a longitudinal ridge produced by the deep invagination of the fissura collateralis. Behind, toward the pos terior horn, the eminence continues into a triangular, slightly convex field, the trigonum collaterale. The medial wall of the inferior horn is occupied by a remarkable semilunar curved protuberance, the hippocampus or cornu Amman is, for whose production the deep fissura hippocampi is responsible. It begins behind the pars centralis or body of the lateral ventricle, in advance of the front end of the calcar avis, and extends in a laterally convex curve downward and forward. Towards the anterior extremity of the inferior horn, the hippocampus broadens and then ends in several claw-like elevations, the digitationes hippocampi, which vary in development, in some cases being merely suggested, while in others they may number four or five or even seven. The indentations lying between the digitations are called the sulci interdigitales. The marginal portion of the fornix, unattached to the corpus callosum, the dorsal surface of which has been mentioned in relation with the pars centralis of the lateral ventricle, continues backward and outwards; it accompanies the hippocampus medially into the inferior horn. The plexus chorioideus ventriculi lateralis, which is directly prolonged from the pars centralis into the inferior horn, where it forms part of the medial boundary, is especially well developed—glomus chorioideum—at the juncture of the pars centralis and the inferior cornu. If the plexus be separated from the fimbria, a thin lamina, the taenia fimbriae, remains. At its front end, the wall of the inferior horn constitutes a delicate occluding lamella, which is clothed with the ependyma and termed the velum terminale of Aeby. The fornix and the hippocampus now merit closer examination (Figs. 44, 45 and 46).
The fornix represents a paired structure, that extends in a bold curve from the uncus gyri hippocampi as far as the corpora mamillaria. From the inferior horn of the
lateral ventricle on each side, the fimbria, at first narrow, extends backward toward the splenium and here passes into the posterior limit of the fornix, the crus fornicis, which runs forward beneath the callosum. The two crura fornicis with the under surface of the corpus callosum form an equilateral triangle, whose apex is directed forward. The two limbs of this triangle are connected by strands of fibres running crosswise and constituting the fornix transversus or commissura hippocampi. The entire triangular fibre-plate, also designated as the psa/terium or Lyra Davidis, is often separated from the under surface of the callosum by a small cleft, the cavurn psalterii, sometimes mis leadingly called Verga's ventricle.
The crura of the fornix, which curve around the posterior parts of the thalami and pass toward the under surface of the callosum, by their union form the corpus fornicis. In its posterior part, the body is attached to the corpus callosum, while it extends as far forward as the vicinity of the foramen interventriculare. The under surface of the fornix exhibits a median groove, the sulcus mcdianus fornicis. In front, the corpus fornicis divides into two anterior columns, the columnac fornicis, which, as white cylindrical cords, sweep downward in forwardly directed curves, in advance of the thalami and behind the commissura anterior, and on each side disappear in the hypothalamic region. They contribute the anterior boundary of the foramen interventriculare and eventually end in the corpora mamillaria.
The hippocampus or corriu Ammonis is, as already mentioned, produced by the deeply invaginating fissura hippocampi. These relations are best understood by examining a frontal section passing immediately behind the uncus gyri hippocampi (Figs. 45 and 46). It will be seen, that the entire cortical formation is pushed in toward the ventricle by the penetration of the hippocampal fissure, thereby producing, in a sense, an almost completely closed hollow cylinder, in which lies the gyrus dentatus as a gray cord. The upper end of the scrolled plate bends sharply outward and terminates as a thin lamella. This invaginated cortex, protruding into the ventricle, is the hippo campus. Since the latter at the same time overlies the gyrus hippocampi, this con volution is also called the lum cornu Ammonis. The white fibre-layer covering the ventricular surface of the invaginated cortex is the alveus. At the sharp outward bend of the cortical plate, the alveus becomes continuous with the fimbria.
On following the entire structure backward toward the splenium (best accomplished by making several consecutive vertical sections behind the uncus), it will be seen that the cortical formation of the gyrus hippocampi passes over into the cortex of the isthmus gyri fornicati and, farther along, into that of the gyrus cinguli. The gyrus dentatus separates from the fimbria and, as the fasciola cinerea, passes around the splenium to continue over the corpus callosum as the induseum. Alveus and fimbria are prolonged into the fornix, the alveus going into the medial and the fimbria into the lateral part of the fornix.