The chorioid plexus (plexus chorioideus, Figs. 44, 47 and 48) of the lateral ventricle is the vascular border of the chorioid tela of the third ventricle. It projects, laterally, from beneath the fornix and its crus through the chorioidal fissure into the floor of the central part of the ventricle and the inner wall of the inferior horn. The epithelium, above mentioned, invests it; and it borders the fornix like a ruffle. It is called chorioid plexus (chorion, a membrane) because it is membrane-like. At the junction of the central part and inferior horn of the lateral ventricle the chorioid plexus presents a large skein like mass called the glomus chorioideum (Fig. 47). The an terior choroidal artery from the internal carotid and the postero-lateral chorioidal, a branch of the posterior cerebral, supply the plexus. The former pierces the temporal lobe and enters the apex of the inferior horn of the ventricle; the latter passes in through the transverse and chorioidal fissures of the cerebrum, following the chorioid tela. The chorioidal vein the blood away. At the foramen interventricu lare, it is joined by the terminal vein of the striated body and the veins of the septum pellucidum and forms the internal cerebral vein. The internal crebral vein courses backward in the chorioid tela and unites with its fellow of the opposite side to form the great cerebral vein, proximal to which union it receives the basilar vein; and then the great cerebral vein (of Galen), uniting with the inferior sagittal sinus, forms the straight sinus.
F. W. Mott calls the chorioid plexuses the chorioid glands. The name is justified by their structure. The chorioid glands, and especially those of the lateral ventricles, are made up of numerous invaginations of the chorioid epithelial lamina taining loops of blood-vessels, arteries and veins joined by capil lary plexuses; the epithelial cells are large and granular, cubical, polygonal or pyramidal in shape and very different from the adjacent ependymal cells; and around the vessels and among the epithelial cells there are many fine nerve fibers.
The chorioid plexuses secrete the cerebrospinal fluid; and, according to the late Prof. E. E. Goldmann, they exclude many noxious substances circulating in the blood from the central nervous system.
The floor of the central part of the lateral ventricle is com pleted by the superior surface of the fornix.
The horns of the lateral ventricle are three in number; the anterior, inferior and posterior (Figs. 45, 5o and 53)• The anterior horn (cornu anterius, Figs. 46, 47 and 96) pro jects from the central part of the ventricle forward and out ward around the head of the caudate nucleus. It is the ventricle of the frontal lobe and is deep and narrow. Its boundaries are as follows: Roof—Corpus callosum (forceps minor).
Floor—Rostrum.
Anterior wall--Genu.
Inner wall—Septum pellucidum.
Outer wall—Caudate nucleus.
The posterior horn (cornu posterius, Figs. 45, 46, 47, 49, 50 and 53) is directed backward and downward in a curve concave inward, from the ventricular center into the occipital lobe; and, like. the occipital lobe, it first makes its appearance in the fifth month of embryonic life. Its extremity bends medially toward the posterior calcarine fissure, with which the horn is parallel. The anterior calcarine fissure produces the ridge along the inner wall called the calcar avis. The posterior horn is roofed over by fibers from the splenium of the corpus callosum, which turn down outside the horn and also form part of the lateral bound ary. In the lateral wall and in the roof and floor is also the optic radiation. A well-marked bundle of fibers from the splenium, forceps major, is found passing along the medial border of the roof into the occipital lobe. It produces an eminence above the calcar avis, called the bulb. The anterior extremity of the posterior horn is continuous, inferiorly, with the beginning of the inferior horn. At the junction of the two is a triangular area, the trigonum collaterale.
The inferior horn (cornu inferius, Figs. 39, 45, 49, and 53) is the ventricle of the temporal lobe. Its course is crescentic, as it follows the perimeter of the internal capsule. It first runs outward and backward from the body of the ventricle, then it turns downward, and finally it proceeds horizontally forward and inward to within an inch of the pole of the temporal lobe. In horizontal section just below the general cavity of the ventricle, the inferior horn is triangular. In that position it has a posterior wall (or floor in the horizontal part), a medial wall, and a curved antero-lateral wall (or roof in the horizontal portion) which is continuous above with the outer wall and floor of the central part of the ventricle.
The parts found in the walls of the inferior horn may be enumerated as follows: Roof (or antero-lateral warn— Inferior lamina of internal capsule, partially covered by tapetum, tail of caudate nucleus, stria terminalis and arnygdala.
Floor (or posterior wall)— Eminentia collateralis (trigonum collaterale), Hippocampus, Crus of fornix.
Inner wall (medial)— Epithelium (of hemisphere wall) covering Pulvinar, Chorioid plexus, Chorioidal fissure, and Dentate fascia.
The structures in the roof of the inferior horn have been suffi ciently described. They are easily understood when it is re called that the roof of the horn is continuous with the outer wall and floor of the central part of the ventricle; the tapetum, the internal capsule (inferior lamina), the cauda, amygdala and the stria terminalis form it.