THE LATERAL VENTRICLE.
In each lateral ventricle we distinguish three outpouchings or horns, the cornu ante rius, the cornu posterius and the cornu inferius, invading the frontal, occipital and temporal lobes respectively, and the middle chief part or body, the pars centralis, uniting the three horns.
The front horn, the cornu anterius, is bounded in front, partly below and above by the fibres of the corpus callosum. The radiation of the callosal knee closes the ante rior horn in front and in addition forms a part of the floor. The medial wall and, at the same time, the partition between the two anterior cornua are contributed by the sep tum fiellucidum. The latter consists of two thin plates, the laminae sefiti pellucidi, between which lies the completely closed cavum sefili pellucidi. A part of the floor and the lateral wall of the anterior horn are formed by a gray protuberance, the corpus striatum. The thickened front part of the latter, which projects into the anterior horn, is known as the head, or caput ; passing backward, the striatum markedly narrows and, as a narrow tail-like band, the cauda corporis callosi, continues through the pars centralis into the cornu inferius, of which horn it contributes a portion of the roof.
The pars centralis is a thin horizontal cleft, roofed in by the radiation of the callosum. On the floor, laterally, is the corpus striatum ; next follows the stria fermi nalis or stria cornea. This structure forms the floor of a groove, the sulcus inlerme dius, situated between the corpus striatum and the adjoining thalamus. The stripe is called stria cornea on account of its bluish coloration, produced by the underlying vena terminalis. Medial to the stria terminalis comes a thin lamella, the lamina ajfixa, that covers the lateral part of the thalamus, to which it is attached. Farther medially, follow the plexus chorioideus ventriculi lateralis and the dorsal surface of the part of the fornix which is free and unattached to the callosum.
Regarding the plexus chorioideus, it must be especially emphasized that this struc ture, composed of pial tissue, really only seemingly lies within the lateral ventricle. As
in all other parts, so also here the ventricle is lined with ependyma which invests the choroid plexus with a thin epithelial layer, the lamina chorioidea epiIhelia1is ; the plexus lies, therefore, extraventricular. Laterally the lamina chorioidea begins at the lamina affixa ; medially it is continuous with the epithelium covering the fornix (Fig. 63). On removing the plexus chorioideus, the lamina chorioidea epithelialis is taken away with it, the epithelial layer tearing through the medial border of the lamina affixa and along the lateral border of the fornix. In these locations delicate white stripes, called taeniae, mark the lines of separation ; hence, the taenia chorioidea and the taenza fornicis are distinguished.
These taeniae, evidently, as such do not exist in the normal and undamaged brain ; they are, therefore, artefacts as are also the taenia fimbriae, taenia thalami and taenia ventricull quartz', to be described later. Their true relations are to be under stood only by reference to embryology. While the original wall of the embryonic brain-tube for the most part thickens during development and becomes nervous sub stance, in certain places, namely in the roof of the third and of the fourth ventricle and along a band-like area on the medial wall of the hemisphere, such conversion into nervous tissue never occurs, the brain-wall there being represented by only a thin epi thelial plate, the lamina chorioidea epithelialis. Where the latter joins the typical wall, the nervous substance is thinned out to a slender wedge.
The lamina chorioidea, moreover, at certain places undergoes a complicated invagi nation toward the cavity of the ventricles, accompanied by the superimposed pial tissue, the process leading to the formation of the plexus chorioidea. When later the brain-membranes are removed, as when, for example, the plexus of the lateral ventricle is taken off, the epithelial lamina is also removed and there remain only delicate linear borders, known as the taeniae, that mark the torn edges along those lines at which the brain-substance joins the epithelial plate.