Velum interpositum. ( Toile Choroidienne, Vicq d'Azyr.)—The choroid plexuses are con nected to each other by the velum interpositum, which is a triangular fold of pia mater that passes in at the transverse fissure between the upper surface of the tubercula quadrigemina and the posterior reflected portion of the corpus callosum. This process is continuous with the pia mater of the inferior surface of the posterior lobes of the brain, and with that of the superior surface of the cerebellum, and it therefore con sists of two laminw ; as it passes forwards, it sends downwards a little process which em braces the pineal body ; it forms the roof of the third ventricle, being interposed between that cavity and the fornix, (hence its name,) and at its sides as well as its apex its continuity These internal processes of the pia mater contain minute crystalline formations, a kind of very fine sand, which, however, is not con stantly present in all brains.
The grains are deposited in the meshes of the vascular plexuses. Sometirnes they accumulate in masses so as to be visible to the naked eye or easily recognized by the touch. In general, however, they are microscopic, in form glo bular, and connect themselves with the minute vascular ramifications like little bunches of grapes. They are found principally in the choroid plexuses of the lateral ventricles, and in that portion of the velum interpositum which embraces the pineal body. In the for mer they are most numerous at that part which was called by the Wenzels gloms, where the choroid plexus turns up from the inferior cornu into the horizontal portion of the lateml ven tricle.* As regards chemical composition this sabulous matter consists chiefly of phos phate of lime with a small proportion of phos phate of magnesia, a trace of carbonate of lime, and a small quantity of animal matter.
The pia mater adheres very closely to the surface of the brain, coming for the most part into contact with grey matter. When a portion of it is raised carefully in a fresh brain, num berless extremely minute bloodvessels are seen passing from it into the cerebral substance. These are the principal nutrient vessels of the brain. On its outside the pia mater adheres partially to the arachnoid membrane. At those points which correspond to the convex portions of the convolutions the adhesion of arachnoid to pia mater is close ; but at other places the latter membrane separates completely from the former.
The pia mater of the brain differs from that of the spinal cord in its great delicacy and tenuity ; it wants the strength and density of the latter membrane. This is owing to its being composed almost entirely of extremely minute and delicate bloodvessels whilst the spinal membrane consists chid), of white fibrous tissue. The bloodvessels of the former are in finitely more numerous than those of the latter, and the reason of this probably is that the cerebral membrane is chiefly in contact with grey matter, which requires a great quantity of blood, but the spinal membrane immediately embraces white matter, which is inuch less vascular.
It is important, in a pathological point of view, to nonce that this membrane is the me dium of nutrition, not merely to the nervous matter of the brain and cord, but also to the arachnoid membrane which is immediately adherent to it, and to which it bears the same relative position as the sub-serous areolar tissues elsewhere to their respective serous membranes. Hence the difficulty, if not the impossibility, of adopting distinctions which systematic wri ters endeavour to make out between arachnids and superficial inflammation of the brain. It is physically impossible that there shall be arachnids without serious disturbance of an inflammatory kind in the circulation of the pia mater, nor can this exist without affecting the superficial layers of the grey matter of the convolutions. It may, theretbre, be confidently affirmed that arachnitis, when affecting that portion of the arachnoid membrane which co vers the hemispheres of the brain, is synony mous with inflammation of the superficial layers of the grey matter of the convolutions. Whatever be the point of departure, it seems impossible that inflammation of the one can exist without a similar and equal affection of the other. And thus we may explain the ap parently anomalous statement of authors that inflammation of the arachnoid should give nse to a more violent train of symptoms than deep seated inflammation of the brain. The real difference is, not between membranous and cerebral inflammation, but between an inflam matory affection of the superficial grey matter of the convolutions, the great source and seat of the physiological activity of the brain, and a similar morbid action of the more central white substance, the function of which is in a certain sense subservient to that of the super ficial grey matter.