With respect to the origin of the cerebro-spinal fluid and its wave of flow, the researches of Leonard Hill tend to prove that it is formed from the lining ependyma and choroid plexuses of the three main ventricles of the brain, passing thence into the spinal cerebro-spinal region and, by means of the foramina of Majendie, Key, and Retzius--apertures in the roof of the fourth ventricle--on to the surface of the brain. From this latter region there appears to be a general tendency for the fluid to pass upwards from the basal aspect of the brain towards the superior longitudinal sinus where it is again absorbed, probably through the medium of the Pacchionian bodies and lacunæ laterales of the venous sinus. [Illustration: FIG. 84. BASAL MENINGITIS SECONDARY TO TEMPORO-SPHENOIDAL ABSCESS.] From these few anatomical facts it is manifest (1) that a subarachnoid purulent effusion has every opportunity of spreading widely over the surface of the brain, and (2) that inflammation and its results in the immediate neighbourhood of the roof of the fourth ventricle will tend to impede the normal flow of cerebro-spinal fluid through the foramina in that region, thus inducing a degree of internal hydrocephalus directly proportionate to the degree of obstruction. Furthermore, it is obvious that the anatomical arrangements are entirely opposed to the possibility of providing adequate drainage in cases of general meningeal infection. It has been suggested that trephine holes should be made on either side of the skull, and that the intervening meningeal space should be washed out between the two apertures. It is, however, impossible to attain such a result, not only because of the futility of attempting to wash through the subarachnoid meshwork, but also on account of the outward bulging of the brain substance through the trephine holes and the corking up of those apertures. The pus rapidly spreads over the surface of the cerebrum and over the base of the brain, along the sheaths of the emerging cranial nerves, and down the spinal canal. Associated with vascular dilatation there is extensive leucocyte extravasation into the perivascular spaces, the course of the line of the attack being mapped out by white and yellow lines and areas of exudation. Insomuch, also, as the meningeal vessels are in direct continuity with those of the cortex itself, it follows that a well-developed case of meningitis implies a corresponding degree of cerebritis, with red or white softening of the brain substance according to the degree of hæmorrhagic extravasation.