The condition of the pia mater subjacent to an inflamed arachnoid membrane, produces sundry peculiarities in inflammation of the cerebral layer of the arachnoid.

In the first place, inflammations of the parietal layer of the arachnoid are, on the whole, of frequent occurrence; their pathological import, however, is mostly subordinate, for they are a secondary appearance, or, as it were, a complementary localization of the general disease; they occur in the course of processes attended with extensive exudations, particularly of those which take place on serous membranes; they are met with also, in cases of what is called a phlogistic state of the blood, and in pyaemia, in the course of acute exanthemata, of Bright's disease, of acute biliary dyscrasia, etc.; and they are commonly slight in degree. The inner surface of the dura mater appears streaked with delicate vessels, and is of a clear, rosy red tint; it is lined with an exudation, that may be delicate, grayish, and soft, like a layer of mucus, or more consistent, and membranous, or yellow, loose, and puriform.

In some rarer cases, the inflammation of the parietal layer has all the appearance of being primary, and, judging from the amount of its products severe also. The inner surface of the dura mater is found lined with a false membrane, from half a line to several lines in thickness; it is of cellular structure, and is mostly remarkable for its vascularity, and for a corresponding degree of redness. It is attached to the dura mater by means of its vessels. Sometimes it contains yellow cheeselike masses of disorganized fibrin, which vary in size, and in shape are rounded or irregular, and branching and coalescing. Still more rarely, the exudation becomes a dense, fibroid membrane, in which plates of bony concretion are developed, as so often occurs in false membranes on the pleura.

These processes usually take place, and furnish their products without being accompanied by any similar disease in the cerebral layer: even in the intense primary inflammation mentioned last, the change which takes place in the cerebral layer, is limited to cloudiness and thickening; the false membrane very rarely produces any adhesion between the two surfaces.

Inflammations of the cerebral layer of the arachnoid membrane present peculiar characters in respect to the condition of the pia mater. We find, on the one hand, that, as arachnitis is usually not fatal of itself, or at least not in an early stage, it sometimes leaves traces of its previous existence, in pretty extensive thickenings of the membrane, in free exudations on its surface, which become converted into circumscribed tendinous patches, or diffused false membranes, etc.; whilst very trifling changes are discoverable in the pia mater, to indicate that an inflammatory process occurred at the same time in it. When, on the other hand, the pia mater is acutely inflamed, and there is profuse exudation into its tissue, the superjacent arachnoid is in no marked degree affected, and its surface is entirely without any free exudation.

The products and consequences of these processes are, considerable thickenings of the cerebral arachnoid, adherent exudations of areolar or dense fibroid texture, which may be smooth, or are granulated like clusters of Pacchionian bodies, insulated shreds of false membrane, or broader and more extended membranes of the same nature, and adhesion of the cerebral arachnoid to its parietal layer. At a later period, plates of bone are sometimes formed in these exudations.

Purulent exudation on the free surface of the arachnoid takes place on the parietal layer, only when the dura mater is very acutely inflamed in consequence of injury of the skull, and caries; and on the cerebral layer, only when a simultaneous acute inflammation of the pia mater also gives rise to an exudation of pus.

It is remarkable that true tubercular exudations do not occur in this membrane.

On whichever layer of the arachnoid these processes take place, that portion of it is exclusively affected which corresponds to the convexity of the hemispheres; and, in proportion as they approach the base of the brain (which they occasionally do), the intensity of the processes, and the quantity of their products, is palpably diminished.