The arachnoid is a shut sac, the visceral or cerebral layer of which is, for the most part, blended with the pia mater in the same manner as serous membranes are with the tissue which lies beneath them: but in some parts, which vary in extent, it departs from this relation to the pia mater, and is thereby distinguished from serous membranes; that is to say, by being unconnected with any subjacent tissue, and by having a double smooth surface. And corresponding to this peculiar anatomical arrangement, there are various features in diseases of the inner membranes (arachnoid and pia mater), when regarded as one organic whole, by which they are distinguished from diseases of other serous membranes. It is desirable that the diseases of the arachnoid should be considered apart from those of the pia mater, because the former, especially in its parietal layer, is subject to many affections which are peculiar to itself, and because the diseases of its visceral layer also are in many respects independent of the pia mater: moreover, there appears to be less confusion in studying the relations of two things in separate and adjoining sections, than in overwhelming one section with differences and peculiarities, and speaking sometimes particularly of one, sometimes of the other, and sometimes of both.

1. Anomalies In Size

To this head belong congenital enlargements of the arachnoid; they are produced by what is called external or meningeal hydrocephalus, and are either partial (local) or general. The former are cases in which sacs of a dropsical arachnoid protrude through an aperture in the skull. I shall have to treat of these cases among the Anomalies of the contents of the arachnoid sac.

2. Diseases Of Texture

1. Hyperemia

We very rarely have an opportunity of seeing the vessels of the arachnoid in a state of congestion, or, indeed, of detecting any injection of them that is perceptible with the naked eye. But changes, nevertheless, are frequently discovered in the membrane, which can be attributed only to congestion, or to slight and passing attacks of inflammation, and which are always accompanied with changes in the pia mater of a similar nature, and arising from the same source.

These changes increase in degree with each recurrence of the congestion, but they are found to be most marked when it has become habitual. The cerebral arachnoid is the most frequently affected, especially that portion of it which covers the convexity of the cerebral hemispheres. The changes consist in opacity, thickening, and hypertrophy. Sometimes large extents of surface are pretty uniformly affected, at other times separate spots are more prominently, or are alone altered. The arachnoid tissue is opaque, dull like whey or milk, tumid and white, and it has the appearance and density of tendon. One form which the affection assumes is remarkable for its frequency; it is well known by the name of the Pacchionian bodies. They are merely a granulated form of fibroid thickening of a serous membrane. The granulations are found both singly and in groups, and when they exist in any number, they are generally situated upon an arachnoid membrane that is cloudy and thickened. By their pressure they force the fibres of the dura mater asunder, and become imbedded in it; then perforating that membrane, they occupy small pits and fossae of their own in the cranial bones, and in this manner give rise to an unnatural adhesion between the cerebral arachnoid and the dura mater. They are usually found at the margin of the hemispheres adjoining the falx, and in that situation they often perforate the wall of the superior longitudinal sinus, and project within its cavity.

Opacity and thickening of the arachnoid are very common post-mortem appearances: after middle life, a moderate degree of them is almost constantly found, and their absence is the exception; for at that period every one must have been exposed to repeated congestions of the brain and its (inner) membranes. This is especially true of the Pacchionian bodies, which, as is well known, are scarcely ever absent, even in earlier life. In persons who have suffered from frequent, intense, and protracted congestions, they are more considerable; but the most marked examples succeed the congestions produced by frequent intoxication, and repeated attacks of delirium tremens.

Moreover, the arachnoid membrane is found augmented in actual bulk, and containing within its cavity an increased quantity of serous effusion, in cases of atrophy of the brain, and, indeed, in combination with various other appearances, which are all occasioned by the tendency to a vacuum within the cranium, and will all be detailed in the sequel.