Spontaneous extravasations of blood into the sac of the arachnoid are by no means uncommon. They mostly happen on the convex surface of the hemispheres; at least, in the more extensive effusions, it is always in this situation that the largest quantity of blood is accumulated; small effusions frequently take place at the base of the skull, but larger ones occur there very rarely. They are seldom met with while in the recent state; and from their having usually existed for a long period before they are examined, the effused blood is found to have already undergone considerable changes. I will begin by depicting a well-marked example, and afterwards analyze its appearances.

Lying beneath the dura mater that covers one of the hemispheres, is found a sac or cyst, which resembles in form a flattened cylinder, somewhat curved from before backward in correspondence with the arch of the cranial vault, or resembling in shape what results from the forcible separation of two layers of a tissue by an effusion which commenced at some single point, and then spread out between them. The figure first described involves an excess in the measurement from before backward over the transverse and the vertical diameters. The sac adheres by its outer surface to the dura mater, but its inner wall is free, or nearly free, from any connection with the cerebral arachnoid, and is consequently more or less smooth, and moist. Its adhesion with the dura mater, too, is but loose; it partly sticks on, and partly is connected with the membrane by a few small vessels. Both walls of the sac are usually of a brown, rusty color, and tenacious; they may often be separated into several layers, which vary in thickness, but the inner of which are the more thin: at the margin of the sac they coalesce and form one lamina, which soon becomes reduced to a thin, brown, rusty-colored membrane, and spreading out further on the cranial vault, reaches to the base, and at length terminates in a thin, rusty-colored, gauze-like film. Loose shreds of plastic lymph hang on the inner surface of the walls, and (which is remarkable) principally on the wall which adheres to the dura mater; within these the sac contains a more or less thick fluid, of a dark and various color, like chocolate, or plum-sauce, rust, or yeast: in course of time the lymph is gradually removed, the inner surface of the sac becomes smooth and polished, and the contents are changed into a colorless, thin, clear, serous fluid. The corresponding hemisphere becomes plane or slightly hollowed, its convolutions flattened, and its ventricle narrowed, while the serum of the ventricle is forced over into that of the opposite side. The pressure of the cyst sometimes diminishes the thickness of the parietal bone.

On closer examination, the outer layers of the wall of the sac are found to be vascular. At the margin of the sac, just where its two walls unite into one stratum, numerous small trunks may be seen, sending branches and twigs both upon and into the walls, especially that wall which adjoins the brain. Here and there also other vessels pass into the convex surface of the sac from the dura mater. The walls are composed of plastic lymph, and contain various quantities of modified red particles of the blood; in their outermost layers fibrils like those of cellular tissue are being developed. The coagulation of this lymph at the periphery of the effusion, is the occasion of its becoming encysted.

These sacs are usually borne for a long time, and they do not present any proof of having diminished from their orginal size; for the impermeability of the wall of the sac materially interferes with the absorption of its fluid contents. Occasionally, however, cases do occur, in which a sac that originally was of large size, has undoubtedly become smaller, some of the fluid part of the contents having been at length absorbed, and the cavity diminished by the approximation of the two walls to each other, and by their cohesion at the margin of the sac. In some cases it has even completely closed in this manner, and is wasted.

The walls of the sac, especially that one which adheres to the dura mater, sometimes become the seat of ossification, that is, of bony concretions in the form of plates.

In a considerable number of observations, I have seen but one case in which the sac was of a cylindrical form, and filled, like a sausage, with a dark, reddish-brown plug of coagulated blood.

Some rare instances occur of large circumscribed extravasations into the arachnoid sac, in which the fibrin does not disengage itself, and coagulate at the surface, and thus enclose the extravasation; such cases present to view a rounded and irregular mass of coagulum.

Small effusions, which spread out and form a thin stratum, leave behind them a correspondingly thin, single lamina, of a rusty brown, or yeast-yellow color, which lines the dura mater. It is very often a mere film, and can be detected only by the closest examination.

According to my observations, these extravasations occur pretty often in adults, and especially during and after the best years of manhood: Barthez, Rilliez, and Legendre, have not unfrequently noticed them in young persons and children. It has been my uniform experience, that the large encysted extravasations over the hemispheres, induce a marked degree of weakness of intellect. They have much general interest, as rare examples of hemorrhage into a serous cavity. The source of the bleeding cannot usually be discovered. In persons who are advanced in life, it may, perhaps, arise from the congestions which are occasioned within the skull, by atrophy of the brain.

It should be remarked, that old effusions may be mistaken for hemorrhagic exudations; but such an error may be obviated by observing the following particulars: a. At whatever period the extravasation may be examined, no change of structure, such as accompanies inflammation, will be found in the arachnoid, even in its parietal layer, to which the extravasation is attached.

In the recent state, it is clear that the effusion is pure blood.

r. Further distinctions may be noticed, in the peculiar change of structure which has been described as taking place in the extravasated fibrin, and in the want of any intimate organic connection between the sac and the dura mater.

Lastly, it is these encysted extravasations which have hitherto, for a long time, been erroneously looked upon as extravasations between the two layers of the dura mater.