Cysts, and lipomatous tumors, are rarely formed in the arachnoid; but both they and the fibroid growths, ascribed to the dura mater (p. 250), when they occur, may sometimes belong rather to the parietal layer of the arachnoid.

a. Besides them, concretions of cholesterine, cholesteatoma may be mentioned here. I have repeatedly met with them in the arachnoid at the base of the brain, forming aggregations of delicate white scales, that shine like tendon or asbestos, and are as large as a bean or a hazel-nut, or of still larger accumulations enclosed within epithelial cysts. In one case they were interwoven with extremely fine (microscopic) hair.

b. Fibroid tissue is developed in the diffused and circumscribed opaque thickenings of the cerebral arachnoid; in the instance of the Pacchionian bodies it constitutes a granular form of thickening of the membrane.

E. Bony Formations

Independently of the concretions of bone, which are found in the walls of encysted extravasations of blood, and in fibroid exudations, osseous growths are also developed in this membrane. They are commonly known as ossifications of the dura mater, but in my opinion they appertain rather to the arachnoid. I gather this both from direct examination, and from the fact, that they occur also on the cerebral arachnoid, and on the free layer of the spinal arachnoid. They are almost always situated on the parietal layer of arachnoid lining the dura mater, and much more rarely on its cerebral layer. They occupy the falx usually, the convex part over the hemispheres, and the tentorium very seldom, other parts almost never. Their form is mostly that of plates, as broad as a lentil or a zwanziger1 piece; their attached surface is smooth and shining, on their free surface they are convex, uneven, and nodulated; their thickness is greatest in the middle, and amounts to two or three lines, their edges are bevelled, irregular, and indented; they are sometimes of a reddish or bluish-red color, sometimes yellowish-white, or white, like a compact bone. They often resemble needles lying singly or joined in groups together; and in this form particularly they are found lying beside the vessels of the falx, or in that part of the dura mater over the hemispheres which adjoins the convex margin of the falx. They may easily be separated from the dura mater; and under the larger plates that membrane appears atrophied.

Bone is rarely formed upon the cerebral layer of the arachnoid; when it is found there, it is almost always over the convexity of the hemispheres, and in the shape of plates, which are smooth on their free surface, and rough on that by which they are attached.

The so-called ossifications of the dura mater are met with mostly in advanced life; their essential importance is far less than that which is usually attributed to them, and they become still more insignificant in proportion as the atrophy, to which the brain is liable at this period of life, increases. As a general rule, they are certainly of moment, when found before the thirtieth year of life. They very commonly coexist with hyperostosis of the cranium, thickening of the vitreous table, especially near the forehead, with adhesions between the dura mater and the skull, cloudiness, and thickening of the inner membranes, etc.

They are composed of true bone, and usually have also a very compact texture.

In some very rare cases bony formations of other kinds are found scattered over various parts of the inner surface of the dura mater, and the opposite layer of the arachnoid. They are crumbling or firm concretions of a reddish, or a white color, and resembling mortar, and are most probably cretified fibrinous exudations.

D. Cancers

Adventitious growths which belong to the present section, frequently occur on the inner surface of the dura mater; they are quite remarkable for the variety of their external appearance and of their elementary structure. Thus, in regard to the former particular, we meet with delicate villous, vascular, fungous growths, with thoroughly encephaloid formations, with tumors minutely divided into acini, like some glands, with lobulated masses variously streaked with fibres, and so forth.

1 [The third part of a florin, and of about the size of an English shilling. - Ed].

E. Tubercle

The rare occurrence of exudations to any amount on the free surface of the arachnoid, while they are quite common in the tissue of the pia mater, probably explains why the arachnoid, unlike other serous membranes, should scarcely ever be the seat of tubercular deposit, - why meningeal tuberculosis in every form is restricted to the deposition of tubercle in the tissue of the pia mater.