It is very frequent, and is, of course, one of the most important of all the tuberculoses. It is met with as a chronic disease, but more frequently it assumes the forms of acute tuberculosis, and of meningitis, with tubercular exudation.

A. Chronic Tuberculosis

In this case the tubercle is found in the form of gray granulations, which, sooner or later, become yellow tubercle. The granules are always grouped or clustered together. They are confined to no particular region. They are constantly combined with tuberculosis of other organs, and they commonly form the starting-point for meningitis and acute tuberculosis of the pia mater. I have never met with complete softening or suppuration of this form of tubercle.

B. Acute Tuberculosis

Tubercle of this kind appears in the form of the finest granules, scarcely as large as poppy-seed; it may be grayish and opaque, or quite clear and pellucid, like a vesicle. More rarely it assumes the form of larger, miliary, and separate granulations. It needs not merely familiarity with their appearance, but also a close search of the pia mater in order to find them. The congestion to which they owe their origin is often no longer discernible on the dead subject; but they remain always in large numbers, and always occupy a large section, and not unfrequently, indeed, the whole cerebral part of the pia mater: while, together with them, and as the product of the same congestion, a considerable quantity of serous, sero-albuminous, and more or less turbid exudation are found at the base of the brain, within and about the circle of Willis.

The base of the brain is the chief seat of this form of tuberculosis: from thence it extends towards and over the hemispheres; it is rare to find the convex surface of the hemispheres the principal seat of its development. With a little attention the granules may Be easily discovered at the base of the brain by removing the bridge of arachnoid, but it requires the closest search to detect them in the pia mater, covering the convexity of the hemispheres: in this situation they are clustered in the intervals between the convolutions, and are very often further concealed by a good deal of congestion of the membrane.

The amount of the accompanying (acute) hydrocephalus and oedema of the cerebral substance bears a direct relation to the extent to which this form of tuberculosis is developed. It is most frequent in children and young persons, but it may occur at any period of life. It is very rarely the primary disease, but almost always depends upon some previous tuberculosis, either chronic tuberculosis of the pia mater, tubercle of the brain, or the like disease of the lymphatic system, or lungs. Not unfrequently it forms part of a general acute tuberculosis which has attacked the most different organs and tissues, either all at once, or quickly one after another.

c. Meningitis, with tubercular exudation, so called tubercular meningitis. This is the second form of meningitis with a fibrinous, yellow, granular product, which was described at p. 262. Sometimes it is combined with an acute exudation of gray granular tubercle (acute tuberculosis). Sometimes also it appears in the form of isolated spots of inflammation of the pia mater covering the convex surface of the hemispheres (described at p. 264). In both forms it has the same combinations and starting-point, as the acute tuberculosis.

It is remarkable that the pia mater of the cerebellum is very rarely the seat of tubercle.

On one occasion I met with a tumor in the pia mater consisting of a cavernous tissue; it was a specimen of teleangiectasis (splenic naevus, Aftermilz), and was situated at the upper part of the left cerebral hemisphere.