Tubercle is the most common of the adventitious products in the brain, and it is frequent in comparison with other diseases. Tuberculosis presents several peculiarities in the brain.

The number of tubercles is usually very small, one or two being met with in most cases, and more rarely three, four, five, or a few more: some extremely rare exceptions do occur, in which twenty and more are found. When there is a small number of tubercles, each separate one acquires a considerable size; but when they are more numerous, no single one becomes much larger than a hemp-seed or pea. There is also a further difference between tubercles in the brain and those in other organs, that the former are usually found not aggregated together, but widely apart from each other.

Tubercle in the brain generally attains a considerable size, and is larger in proportion as the number of deposits is small. Its volume varies from that of a millet-seed to that of a hen's egg, or more; the most usual size is that of a hazel-nut or walnut.

It appears, therefore, that tubercle in the brain presents considerable difference, in respect to the size and number of the deposits, from the same disease in other organs, especially the lungs. These conditions of its development correspond most nearly with those by which tubercle is developed in the lymphatic glands, - with which, moreover, it is often found combined.

There is no peculiarity in its form, by which tubercle of the brain can be distinguished from large tubercular masses in other organs. It may be round, or may assume the most irregular, branched, or lobular shapes.

Every part of the brain is occasionally the seat of tubercle. It is very common in the cerebrum, and less so in the cerebellum; while it is rarely found in the pons, and still more rarely in the medulla oblongata. As a general rule, it is deposited in or near gray substance: its usual situation, therefore, is the cerebrum, either near the periphery, or more deeply amongst the gray portions of the corpora striata and optic tha-lami. The corpus callosum, fornix, septum lucidum, and crura, scarcely ever contain any.

Tubercle of the brain, when it is situated near the surface, and is of large size, frequently encroaches on the superficial layer of cerebral substance, breaks through it, and fixes in the tissue of the pia mater. By giving rise to exudations in this membrane and the arachnoid, it sometimes becomes adherent to the inner surface of the dura mater, and though in reality originating in the brain, it is then very liable to be looked upon as tubercle of the membranes.

The usual condition in which tubercle is found in the brain, is that of an adventitious mass of the size and form already mentioned, of a yellow or yellowish-green color, of the consistence of lard or cheese, and firm, but easily lacerable; when situated deeply in the brain it is often fissured, and spreads out in various directions. The greatest part of the mass, and sometimes apparently the whole of it, are thus constituted. It is generally, howener, surrounded by a considerable layer of pale reddish vascular substance, which is also firm, and which when minutely examined, is found to consist of nuclei, of cells in different stages of development, and isolated shreds of cerebral tissue. In some parts it often appears paler than elsewhere, of a dull reddish-white or white color, dense and resistant like a cicatrix, and in fact, in course of conversion into a fibroid callus. Outside this layer an extremely delicate, moist, and jelly-like cellular structure connects the tubercle with the surrounding cerebral tissue; but its texture is so slight that the whole morbid mass may be easily loosened and turned out of the brain. This stratum further contains, scattered mostly through its inner part, some small gray or grayish-yellow tubercles, which occasionally unite with the great central mass. Although this cellular stratum surrounding genuine tubercle in the brain, is sometimes very thin, yet it is scarcely ever absent; it is the product of a moderate inflammatory process tending to induration, and corresponds to the like process which occurs around large masses of tubercle in other tissues.

As tubercle in the brain is generally discovered in the form depicted above, bearing marks of a previous process of softening, it may be asked, whether it has no crude stage in that organ, that is, a stage of gray translucent granulation, or whether it is at once secreted as the yellow lardaceous, or cheesy tubercle. There are some rare cases which prove that tubercle in the brain does, in part at least, commence in the gray, translucid form, for portions of a tubercular mass are sometimes found in that state. In any case, however, it may continue for a short period only in that form, and soon pass into the stage of the yellow cheesy tubercle Tubercle is usually fatal when it occurs in the brain, before the further changes of which it is susceptible, viz., softening and cretification, can occur. The former, however, is not extremely rare. When the tubercle is entirely softened, its place is found occupied by a cavern, mostly of a spherical form, which is enclosed in the peripheral vascular reddish stratum before described, and has the characters of an encysted abscess. The interior of this stratum is in a state of suppuration; and sometimes secondary deposits of tubercle, which may also soften, are found in it. As, after some time, the early characters of the tubercular matter may disappear, it is often very difficult to distinguish the abscess which has originated in tubercle, from that which has resulted from inflammation; and it is only by the concurrence of tuberculosis in other parts, and more especially in the brain, that the diagnosis can be rendered certain. Cretification of a tubercle in the brain is one of the most uncommon metamorphoses which tubercle ever undergoes; but yet it does occur in some cases, not only in one, but even in several extensive tubercles.

Tuberculosis is a chronic affection in the brain. An acute form of the disease is unknown in that organ.

Brain-tubercle, as well as tubercular abscess, may exist for a long time before the changes which it occasions, either in the whole brain, or in the adjoining cerebral substance lead slowly or rapidly to a fatal termination. A general description of these changes is given in the Introduction.

It must also be remarked that death may ensue from inflammation of the membranes with tubercular exudation, and from acute tuberculosis of the membranes at the base of the cerebrum, combined with acute hydrocephalus. Both these are processes which originate in tuberculosis of the brain.

Tubercle in the brain, like tubercle generally, may occur at any period of life; but it is notorious that in this situation it is especially a disease of childhood and youth. In some very rare cases it is the only instance of tubercle in the organism, but, as a rule, it is combined with tuberculosis of other structures. Its most frequent combination is with tuberculosis of the absorbent glands, which prevails most at the same period of life. Next in order of frequency to the glands, is its association with tubercle in the lungs.