Although there are some forms of adventitious products which are rarely seen in the brain, yet there are others which are comparatively frequent: so that this class, on the whole, supplies an average number of the diseases of the organ. Among the most frequent are tubercle and cancer.

Their importance is proportioned to their size, to the vascularity and looseness of their texture, and their consequent liability to swell, to the rapidity of their growth, to the degree in which they give rise to congestions in their own immediate neighborhood, or in the whole brain, whether it be by acting as foreign bodies, or in consequence of the change of material going on in them, etc. They lead to displacement of the brain and pressure; and thus interfering with the injection of its vessels, they produce anaemia in it; they cause congestions also, and swelling of the organ, and finally hypertrophy, oedema of the brain, and hydrocephalus, inflammation and yellow softening in the neighborhood, etc. And so death occurs, sometimes gradually, at other times in a rapid and unexpected manner.

A. Fatty Tumors

This disease is very rare in the brain: when it does occur it will be found to spring rather from the tissue of the pia mater, and lining membrane of the ventricle than from the cerebral substance. This is especially true of the genuine lipoma. Examples of encysted cholesteatoma (adipocire-like tumors) are also met with.

B. Cysts

Though these growths are rare, they are less so than the lipoma; the only form in which they occur is that of the sac of the acephalocyst. The mother-sac of acephalocysts in the brain generally has such remarkably thin coats, that it may be easily overlooked; and a single acephalocyst, when it completely fills the mother-sac, might readily be regarded as the sac itself. The cases which I have met with have all been of that kind, namely, a mother-sac, completely filled with a single acephalocyst. Other observers have seen cases in which the acephalocysts were more numerous, and different museums contain examples of them.

Neither simple nor compound cysts seem ever to be formed in the brain.

C. Fibroid Structures

Tissues of this kind, as well as newly-formed cellular tissue, are met with in various stages of development, in apoplectic and inflammatory spots, in the wall of the apoplectic cyst, and of a healing inflammatory spot, and in their cicatrices, in the membranous wall of an abscess, or of a tuberculous cavern in the brain, in the capsules of many of the adventitious products, etc.

Fibrous tumors are very seldom found in the brain: there are many cancerous growths which bear a deceptive likeness to them, and which are very commonly mistaken for fibroid growths.

D. Chalky Concretions

These concretions are found in the form of circumscribed accumulations of a dull white chalky powder, or of a yellow mortar-like, or coherent solid mass. They are generally enclosed in a cyst, which may be thin or thick. They consist essentially of the thickened and cretified contents of an abscess, of cretified tubercle, of the chalky contents of the bag of a dead cysticercus, or the like.