There is no organ in which growths of a cancerous nature occur in such number, and in such variety of internal elementary structure, as in the brain. I must refer the reader on this subject to the general remarks on cancer, and the morbid products allied to it. They are met with either infiltrated through the cerebral substance, so that the transition from diseased to healthy structure is gradual and insensible; or, as is far more common, in the form of an independent and abruptly circumscribed tumor, which is enclosed in a delicate areolar and vascular investment.

The infiltrated cancer generally involves a very considerable portion of the brain, while the tumors attain various dimensions; sometimes equalling a hen's egg in size, sometimes exceeding the fist. They are generally spherical; but when their texture is soft, they readily accommodate themselves to any firm adjoining structure; and those even which are dense and tough, when situated at the periphery of the brain, become flattened on their surface, or excavated like a navel. They sometimes advance from the brain to the membranes, and through them to the cranium.

The infiltrations sometimes render the diseased portion of brain tough and firm, like a cicatrix. (Sclerosis).

Highly vascular adventitious growths are particularly serious, from their liability to swell.

Cancer of the brain very often occurs quite alone in the organism; but medullary cancer, and melanosis, amongst others, are usually associated with cancer in other organs, or with a general development of cancer.

Lastly, there is usually but one adventitious growth in the brain; but when there are two, it is interesting to notice, that they not uncommonly occur symmetrically in corresponding portions of the brain.

G. Entozoa

The acephalocyst, with its inhabitant, the Echinococcus, and the Cysticercus cellulosae, are pretty frequently found in the brain. There are often some few only of one or the other, while in some rarer instances they abound in almost countless numbers. In the former case they are usually confined to the brain, in the latter the muscles also are often crowded with them.

They occupy the gray substance almost exclusively, selecting the peripheral much more than the central masses of it. When situated at the surface of the brain, they mostly project beyond its level, and become partially imbedded in the tissue of pia mater.

Should the animal die, the cyst becomes thickened and shrunk, and its contents inspissated, and at last chalky; so that a chalky concretion is afterwards found enclosed in a fibrous bag. It is very difficult to distinguish between this and cretified tubercle, and the diagnosis can be established only by the presence of other living animals.