Inflammation of the brain (encephalitis, the phrenitis of old writers) is not, on the whole, a rare disease, although it is so in comparison with other diseases of the brain: it is seldom found as a primary affection. It never occupies the whole of the brain, but appears in the form of circumscribed spots, which are sometimes very extensive.

It may be acute or chronic; it may produce disorganization of the brain, and reach its terminations by a rapid or a slower course. Some of its terminations have been most improperly regarded as chronic softening, that, for instance, which Durand-Fardel calls Cellular Infiltration.

When the disease occurs in the white substance, it presents in its various stages and degrees the following anatomical characters.

a. The diseased part is injected, streaked red, and swollen: the bleeding points seen on making a section of it are more numerous than natural, and not unfrequently its surface rises above the level of the incision. At the same time, the cerebral substance is unusually moist, and is already, in some degree, loosened or softened in its texture.

b. If the congestion continue, the redness not only acquires a darker hue, but also gradually discolors almost the whole texture of the part: for, besides being reddened by the injection, it is further discolored by dots and streaks which are situated outside the vessels, and result from numerous small extravasations into the tissue. Capillary apoplexy, as it is called, is thus found associated with inflammation. The swelling, moistness, and loosening of the texture, are all increased.

c. After this, exudation takes place, and, at the same time, evident modifications of the existing redness. A slightly plastic exudation pervades the cerebral texture uniformly, and, if not mixed with many of the red particles of the blood, it alters the dark redness of the part to a perfectly uniform and somewhat paler color. Exudations, which contain a large proportion of coagulable material, are still more remarkable: their coagulable part solidifies in masses of irregular shape and various size, but it is generally found, along the course of the vessels, in streaks or stripes of a yellow and greenish color. At the same time ruptures of vessels and hemorrhages take place, which are proportioned in number and extent to the violence (tumultuousness) of the process; and the fluid portion of the exudation percolates through the adjoining tissue in the manner before described. Hence there results a coloring, which is composed of several tints, associated and blended in the most different ways: - of dark red, from blood which has been extravasated at different periods; of paler reddening from infiltration of the cerebral tissue with the fluid part of the exudation; of yellow and green from the coagulated fibrin of the exudation; and of white from an occasional piece of cerebral tissue, which has escaped the process. The prevailing color, however, is the paler red of the cerebral tissue which is pervaded uniformly by the exudation.

At a later period, when this condition has existed for some time, the red color is further modified, by the coloring matter of both the extravasated and exuded blood gradually changing to a rusty brown, or to the yellow color of yeast.

d. The process of exudation, moreover, alters the cohesion of the diseased cerebral tissue. At first, as I have remarked, it is softened, but afterwards it becomes entirely disorganized. Its texture is broken up mechanically, partly by the extravasations, but principally by the exudation; and, very probably, it is also dissolved by the exudation. This is the condition for which the usual color of the diseased spot has procured the name of "Red softening of the Brain;" but the foregoing description of it, which has been drawn from repeated investigations, leaves no question as to its inflammatory nature. The softening is more marked, the greater the quantity of aplastic matter contained in the exudation.

During the stage of exudation (c), the inflamed spot might be described, especially by the softening, but further by the following characteristics: the substance of the brain is converted into a pulp, which is red, and variously shaded with dirty violet, brown, and yellow, or of a rusty or yellow color, like yeast; fragments of healthy brain, coagulated extravasations, and coagulated fibrinous exudation are scattered through the mass.

When the gray substance is the seat of the process, the anatomical characters are essentially the same; but the great vascularity and looser cohesion of this substance give rise to more of the redness from injection in the first stage, as well as to darker red coloring at later periods: the swelling, and loosening of tissue, the moistness, and the subsequent softening are also more marked.

Upon microscopic examination, the elementary tissues of the brain are seen broken up and dissolved, and amongst them the component elements of the different extravasations and exudations are met with in every stage of their progressive changes, both of degeneration and development. Amongst the latter are found, partially dissolved blood-corpuscles, shapeless masses of red, brown, and yellow pigment, single and conglomerate nucleoli in large quantity, globules and crystals of fat, nuclei, primitive cells, etc.

This description has been taken from cases as they ordinarily occur; but inflamed portions of brain are occasionally seen presenting much that differs from it.

a. Sometimes spots are found both in the white substance and in the masses of gray matter, which would be scarcely recognized as spots of inflammation: their nature can be determined only by a close examination, and from their analogy with similar inflammatory processes in other tissues. The redness resulting from injection of the part, is scarcely perceptible; other coloring is still more deficient, and the cerebral substance is loosened and softened to a uniform dull white pulp. On minute examination the texture of the brain is perceived to be broken down, and single and conglomerate nucleoli, nuclei, and pigment-molecules are found scattered throughout it. The inflammatory process, when at a moderate degree of intensity, has led to the effusion of a product containing very little plastic material.