These are the appearances in a well-marked case; but there are different degrees of the affection as may be seen in the immediate neighborhood of a part which is thoroughly disorganized. They are distinguished by the different amounts of discoloration, i. e. of saturation with color, and by the degree in which the texture is infiltered and disorganized.

In slight degrees of the disease, the portion of brain affected is of a dull white color, inclining to yellow, and is evidently moister and softer than natural.

When it has reached a higher degree, the moistening and softening are more considerable; but a further circumstance may be noticed, which is important, both generally and specially: it refers to the degree in which the cerebral texture is disorganized. When a section of the part is made, or portions of it are squeezed in the course of the examination, a considerable quantity of clear, or nearly clear, yellow, thin fluid gradually oozes out; or it may collect in fissures, which form spontaneously: it leaves behind large coherent masses of softened, but still of white cerebral substance.

The highest degree is characterized by the complete or saturated yellow discoloring of the diseased part, and by its containing the briny pulp before alluded to. Even under the most favorable circumstances, the yellow fluid yielded by this pulp does not strain off from the cerebral mass, it always brings away a sediment composed of delicate shreds of broken-down cerebral tissue.

There is neither vascularity nor reddening in or around the seat of disease, though sometimes, indeed, small extravasations give a dotted or streaked appearance of redness to the part (ecchymosis).

The presence of a spot of softening occasions more or less turgescence of the diseased portion of the brain, or of the whole organ. The swelling is due partly to displacement, and partly to congestions or oedema of the brain. It is remarkable how much swelling of the brain yellow softening produces. Spots of the size of half a cubic inch, or of a nut, give rise to quite a disproportionate turgescence. Though this may depend in great part upon cedematous infiltration of the cerebral mass, yet the other cause, viz. the displacement, is not to be overlooked; for the much greater space occupied by the softened portion than by that which is healthy, is quite palpable. How great this enlargement is, and the tendency of the diseased portion of the brain to expand, is manifest also from the appearances observed when a section of the part is made.

The most common seat of yellow softening is the cerebrum; it is less usual in the cerebellum, and is scarcely ever found in the pons and the other structures at the base of the brain. It attacks both the fibrous structure and the central masses of gray matter, but it is extremely rare in the periphery of the brain, and when it does occur there, is almost always secondary. There is no essential anatomical difference between yellow softening of the gray matter and that of the white.

The affected spot varies in size from that of a bean to that of a walnut or a hen's egg: I have as yet seen no examples of primary and uncomplicated yellow softening which exceeded those dimensions. There is generally only one spot, and its shape is round.

Yellow softening is met with both as a primitive idiopathic disease, and as secondary and symptomatic. The foregoing remarks apply principally to the former kind. The latter is the more frequent. It accompanies inflammation, hemorrhage, and adventitious products of the most different kinds in the brain, affecting the cerebral substance immediately around them. In cases of inflammation, it not only encircles the diseased spot, but is seen also at different parts in its interior; for the portions of cerebral tissue which sometimes remain uninjured within large areas of inflammation may afterwards be attacked with yellow softening.

It is sometimes noticed in the neighborhood of the ventricles, forming the central spot from whence acute oedema radiates in cases of acute hydrocephalus.

The cerebral substance immediately around an apoplectic clot, or adventitious product, is occasionally found in a state of yellow softening; but far more commonly there is an interspace of inflamed tissue (red softening) between the central disease and the yellow softening.

The diseases of the brain which have just been mentioned are the usual pre-existing and the most important diseases with which yellow softening is found combined.

Yellow softening appears to be always and rapidly fatal: and, in those diseases of the brain which have been mentioned, it is in fact the immediate cause of the fatal result. According to my observations it runs a rapid course, especially in its secondary form, though no doubt cases of idiopathic yellow softening may last for a longer time. There does not appear, a priori, to be any reason why these spots should not heal; they might do so in the same manner as the apoplectic cyst, or inflammation; but, as yet, we have no facts to prove it.

Yellow softening, primary as well as secondary, may occur at any period of life; but, under both its forms, it is most common in middle and advanced age.

Its nature is still quite problematical.

It has been remarked above, that in a slight degree of softening, a thin yellow fluid oozes from the diseased portion of brain. This fluid, on minute investigation, presents a number of broken, extremely varicose, primary tubes, with their contents, of swollen blood-globules, very transparent fat-globules of various size, and some yellow amorphous pigment. I have repeatedly tested the fluid, and been convinced that it is somewhat intensely acid.

The first question which arises is, whether yellow softening be of inflammatory origin. The following points must be considered in the attempt to solve it.

a. At no stage of the primary form of the disease, from the earliest period at which it can be recognized as yellow softening, is any injection or reddening perceptible; nor is there any vascularity worthy of notice in the parts adjoining or more distant from it, if we except the congestions, which, from the existing oedema, we may infer to have previously taken place.