This form of softening occurs chiefly in old persons, in whom the arteries of the brain have been more or less ossified, or in whom the vessels leading to the seat of softening have been so diseased as materially to interrupt or diminish the quantity of blood flowing to the part. It has occurred after lig,ature of the common carotid artery, as must be inferred from the numerous cases of hemiplegic para lysis after this operation, on the side opposite to that of the tied artery; and I have myself re corded a remarkable example in which it was produced throughout nearly the whole hemi sphere of the brain by the plugging of the com mon carotid artery by a dissecting aneurism.* Any condition of the arteries of the brain, or of the general system, which may impair the nutrition of the brain, is favourable to the pro duction of this form of softening. I have seen instances of it after inveterate contamination of the system by lead, in house-painters, who have had epileptic seizures before death, chemi cal examination shewing that the nervous mat ter of the brain contained lead in considerable quantity. It also occurs in persons dead with cerebral symptoms, epilepsy, coma, &c., in Bright's disease of the kidneys.
This softening frequently surrounds apoplec tic clots, and in such cases is most probably the precursor of the apoplectic effusion. Fre quently small apoplexies are found throughout a patch of softening of this kind, and sometimes the effused blood is infiltrated throughout the softened part to a great extent, and puts on an appearance which has been likened by Rostan and many others to one of the ecchymosed spots which are seen in scurvy.
A colourless softening is found in hydroce phaloid brains. This is probably due to an arrest in the nutrition of the organ, in conse quence of which that condition of it which be longs to infancy (when a much larger quantity of water enters into its cornposition than in the later periods of life, and when the brain-sub stance is naturally very soft) becomes unduly developed, and vvater accumulates in the sub stance as well as in the cavities of the brain. The softening under these circumstances is generally seen most distinctly in the thin lamellar portions of the brain, such as the corpus cal losum, the fornix, the Vieussenian valve, the anterior and middle commissures, the septum lucidum. These parts are so soft that unless the greatest care and gentleness are used in the removal of the brain they give way. The softening is not limited to these parts, although greatest in them ; it is general throughout the brain. When, however, the hydrocephalic state has been very chronic, the substance of the hemispheres becomes condensed by the pressure from within the ventricles, and the water having thus been pressed out of it, it appears of a natural consistence, or even more dense than natural.
In all .the cases of general paralysis of the insane which I have examined, the consistence of the brain generally has been considerably less than natural. There have also been marks of chronic disease of the arachnoid in various ratehes of opacity, which I am disposed to view rather as a deposit frorn an abnormal blood than as the result of what is called chronic inflammation. The softened condition of the brain is doubtless due to a similar cause, the blood yielding vitiated materials for the nutrition of the organ. In brains of this de scription the dilated and congested state of the veins, and the enlarged and lax condition of the arteries, abundantly demonstrate how sluggish had been the force by which the circulation is maintained in the capillaries, that force of at traction between the blood and the nervous matter, by which more than by any other means active nutrition is maintained.
The parts in which softening when partial is apt to occur in the bmin may be thus enume rated according to the order of their frequency —the fornix and septum lucidum, the corpus striatum and Optic thalamus, the mesocephale, the corpus callosum and other transverse com missures, the hemispheres of the brain, the cerebellum, the medulla oblongata.
ty the iViommotory or red softening.— Another form of softening of less frequent oc currence than that just described possesses very distinctive characters. It is generally pretty circumscribed in extent, of a diffused redness, most commonly of a bright hue; the consistence of the part is much diminished, and it readily breaks up under the stream of water. Nerve tubes are found in it, more or less varicose and friable, also red particles of the blood, and many of those large nucleated cells commonly known as exudation corpuscles, within which an active molecular motion may be often seen.
The red colour of this form of softening is due partly to the injection of the bloodvessels, and partly to the extravasation of the red parti cles of the blood throughout the softened part. Sometimes the red colour is absent, although the lesibn is essentially the same. In such cases the colour may be yellowish and due to the presence of a less injection of the bloodvessels and a slighter extravasation of the colouring matter of the blood. Dr. Bennet states that he has found exudation corpuscles in a softening of a brilliant white colour, a fact which seems to indicate that the products of inflammation may be present without discoloration, and that all instances of white softening ought not to be considered non-inflammatory.