The destruction of brain, and the loss of its substance resulting from the inflammatory process, give rise to a remarkable consequence. The nerve-fibres which are involved in the inflamed spot become atrophied, more or less plainly, according to circumstances. And, after repeated attacks of inflammation, the whole brain becomes also tough and wasted, in the same manner as after the frequent occurrence of cerebral hemorrhage.

Encephalitis may be a spontaneous disease, or it may come on as a traumatic affection from a wound, or concussion of the brain. In the former case, it is sometimes the primary disease; but very frequently it is secondary, or consecutive. Thus it is often developed in the neighborhood of malignant deposits, around the bed of hydatids, or about foreign bodies which have lain for an indefinite period in the brain without producing injury, etc. When it happens as the primary disease, it is very frequent in persons advanced in life, and in the aged; it is, however, far from being peculiar to them; no period of life, not even childhood, is exempt from it, and I have unquestionably observed it in the immature foetus. It seems to depend, principally, upon a liability to passive congestions, for it is common in old and much-reduced persons, and is not uncommon in the period of debility following the exhausting acute dyscrasiae, especially typhus. Hypertrophy, or dilatation of the heart, is not unfrequently also present, and may favor the production of the disease.

After these remarks on encephalitis in general, some special notice is required of inflammation of the periphery of the brain. The same reasons which make peripheral apoplexy important, render this so too; and, indeed, peripheral inflammation is, if possible, the more serious, from its not unfrequently occupying a great part or even the whole of a hemisphere, and often giving rise to secondary imbecility.

It presents the same general appearances as inflammation of the gray matter elsewhere. Even when slight in degree, and when but just commencing, it may be clearly recognized, on exposing the surface of the brain, by the dark-red coloring and swollen state of the inflamed part, and by large pieces of cerebral substance adhering to the pia mater, and coming away when the membranes are peeled off. Higher degrees of it are generally combined with inflammation of the pia mater itself; and sometimes it is not confined to the peripheral substance of the brain, but it involves also the pia mater, the arachnoid, the dura mater, and even the cranium, as is proved by the adhesions formed between those different structures, and by their increase in size.

Peripheral encephalitis is mostly situated in the gray substance of the convolutions on the convexity of the cerebral hemispheres; it is less frequent at the base, and still less on the cerebellum. It is also generally remarkable for its great extent.

Peripheral inflammation terminates in resolution, or in a state of looseness of the outer layer of the cineritious structure, in which it clings to, and may be stripped off with the pia mater.

Or it may terminate in atrophy, and absorption. The cineritious layer then becomes thinner, and either whitened, or of a dirty fawn or yellow color; or else it is completely removed, and then the white substratum of the convolutions is laid bare, and appears condensed and callous. My observations correspond with those of Durand-Fardel, in having never found in the peripheiry of the brain, that condition of parts which he has named cellular infiltration.

The last termination of the disease is induration. The convolutions are found tough and callous through their whole thickness; they are also paler than natural; and the pia mater covering them, having become condensed and tendinous, adheres closely to their surface.

I have already spoken (at p. 298) of the so-called yellow plates, which Durand-Fardel attributes to chronic softening of the surface of the brain, and of the ulceration, as it is called, of the surface of the brain, which is associated with it. The former are the remains of the extravasation: and the so-called ulceration is produced by absorption of so much of the substance of the brain as had been broken up by the hemorrhage. Both appearances certainly may be found combined with some remains of inflammation, especially with cellular infiltration in the adjoining white substance.

A later and a most important consequence of these terminations of peripheral inflammation is a gradual atrophy in the interior of the brain, which, when the original process has been very extensive, involves the entire hemisphere. It is accompanied by induration and a dirty white discoloration of the fibrous substance. These appearances are sometimes so marked that the whole disease might be taken for an original and congenital arrest of development; and the more uniformly the several parts of the hemisphere are wasted, the more likely is the mistake. Moreover, the atrophy is usually concentric, so that the lateral ventricle is diminished in size. The vacuum is filled up by thickening of the membranes, and an effusion of serum around the hemisphere; and in the case of the cerebrum, by an enlargement of the opposite hemisphere, which is due to the dilatation of its lateral ventricle.