I would here offer the following general remarks: that, in accordance with what has been already said, by the term hydrocephalus is meant "Hydrocephalus stricte sic dictus internus," - dropsy of the ventricles: and that I retain the usual division into acute and chronic hydrocephalus, as it appears to be that which still possesses the most practical value. The chief seat of hydrocephalus is generally the lateral ventricles, and they are for the most part symmetrically affected. Exceptions will be pointed out in their places.

A. Acute Hydrocephalus

This is both the most frequent and the most important of the forms of hydrocephalus - the acute dropsy of the ventricles. Anatomy discloses two essentially different forms of it.

A. The First Form

Its anatomical characters are a. The effusion of a fluid which is thin or somewhat thick, of a grayish color, or grayish-yellow tinged with green, and more or less turbid, in proportion to the quantity it contains of plastic matters capable of assuming some of the primary forms of organization. It is very often found to have separated into two parts - one fluid, and the other of more consistent and deeper color: it has, in fact, become clear from certain of its elements having fallen to the bottom as a sediment. These elements are usually found in the most dependent part of the lateral ventricles; viz., their posterior cornu. On minute examination of the effusion, its opacity is found to arise from the presence of the elements of a plastic exudation, nucleoli, nuclei, cells at various stages of development, and true pus-cells; and of them the sediment, distinguished by the deeper yellowish or greenish color which it presents, is, for the most part, composed. But cast-off epithelium in course of solution, shreds of the lining membrane of the ventricles, and even shreds of nerve-tubes, are also found in the effusion, and all contribute to its opacity.

In some few cases a more solid exudation is observed besides the fluid effusion; it adheres here and there, especially on the corpora striata and optic thalami, in membranous plastic flakes to the lining membrane.

B. The quantity of the morbid effusion cannot be accurately determined, because of our uncertainty as to the pre-existing or normal quantity of fluid in any particular case. The whole quantity, however, inclusive of that which previously existed, is generally not considerable, and usually does not exceed an ounce; oftentimes it is scarcely half so much. On the other hand, in a few cases in which acute hydrocephalus supervenes upon chronic (whether the latter be congenital, or have come on early in life), the quantity of opaque contents of the ventricle is very considerable, and the greater part of it appears to have been produced by the recent process. The enlargement of the ventricles corresponds to the quantity of their contents, and in ordinary cases is but slight.

y. The lining membrane of the ventricles becomes opaque, soft, and dissolved, and shreds of it consequently appear in the effusion.

The choroid plexuses become opaque and softened, and are very commonly enveloped in a villous and slightly shreddy layer of grayish, or grayish-yellow exudation. d. From this point further changes extend in two different directions, to the cerebral substance, and to the inner membranes at the base of the brain. The affection of the membranes is the more essential part with respect to the nature of the process.

(1.) In the neighborhood of the ventricles the cerebral substance is percolated with serous fluid - infiltrated - to such a degree, that it seems as it were in a state of watery softening; very often, too, it is streaked or dotted with ecchymoses. The cerebral substance is thus affected wherever it adjoins the lining membrane of the ventricles; but very commonly the fornix and septum are softened to the greatest degree, and the latter is sometimes quite broken down and perforated. From this spot the oedema extends through the whole of the cerebral hemispheres, always, however, diminishing gradually as its distance from the ventricles increases, and always being greatest close to them. Hence the cerebrum swells, and increases in actual volume; its convolutions are forced against the walls of the cranium, and flattened; and in the same manner the cerebellum and pons are depressed and flattened in a marked degree. The inner membranes covering the convex surface of the hemispheres, being involved in the pressure against the cranium, appear bloodless. The cerebral substance also is bloodless and pale; it has a singular dull white appearance, and a peculiar soft and doughy consistence arising from its uniform moistness.

(2.) With the affection of the lining membrane of the ventricles and choroid plexuses, the diseased condition of the inner membranes at the base of the brain, forms one entire disease, not only by continuity, but in essence also. It takes the form of meningitis, especially of that described at page 262 as the second form of meningitis; or else it appears as acute tuberculosis of the pia mater at the base of the brain. In speaking of the serious character of these processes, I have already grounded it on their extension to the lining of the ventricles, that is, to their combination with acute hydrocephalus.

B. The Second Form

Its anatomical characters.

a. Effusion of a clear, colorless, serous fluid into the ventricles; sometimes it is slightly turbid, from being mixed with shreds of the lining membrane and of cerebral matter.

B. The effusion varies in quantity, being sometimes slight, but more frequently considerable, amounting even to six ounces.

r. The cerebral substance around the ventricles is generally in a state of watery softening, in which the rest of the cerebral mass shares, only in a less degree, the change in it not exceeding ordinary oedema. The whole brain presents the same swelling and the same general condition as have been described of the first form, but generally, even to a more marked extent. The cerebral substance surrounding the ventricles and the lining membrane, may sometimes be found in the dead subject in a normal or nearly normal condition; but this happens only in some extremely rare cases, in which the disease is known to have run an acute course.