When an empty space is formed within the skull by a reduction of the volume of the brain, it is filled up (as already pointed out, pp. 253 and 260), by an increase of the volume of the inner membranes of the brain, and especially by an extraordinary exhalation of serum into the tissue of the pia mater, the sac of the arachnoid, and the internal cavities of the brain, more particularly the lateral ventricles. These changes result from the congestion of the vessels which the vacuum produces. The most common instance of effusions of this kind into the ventricles, is that which occurs when the brain is atrophied in old age; a condition which has obtained the title of hydrocephalus senilis. From the process of involution of the brain, which gives rise to the affection in this instance, being so free from complication (einfach), from its uniform occurrence in both halves of the brain, and, lastly, from its frequency, it may serve as an example of this species of hydrocephalus. But the same condition is met with also in all cases of premature senility of the brain (senium praecox cerebri) and in every spontaneous and primary, as well as in every consecutive atrophy. Examples of the latter kind occur after recovery from repeated attacks of apoplexy, after inflammation of the substance, which has terminated with induration and wasting of the diseased portion, after the closure of an abscess, the healing of a wound attended with loss of substance, etc.

Hydrocephalus is symmetrical, or otherwise, according to circumstances. The quantity of serum contained in the ventricles, and the dimensions to which these cavities are enlarged, are measured by the degree to which the brain is atrophied: the former very commonly equals an ounce and a half, it often amounts to 2 or 4, and may reach even 6 ounces, or more. As the quantity of serum increases, it accumulates in greater proportion in the dilated third ventricle, and produces especial attenuation of the gray commissure, the pillars of the fornix, and the septum ventriculorum: the septum may even be more or less perforated.

The serum in the ventricles, like that contained in the sac of the arachnoid, and that infiltrated through the tissue of the pia mater, is remarkably clear.

As the membranes at the periphery of the brain increase in volume, so also does the lining membrane of the ventricles become thicker than natural; and it often bears on its surface some analogue of the Pacchionian corpuscles of the arachnoid; for the membrane is covered over with fine granules, which are either clear like crystals, or opaque and white. This thickening is the principal cause of the resistance which is experienced in slitting up the walls of the ventricles.

Serous cysts on the choroid plexuses are frequently found in hydrocephalus senilis.

Though, from what has been said, it will be perceived, that in none of these cases is the watery effusion the essential disease, yet the false meaning which is often attached to it, especially in senile hydrocephalus, renders it necessary to remark expressly, that the whole gravity of the case rests with the disease of the brain to which the effusion is owing. And so, too, the symptoms during life, and the usual mode of death, are to be comprehended and estimated by the same rule; the latter, for instance, is not to be sought in the effusion, but is to be looked upon as the final consequence of an atrophy of the brain, which has arrived at its relative maximum.

All these forms of hydrocephalus, as I have already partly explained, combine with one another. Thus the acute disease not unfrequently supervenes upon the chronic, whether the latter be the congenital or the acquired; the hydrocephalus ex vacuo may associate itself with any of the other chronic forms.

The chief seat of all the forms of hydrocephalus, as I remarked at the commencement, is the lateral ventricles; when the effusions are large they always advance into the third ventricle, but the fourth is even then involved in a very subordinate degree, and may not be involved at all.

An accumulation of serum in the ventricle of the septum, is far more frequently met with than is generally supposed, especially in cases of chronic hydrocephalus; but it is decidedly rare for the accumulation to be at all great, and for the cavity to be very considerably enlarged.

The enlargement of the lateral ventricles is chiefly confined to their body, and the anterior and posterior horns; that of the inferior horn is usually less in proportion.

As a general rule, hydrocephalus is a symmetrical disease; but in some of the chronic forms the dilatation predominates on one side, and in the hydrocephalus ex vacuo it is sometimes entirely confined to one side. Slighter degrees of inequality on the two sides not unfrequently exist in senile hydrocephalus, especially in the instance of atrophy of the brain following the closure of an apoplectic cyst.