The most remarkable, and one of the most important of the post-mortem appearances, in both the principal forms of acute hydrocephalus, is this almost constant softening, or, as it is called, maceration of the cerebral substance about the ventricles. Before proceeding to any general remarks, some notice of the nature and import of this appearance is indispensable.

In so far as regards anatomical disorganization, I hold it to be in itself no very essential part of the disease; it is, however, certainly very important, and perhaps even more so than the effusion into the ventricle itself. For first, it involves cerebral substance, and secondly, it attacks that substance in a very acute manner, and rapidly produces disorganization of it. It is, in fact, nothing more than an acute oedema of the highly delicate and easily injured texture of the brain, and the equally delicate lining membrane of the ventricles; but so rapid, occasionally, is its progress, and to such degrees does it advance, that it gives rise, for the most part, to countless lesions of continuity, and thus, in the form of softening, disorganizes the brain and destroys life: if it should advance more slowly, or to a less degree, it may very often continue a long time without marked symptoms.

The mode of origin, and the import of this oedema, will be more distinctly understood from the following particulars:

(1.) It corresponds entirely to the oedema which surrounds every spot of inflammation, and to that which ensues upon acute congestions.

(2.) And further, it is in my opinion worthy of remark, that if an effusion takes place so rapidly that room cannot at once be obtained for it in the ventricles, by displacement of the brain, the resistance from within is so great as to hold, or press back, the exudation, and a portion of that which should be exuded from the lining membrane of the ventricle, is poured into its tissue, and into the adjoining part of the brain. The greater the intensity of the process, and the quantity of its products, the sooner does infiltration ensue, and break down the textures; and it will the more readily take place, if the brain have been oedematous before, or the cerebral mass around the ventricles have been distended by a preexisting effusion.

(3.) In the first of the two forms of hydrocephalus, especially in that with which true meningitis is combined, the serous exudation which gives rise to softening of the cerebral substance around the ventricles is sure to contain a portion of coagulable or plastic materials, capable of assuming a primary organic form; in the second it is entirely, or almost entirely, composed of pure serum. This accounts for the circumstance that in many cases, the macerated cerebral substance, when minutely examined, is found to contain the so-called exudation-corpuscles, exudation-cells, nucleated and primary cells (pus-cells), while in other cases these are entirely wanting. (Gluge).

This state of the brain, then, may be suitably classed, as has been done already, with the termination in softening, and may be named white softening, hydrocephalic softening. I shall have some further remarks to offer upon it when treating of oedema, especially in the article of Softening of the Brain.

This white softening of the cerebral substance is sometimes accompanied with yellow softening, more particularly when the case is one of the first form of hydrocephalus, and combined with meningitis.

Moreover, I have alluded above to the softened cerebral substance being sprinkled or streaked, as it were, with red ecchymoses: both forms of this disease present this feature, but it is more common in the first form. It arises from the laceration of the delicate vessels, which are torn when the cerebral texture is broken down; but there is very often far less of it than the degree of disorganization would lead us to expect. The question which this suggests, admits as yet of no other solution than that the simultaneous swelling of the whole brain so obstructs, and precludes the injection of, the cerebral vessels, that those which are torn are empty.

With respect to the nature of acute hydrocephalus, an inquiry which has led to so much discussion, that of the first form of the disease is perfectly clear: it is either an extension of meningitis of the base to the ependyma of the ventricles, - of a meningitis attended with an exudation that contains less than the average of plastic material, that bears traces of a faulty constitution of the blood, and that, in its fibrinous portion, is very often tuberculous; or it consists of a supplemental, and, for the most part, serous exudation, accompanying an acute deposition of tubercle in the pia mater, at the base of the brain. This form of acute hydrocephalus, therefore, is either actual inflammation, or an exudative process having a general connection with it. Although I cannot coincide in the opinion of several French observers, who think that acute hydrocephalus is never anything but meningitis, - by which term acute tuberculosis is also meant, - yet I so far agree with them as to believe, that in the great majority of cases such is the fact, and that the meningitis is very commonly of a tuberculous character.

This form of hydrocephalus occurs both as a primary and substantive, and as a secondary affection. When secondary, it attends the diseases of the brain which have been mentioned already, at p. 263; viz., inflammation, abscess, and a yellow softening, adventitious growths within the skull in general, but more than all, with tubercle of the brain: with that disease it associates itself, either in the form of meningitis with tuberculous exudation, or in that of acute tuberculosis. It is very frequently the means by which those diseases of the brain destroy life. The hydrocephalus which originates with tubercular meningitis, or with acute tuberculosis, is very rarely a primary disease, but supervenes upon some previously existing tuberculosis, upon that in the brain particularly, as well as that in the glands or in the lungs: when connected with acute tuberculosis, it forms one of the many local parts of the general disease. (Compare p. 265).