The subjects of it are, for the most part, children; but adults, and even persons advanced in life, suffer from the secondary form, especially when it is a process connected with tubercle.

The second form, though an acute hydrocephalus, cannot be admitted to be inflammatory: it bears, in fact, none of the characters which mark inflammatory states or products. It arises from congestions of various kinds: such as are connected with the development of the brain in childhood, or those produced by chronic eruptions on the scalp, by the irritation of morbid growths within the skull, etc. These congestions are analogous to those from which acute dropsies of many of the serous and synovial membranes result, - dropsy, for instance, of the tunica vaginalis testis, and the acute oedemas. It may be occasioned, too, by the congestions which follow concussion of the brain, or mechanical obstructions, such as disease of the heart, rickets of the thorax, impermeability of the lungs in tuberculosis and phthisis, chronic catarrh of the bronchi, chronic pneumonia, etc. The result of these congestions is an excessive effusion of serum, first from an apparatus especially adapted for that purpose, viz. the lining membrane of the ventricles, and then into the brain itself.

These effusions, if the process which gives rise to them be very intense, destroy life at once, upon their first occurrence; otherwise they are fatal only after being several times repeated. Hence it is that the quantity of fluid found in the ventricle varies so much: the larger accumulations, those which amount to as much as six ounces, are apparently the sum of several smaller effusions, occasionally repeated. The enlargement of the ventricles corresponds to the quantity of serum within them; and the skull enlarges in an equal degree until the sutures are closed.

This form of hydrocephalus, then, not unfrequently runs a protracted, subacute course; and it tends the more to do so, in proportion as the several exudative processes are slight in degree, and as the skull retains more of its early elasticity. And further, the less distinguishable the several exudations are from one another, the more this form is allied to chronic hydrocephalus.

It is sometimes a primary and substantive disease, sometimes a secondary. When primary, it is in childhood, like chronic hydrocephalus, remarkable for its combinations, of which I have to speak hereafter; and for being intimately connected with a deeply-lodged anomaly of the general vegetative processes. When secondary, it is frequently occasioned by various diseases of the brain, as inflammation, abscess, and by morbid growths in the skull.

Considered apart from chronic hydrocephalus, which stands in close proximity to its subacute variety, it is certainly, on the whole, more rare in childhood than the first form; but then it may occur much earlier, being met with in the first year of life, and doubtless, also, in the foetus. Moreover, it is not very rare at any later period of life, up to old age, for it is occasioned by mechanical congestions in the course of various chronic adynamic diseases, which are attended with a dropsical crasis of the blood. .

Both forms of acute hydrocephalus are attended by certain combinations, some of which are common to both, while others are peculiar to one of them. They are partly constant and essential, and partly neither constant nor essential.

The first form is, in the great majority of cases, combined with a tuberculous diathesis, and the local tuberculoses, mentioned at p. 265: these constitute the fundamental anomaly.

The second form combines with it several abnormal conditions, especially in children; and very often all the disorders enumerated below occur together, and form one complex morbid state, that manifests a thoroughly depraved working of the vegetative process. They are:

(1.) Hypertrophy of the whole system of lymphatic glands, and of the follicular apparatus of the intestinal mucous membrane.

(2.) Arrested decay or involution of the thymus gland.

(3.) Chronic catarrhs, especially of the bronchi.

(4.) Rickets and its attendants.

One coincidence, which deserves special attention amongst these combinations, is that of hydrocephalus with hypertrophy of the brain. The latter is well known to be very commonly associated with general rickets; but the former is so constantly found to be connected with rickets of the thorax, that Engel has given the name of hydrocephalic to that particular distortion of the chest.

A very common and essential combination with both forms of the disease, and one with which the fatal result is frequently connected, is softening of the stomach.