Some of these anomalies have been already detailed: the accumulation of serous fluid in the arachnoidal sac in any beyond the natural quantity, constitutes another instance of them. When the accumulation is considerable, it constitutes the disease which is known as external, or meningeal, hydrocephalus. Sometimes it is congenital, and is then remarkable for the great quantity of the serum.

It appears in two forms: a. In that of dropsical sacs, which consist either of arachnoid membrane alone, or, as is more common, of dura mater also, identified with the arachnoid, and attenuated in an extreme degree: they protrude through an aperture in the skull, and form a diverticulum of the arachnoid sac, which communicates with the general cavity by a narrow canal. If, as is very commonly the case, there be no complication with hernia of the brain, such sacs are undoubtedly curable by being emptied and tied.

b. The second is a uniform accumulation of serum in the arachnoid sac, by which the brain is displaced and compressed towards the base of the skull, and the cranium is, at the same time, uniformly enlarged. Although there are some cases, and one in particular in the Vienna Museum, which show that this hydrocephalus may attain very considerable dimensions, yet such an enlargement is extremely rare.

There is an excessive accumulation of serum in the arachnoid sac, which comes on very frequently during extra-uterine life, and calls for some remark.

The normal quantity of serum in children is just sufficient to keep the free surfaces of the arachnoid membrane moist; it may be a few drops, but it does not exceed a drachm; in adults it amounts to three or four drachms. This quantity is found collected in the posterior fossae of the skull; and if the brain have been carefully removed, it remains in its natural clear and colorless condition: but as in examining and taking out the brain, some blood is usually mixed with it, it appears turbid, and is more or less tinged red.

When there is an excess of this serum, not only has a greater quantity gravitated to the base of the skull, but a part of it also pours out when the dura mater is slit at the mid-height of the brain, along the line where the skull has been divided by the saw. The whole amount may often be estimated at an ounce, an ounce and a half, two ounces, or even more.

The conditions under which considerable quantities are found, are principally two: they may occur,

1. In consequence of frequently recurring habitual congestion of the membranes, or of a varicose state of the vessels of the pia mater.

2. As a result of atrophy of the brain. Under both conditions the excessive accumulation of serum in the arachnoid sac, is combined with thickening of the inner membranes - the cerebral arachnoid, and pia mater, - with infiltration (oedema) of the pia mater in atrophy of the brain, with accumulation of serum in the ventricles, and sometimes with a state of infiltration or oedema of the brain. It is always important to notice that serum is accumulated at the same time beneath the bridgelike expansions of arachnoid at the base of the brain, especially in the cerebellum; and the more so as the fluid escapes from the latter situation when the brain is removed, and augments the quantity which is found in the arachnoid sac.

This serous effusion into the arachnoid sac, is essentially chronic; but it is subject to sudden or to gradual increase with every attack or augmentation of the congestion, with every advance in the varicose state of the vessels, and in the case of atrophy of the brain, as the vacuum becomes greater within the skull. It is an important question whether there are any acute effusions of this kind which are rapidly fatal; and whether it is possible to recognize them on the dead body as the cause, or as part of the cause, of that form of sudden death which is usually designated "serous apoplexy." That there are such effusions, which may quickly destroy life by paralyzing the brain, can certainly, in the present state of our knowledge, not be denied; but the diagnosis of this mode of death is just as uncertain on the dead body as on the dying patient, for the serum which is found in the arachnoid sac, may be a chronic accumulation that has existed there for a long time, just like that which is contained in the ventricles, or in the tissue of the pia mater, or that which produces oedema of the brain itself. Moreover, the coexistent congestion of the membranes of the brain is usually but slight. And lastly, there are, for the most part, in such cases, various other morbid affections elsewhere, by which the sudden death can be otherwise explained. I shall have occasion to make some further remarks on this subject at another more suitable place.

Not unfrequently the quantity of this serum is uncommonly small, and the surfaces of the arachnoid appear to be without moisture. This is the case in marked enlargement of the brain, especially in hypertrophy.