This disease, occurring alone in the choroid plexuses, is one of the very rarest ever met with. I have seen a medullary degeneration of the choroid plexus of the fourth ventricle.

2. Cancer Of The Lining Membrane Of The Ventricles

The internal membrane of the ventricles of the brain is composed of a very delicate continuation of the arachnoid and pia mater, and a layer of epithelium. The most frequent and most important diseases to which it is liable, have, from one most striking characteristic which they present, viz., an excessive accumulation of cerebro-spinal fluid, been included together under the title of hydrocephalus.

Although it is true that the presence of an excessive quantity of fluid in the ventricles, and structural disease of their lining membrane, are not essentially characteristic of all cases of hydrocephalus, but are sometimes secondary, and occasioned by disease of the brain, and therefore that all the forms of hydrocephalus do not rightly belong to the present section, yet I prefer treating of them altogether, for two reasons:

(1.) Because the most important of all the forms, viz., acute hydrocephalus, arises from disease of the lining membrane exclusively; and:

(2.) Because, in spite of their differences, it has become a prevalent custom to associate them together, both in thought and in description.

Though, for the sake of convenience, I thus disarrange the subject, it will be restored again by referring to each of the forms I am about to describe in the place to which it properly belongs.