.Entozoa.—The entozoa found in the brain are the cysticercus cellulose, and the acephalo cyst, with its denizen the echinococcus. Like tubercles, these are always placed near the vas cular surface, and they may be said more properly to infest the pia rnater than the sub stance of the brain; by their growth, however, they encroach more or less upon it. The ani mals sometimes die, and their containing cysts shrink up and become converted into earthy matter, forming calcareous tumors of variable size in the substance or on the surface of the brain.
Morbid states of the ventricles of the brain. —The diseased conditions of the ventricles of the brain are referable, first, to the cavities themselves; secondly, to their contents ; thirdly, to their lining membrane and to the choroid plexus.
The most frequent morbid condition of the ventricles is a state of dilatation, which is always passive, being produced by the accumulation of water in it. This retention of fluid within these cavities appears to be a true dropsy, and is in most cases connected with an extemal meningeal inflammation in a strumous constitu tion. It is in children that we most frequently meet with this dilatation of the ventricles, and in them it constitutes the disease called hydro cephalus internus. In adults it occurs sorne times, but extremely rarely. In the former, when the disease is of a very chronic nature,. the fluid will accumulate to,a very great extent, and enlarge not only the ventricles but the cranium itself to an enormous size.
In persons in advanced age, in lunatics of long standing, and in old epileptics, we frer quently see a dilated state of the ventricles from distension by water. This is always asso ciated with a wasted state of the brain; this fluid, as vvell as the external fluid, serving to fill up the space from which the cerebral matter had receded.
In all these cases the ventricles which expe rience dilatation are the lateral ventricles, the third and the fourth. In a very few instances the fifth ventricle has been found similarly dilated.
The fluid contained in the ventricles is gene rally a clear straw-coloured serum, varying in quantity from half an ounce to several ounces. Sometimes it is milky, and has shreds of lymph floating in it; at other times it may be sero purulent, but this is extremely rare, and only occurs when the lining membrane has been the sou of acute inflammation and of inflammatory deposit.
The lining membrane of the ventricles, which in health is of extreme tenuity, becomes fre quently thickened and partially opaque in chro nic disease of the brain, where the ventricles are more or less dilated. In acute disease
lymph is sometimes deposited upon it in large and loose flakes, easily removeable from it. And sometimes there is a deposit all over its surface of a fine granular semitransparent lymph, which gives to the internal surface of the membrane the appearance of an extreniely fine and delicate reticulation.
As the choroid plexus are covered by a pro longation of the membrane of the ventricles, their investment is apt to participate in any morbid process which may take place in the former. In acute affections it will be covered with lymph, as the membrane lining the ven tricles is elsewhere. When much water has been accumulated in the ventricles, the choroid plexus are pushed against their floor, flattened, and rendered pale by maceration. On the other hand, whatever causes much va.seular congestion in the vessels of the brain will produce the same effect in a marked manner upon those of the choroid plexus.
Earthy concretions are sometimes found in the choroid plexus, which may probably be an augmentation of the crystalline matter found in them in their healthy state. These appear to consist chiefly of phosphate and carbonate of lime.
A very common appearance found in the choroid plexus consists in certain vesicles, very variable both in size and number. These are simple cysts, containing a straw-coloured fluid. Formerly they used to be regarded as hydatids, but they are now known to be essentially dis tinct from thern. They occur frequently in brains which exhibit no other departure from the normal condition. Of their precise nature, and of their cause and mode of formation, nothing is known ; and as they are seldom of a large size they are not likely so to disturb the functions of the brain as to give rise to symptoms by which their presence could be detected.
On the pseudo-morbid appearances of the nervous centres and their coverings.—The ac tual indications afforded by any departure from the normal physical condition of the nervous centres after death are so important to the attain ment of right conclusions respecting the patho logy of the nervous system, that it behoves the anatomist to take fully into account all those circumstances which may give rise to appear ances in the cerebro-spinal centres or their membranes simulating disease. Such appear ances, not inappropriately termed pseudo-mor bid, occur in the greater or less vascular fulness of the membranes and of the centres themselves, in the variations in the quantity of fluid around or within the brain, or around the spinal cord, and in the consistence of the nervous matter.