Many of the serous fluids which are found in the ventricles of the brain and beneath its arachnoid membrane, offer sufficient distinc tions in their nature and causes to merit recognition as a separate variety. They are alike independent of physical obstruction of the vessels, or of a qualitative alteration of their contents ; while their quantity, which is frequently a considerable one, and the corre sponding diminution of the size of the brain, together clearly indicate that they are not due to mere post-mortem phenomena. But while, on the one hand, they are unattended by these, the ordinary causes of such effu sions, and are devoid of all symptoms which would indicate them as in themselves mor bid ; so, on the other, they are not present in the healthy subjects Hence we nnay deduce, first, that they are related to some abnormal condition ; and secondly, that this relation is not an immediate one. This may be confirmed by considering that the organ bathed by these fluids is one which, from its physical and physiological properties, its soft consistence and important functions, is both peculiarly susceptible of disturbance from pressure, and ready to give signs of such disturbance ; so that the absence of these indications betokens a nicety of adaptation of the fluid to the cranium and its contents which is hardly ta be explained in any other way than by sup posing that this adaptation is itself the ob ject which the presence of the serum fulfils, or that the want of it is the condition nhich necessitates the effusion, if indeed it does not rnore immediately give rise to it.
In the cerebro-spinal fluid itself, we are pre sented with a more normal counterpart of this scarcely morbid effusion ; since a fluid of similar constitution, in lesser quantity, is here a constant phenomenon. In. the loose and vascular areolar tissue between the arachnoid and the spinal cord, this perpetual dropsy is the natural condition of the part ;, and is perhaps due to the greater mobility enjoyed by the vertebral column where it surrounds these parts of the nervous centre, a freedom of movement which requires that they, in their turn, should be more carefully protected from external violence.
Physical and chemical properties. — The appearances of the fluid found in the cir cumstances above mentioned are tolerably uniform, and the few variations that occur are chiefly of an accidental nature. It is usually a limpid, colourless, and transparent fluid, of a faintly alkaline reaction ; and, in a state of purity, tt offers no trace of organiz ation, either to the naked eye or the micro scope. In its consistence, however, it is susceptible of great differences. It varies from the limpidity of water to the viscidity of synovia ; and when containing very much albumen, is sometimes even thicker and more tenacious than this liquid. Its colour is very frequently and greatly affected by adinixture with blood, bile, and other matters; or by the partial precipitation of its albumen ; or, more rarely, by the solidification or crystal lisation of fatty constituents. Many of these
causes also affect its transparency, giving it more or less opacity, as well as colour. Its alkalinity is less liable to variation ; but occa sionally it is neutral, and very rarely acid. Its unorganized character is only interfered with by accidental impurities similar to those above noticed.
The chemical composition of these fluids is much more variable; indeed it is very pro bable that scarcely any two of them are exactly alike in this respect. The following table exhibits four analyses, contrasted with that of the serum of the blood : — and two or three others are given by the sante author, which have a very similar com position. In the serous fluids of the cerebral ventricles, the quantity of albumen appears still smaller, as is exemplified in the following analysis by Berzelius.* The difference in the aMount of albumen which these analyses exhibit is very striking ; and the large quantity present in the latter is especially remarkable as offering nearly four times the quantity which is present in the serum of the blood. The anomaly of an unorganized liquid, derived from the blood, possessing more of this important constituent than the parent fluid, has been attributed by Vogel to a reabsorption of the watery parts subsequently to the effusion. The varying methods of analysing these fluids leave less room to remark quantitative differences of their other constituents. The quantity of salts seems, however, pretty constant ; al though the following analysis :it exhibits a bingular increase in one of the most common saline ingredients. It was taken from the dropsical belly of a woman aged-4-0, and the urine is stated to have contained about 6 parts in the 1000 of the same salt, 1000.81 The small number of analyses hitherto rnade, and the incompleteness of the patho logical notice with which they are usually accompanied, render it at present too early to arrange the composition of these fluids in any real connexion with the various morbid states which have regulated their production. But the possible cause of an excessive pre ponderance of' albumen has been already alluded to, and on the whole it seems likely that the cases where this substance is of a less remarkable, but still a considerable amount, belong chiefly to the category of dropsy from mechanical obstruction ; while the dropsies of anwmiin, post-mortem trans udation, and the like, seem to be characterised by the possession of a very small quantity of albumen : thus the second analysis in the table exhibits only three parts in the thousand ; In respect of their diminished quantity of albumen, it is difficult to avoid noticing their approximation to the characters of the cerebro spinal fluid, the vitreous humor, and other healthy effusions.