Bodies, somewhat similar, are also found oc easionally on the choroid plexuses of the lateral ventricles. Very frequently we meet with gra nulations of a like kind in the fringe-like pro cess of pia meter which descends from the velum interpositum to surround the pineal gland, and also upon the little processes of that membrane which go under the name of choroid plexuses of the fourth ventricle.
Wherever these bodies are found, they show a remarkable tendency to congregate in clusters around venous trunks. In examining them along the edges of the heinispheres, we find that they are most numerous around the veins which pass from the pia maw in that situation into the superior longitudinal sinus. This ten dency, probably, explains the occurrence of these bodies in some of the sinuses. They are most commonly met with in the superior longi tudinal slims, as already stated ; they are also found in the lateral sinuses, and soroetimes but rarely in the straight sinus. In all these situ ations these bodies appear to stand in a similar relation to the sinuses; they have penetrated the fibrous tunic of their walls, and pushed before them the inner or venous tunic.
In point of size and shape the Pacehionian bodies resemble minute granulations; their co lour is white, like that of coagulable lymph, and not unlike that which is occasionally secn upon serous surfaces after chronic inflamma tion. A gmnular lymph, taking somewhat a similar form, is occasionally seen on the mu cous membrane of the rectum after dysentery. At some parts the granulations appear simply as isolated elevations of the surface of the arachnoid membrane. At others they are col lected ha clusters round a common stem ; and when the membrane is removed and floated in water, this bothryoidal disposition may be well displayed. A large proportion of them cause, by their pressure, an adhesion between the opposed surfaces of arachnoid membrane ; and those which are attached to a stem are the most likely to project into the interior of the sinuses.
When examined by a microscope, each of these bodies appears to consist of a mass of minute granules enclosed in a membranous sac ; when the body is pediculated, its stalk exhibits a series of strim which take the direction of its length, and probably result from longi tudinal folds of the membrane which forms it. Dilute acetic acid causes them to swell and gelatinifies the bodies, and sometimes displass epithelial scales upon the surface of the mem brane which covers them.
The following explanation of this structure may be offered. The primary deposit of granular lymph takes place among the vessels of the pia mater. The small bodies thus formed push the arachnoid meinbrane before them as a sac or covering ; in some instances the granular mass is only partially covered, and then it causes merely a slight projection on the surface of the visceral layer of araehnoid ; but in others the mass is completely covered, and a stalk is gradually formed ; and when several granular masses have been deposited immedi ately contiguous to each other, they may all be attached in a cluster to the same stem. The fact that epithelial particles may be seen upon the surface of the membranous sac of some of the bodies is sufficient proof that it is derived from arachnoid membrane. If this be ad mitted, then it seems impossible to come to any other conclusion than that the pia maim is the seat of the primary deposit, and this opi nion is confirnied by the fact that we meet with the Pacchionian bodies on the internal pro cesses of the pia mater, when we have no evi dence of the existence of arachnoid membrane.
Or it might be conjectured that these bodies indicate a degenerate condition of the elemen tary particles of the stiperficial layer of the grey matter of certain convolutions, produced by frequent irritation.
Are the Pacchionian bodies naturalstructures? The great frequency with which these bodies are met with in the various situations above mentioned, has induced many, even in the present (lay, to regard them as normal struc tures, the physiological office of which is as yet unknown. But there are many facts which strongly militate against such a conclusion. In the first place it may be observed that Pacchi onian bodies never occur in the earliest periods of life. In the course of a long experience in anatomical investigations I have never seen them at a period antecedent to six years. The brothers Wenzel, who made a series of special examinations with a view to determine this question, make the following statement. In children, from birth to the third year, these bodies, if they ever occur, must be very few. From the seventh to the twentieth year they sometimes are numerous. From the latter pe riod to the fortieth year their number is consi derable, and the nearer we approach the fortieth year the greater doesit become. Lastly, from the fortieth to the one hundredth year these bodies are found in great numbers.