644 



NFRVOUS SYSTEM. (NEUVOUS CENTRES. THE MESINGES.) 



posed in the performance of the experiment, 

 ought fairly to he considered to have a share, 

 and that not an inconsiderable one, in any im- 

 pairment of the nervous function that mi:; 1 it 

 become apparent. The sudden removal of the 

 fluid brings on fainting or even death, effects 

 due to shock, and analogous to those which 

 result from the sudden removal of dropsical 

 fluid in particular cavities, when the organs and 

 the circulation in them have become adapted 

 to its pressure, as in cases of ascites, hydrotho- 

 rax, &c. 



The interior of the arachnoid sac is moistened 

 by an exhalation of a similar kind to that which 

 is found in the other serous membranes. Ac- 

 cumulations of fluid in the arachnoid sac, how- 

 ever, are of very rare occurrence. 



Of the glandula Pacchioni. To these bo- 

 dies we have already had occasion to refer in 

 the description of the sinuses. We proceed 

 now with a more special notice of them. 



These bodies were first formally described by 

 Pacchioni, and were regarded by him as con- 

 globate glands of the dura mater, from which 

 lymphatics proceeded to the pia mater.* They 

 have been recognized by all subsequent anato- 

 mists under the name here assigned to them, 

 although the idea of their physiological office 

 suggested by Pacchioni has not met with ge- 

 neral acceptation. Bichat suggested a more 

 appropriate and scientific appellation in that of 

 cerebral granulations. No anatomists have in- 

 vestigated the history of these bodies so exten- 

 sively as the brothers Wenzel.f 



The Pacchionian bodies are found principally 

 along the edge of the great hemispheres of the 

 brain on either side of the great longitudinal 

 fissure. Here, in general, they cause the obli- 

 teration of the sac of the aiachnoid for a greater 

 or less distance by producing adhesion between 

 the visceral layer of that membrane and that 

 portion of its parietal layer which adheres to 

 the angle along the superior border of the falx 

 cerebri. In cases where these bodies are 

 numerous and well developed, it is found very 

 difficult to separate the dura mater from the 

 subjacent arachnoid by reason of the firmness 

 of the adhesion effected by them ; and when 

 this adhesion exists, the corresponding surface 

 of the dura mater has generally a very com- 

 plicated cribriform appearance. The extent 

 of surface which they occupy is very variable. 

 Sometimes, but very rarely, they extend along 

 the entire edge of each cerebral hemisphere; 

 but generally they occupy its central part for 

 an extent of from one to three inches. Very 

 frequently they extend outwards over the sur- 

 face of the cerebral hemispheres, rarely beyond 

 half an inch or an inch. The arachnoid mem- 

 brane in their immediate vicinity is always 

 opaque. 



Bodies, somewhat similar, are also found oc- 



* Ant. Pacchioni diss. epistolaris ad Luc. 

 Schroeckhium de glandulis conglobatis dura; me- 

 ningis hmnanae, &c. &c. Rom. 1705, et Opuscu- 

 lum Anatomicum de dura rueninfe, in Opera Omnia. 

 Rora. 1741. 



t Wenzel, de penitiori cerebri structura. Tu- 

 bing*, 1812. 



casionally 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 mater 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. 



\\ herever these bodies are found, they show 

 a remarkable tendency to congregate in clusters 

 around venous trunks. In examining them 

 along the edges of the hemispheres, we find 

 that they are most numerous around the veins 

 which pass from the pia mater 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 sinus, as already stated ; they are also 

 found in the lateral sinuses, and sometimes 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 Pacchionian 

 bodies resemble minute granulations ; their co- 

 lour is white, like that of coagulable lymph, 

 and not unlike that which is occasionally seen 

 upon serous surfaces after chronic inflamma- 

 tion. A granular 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 in 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 stria? which take the direction 

 of its leno-th, 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 displays 

 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 membrane 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 arachnoid ; 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 



