What has been described as the movement of this liquid consists in an alternate elevation and collapse synchronous with expiration and inspiration, seen only when a portion of the cranio-spinal . wall has been removed, and caused by the repletion of the venous system of the spine vvhich occurs in the former state of the respiratory movements, and its collapse which takes place in the latter. The distended spinal veins compress the cerebro-spinal fluid, and cause it to rise towards the head in expi ration; their collapse in inspiration favours the movement of the fluid in the contrary direction. We have no evidence from experi ment or direct observation that there is any movement in the fluid of the ventricles; but the discovery of cilia upon the inner surface of these cavities seems to indicate that this fluid is not quite stationary within them.
The following account of the physical and chemical properties of the cerebro-spinal fluid is derived from Majendie's researches. When removed from the body a few monoents after death, this fluid is remarkably limpid, and may be compared in this respect to the aqueous humour of the eye; sometimes it has a slightly yellowish tinge. In temperature it ranks among the hottest parts of the body. It has a sickly odour and a saltish taste ; it is alkaline, restoring the blue colour of reddened litmus.
According to M. Couerbe, some of the secon dary organic products which he has obtained from the brain are to be found in this fluid. The following constituents are enumerated by this chemist : 1. an animal matter insoluble in alcohol and ether, but soluble in alkalis ; 2. albumen ; 3. cholesterine ; 4. cerebrote ; 5. chloride of sodium; 6. phosphate of lime ; 7. salts of potass ; 8. salts of magnesia.
What is the use of the cerebro-spinal fluid ? An obvious mechanical use of this fluid is to protect the nervous centres with which it lies in immediate contact. By the interposition of a liquid medium between the nervous mass and the wall of the cavity in which it is placed, provision is made against a too ready con duction of vibrations from the one to the other. Were these centres surrounded by ma terial of one kind only, the slightest vibrations or shocks would be continually felt, but when different materials on different planes are used, the surest means are provided to favour the dispersion of such vibrations.
The nervous mass floats in the midst of this fluid, being maintained in equilibrio in it by its uniform pressure on all sides, and the spinal cord, as we shall find by-and-bye, is supported by an additional mechanism which prevents its lateral displacement.
By its accumulation at the base of the brain, this fluid must protect the larger vessels and the nerves situate there from the unequal pres sure of neighbouring parts.
It is not improbable also that this fluid may contribute to the nutrition of the brain and spinal cord, by holding in solution their proper nutrient elements preparatory to their absorption or addition to the nervous masses themselves; and this view would receive great support if Couerbe's analysis, which detects some of these elementary matters in the fluid, should be con firmed by the observations of other chemists. Nor must we omit to notice here, the fact as certained by Majendie, that when certain sub stances which find their vray readily into the blood have been injected into the veins, they may be soon after detected in this fluid, such as iodide of potassium.
Majendie observed serious symptoms to ensue upon the removal of this fluid from living dogs, but it is impossible to ascribe such symptoms solely to this cause, for the intro duction of airrinto the subarachnoid cavity, the disturbance and consequent irritation to which the nervous centres must necessarily be ex posed in the performance of the experiment, ought fairly to be considered. to have a share, and that not an inconsiderable one, in any iin pairmcnt of the nervous function that might !Iceman 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 midden removal of dropsiad fluid in particular cavities, when the organs and the circulatiou in them have become adapted to its pressure, as its cases of ascites, hydrotho rax, &c.
The interior of the amelinoid 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 glandalre Pacchioni.—To these bo dies we have already had occasion to refer in the description of the sinuses. We proceed now vvith 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 recoeMzed by all subsequent anato mists tinder thee 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 Wenzat The Pacchionian bodies are found principally along the edge of the great hemispheres of the brain on either side of the great longitudinal fissure. Ilere, in general, they cause the obli teration of the sac of the al achnoid 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 dum mater has generally a very com plicated eribriform 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 au 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 anichnoid mem- brane in their immediate vicinity is always opaque.