NERVOUS CENTRES. (HUMAN ANATOMY. THE MENINCES.) 



640 



pia mater exists in connexion with brain or 

 spinal cord, whether on the surface of these 

 organs, or in the ventricles of the former. It 

 serves to fill up various inequalities in the cra- 

 nial or spinal walls, and it accumulates in 

 greatest quantity in those situations where the 

 sub-arachnoid space affords the greatest ca- 

 pacity. 



Majendie describes four situations at which 

 this fluid accumulates in greater quantity than 

 at other places on the surface of the brain. The 

 most considerable of these, which he desig- 

 nates the posterior conflux, is situated below 

 and behind the cerebellum; it corresponds to 

 the posterior surface of the medulla oblougata, 

 and is covered behind by the layer of arachnoid 

 which extends between the medulla and the cere- 

 bellum. (Vid.supr.p.638.) It is here that, ac- 

 cording to Majendie, a communication takes 

 place between the fluid on the exterior and that 

 in the ventricles, at a point corresponding to 

 the inferior extremity of the fourth ventricle. A 

 second, or inferior conflux is found immedi- 

 ately in front of the pons Varolii ; it is situated 

 between the crura cerebri, and contains the 

 basilar artery. It is, in fact, only the posterior 

 part of what Majendie designates the anterior 

 conflux, which extends forwards to the com- 

 missure of the optic nerves, occupying the 

 central depression between the middle lobes 

 of opposite sides, and bathing in its fluid the 

 commissure, the tuber cinereum, the infundi- 

 bulum, and the trunks of the anterior cerebral 

 arteries. It communicates with the posterior 

 fissure beneath the crura cerebelli. The posi- 

 tion and the extent of this conflux is indicated 

 by the separation of the visceral layer of the 

 arachnoid membrane over the central part of 

 the base of the brain. Doubtless the accumu- 

 lation of fluid around so many parts of impor- 

 tant function and delicate structure, is a va- 

 luable safeguard to them against the communi- 

 cation of shocks from the walls of the cra- 

 nium. A fourth conflux is called superior ; it 

 is situated behind and a little below the level of 

 the corpus callosum, behind the pineal gland, 

 and above the tubercula quadrigemina. It 

 communicates around the crura cerebri with the 

 anterior conflux, and with the posterior conflux 

 by the fissures which separate the superior ver- 

 miform process from the hemispheres of the 

 cerebellum. The fluid contained in it bathes 

 the pineal gland, the tubercula quadrigemina, 

 the superior vermiform process, and the vena 

 Galeni as they empty themselves into the strait 

 sinus. 



As the fluid is in contact with pia mater, 

 it is plain that it must surround and support 

 the roots of all the nerves which proceed from 

 both the brain and spinal cord, and that all 

 the bloodvessels which penetrate or emerge 

 from those organs, or which ramify in the 

 pia mater, must also be bathed by it. The 

 fluid surrounds the nerves as they emerge from 

 the cranium or spine, and maintains contact 

 between the layers of arachnoid membrane 

 which compose the sheaths that accompany 

 them in their passage outwards. Majendie 

 states that this fluid accompanies the roots of 



the fifth pair of nerves as far as the Gasserian 

 ganglion, and that it bathes and mingles with 

 the fibres of the ganglion itself, as well as of 

 the three nerves which originate from it. This, 

 however, I think extremely doubtful. 



That fluid exists in the ventricles of the 

 brain has long been known to anatomists ; and 

 it seems highly probable that this fluid is se- 

 creted by the processes of pia mater which aie 

 found in all these cavities; or possibly by the 

 membrane which lines their surface. Does the 

 internal fluid communicate with that in the 

 sub-arachnoid space ? Majendie affirms that a 

 communication takes place by means of an open- 

 ing which is situated at the inferior extremity of 

 the fourth ventricle. I have not been able to 

 satisfy myself of the existence of such an open- 

 ing; the following is Majendie's description of 

 it : " The true orifice, constant and normal, by 

 which the cerebro-spinal fluid continually 

 passes, either to enter the ventricles or to 

 issue from them, may be seen at the inferior 

 termination of the fourth ventricle, at the place 

 named ' le bee de la plume ' by the old anato- 

 mists. 



" To demonstrate the existence of this ori- 

 fice it is necessary to raise up, and to separate 

 slightly from one another, the lobules of the 

 inferior vermiform process of the cerebellum, 

 and without breaking any of the vascular ad- 

 hesions which unite this part of the cerebellum 

 with the spinal pia mater, we perceive the 

 angular excavation which terminates the fourth 

 ventricle. Its surface is smooth, even (polie), 

 and is prolonged as far as the ventricle of the 

 cerebellum. Such is the anterior part of the 

 orifice : the lateral and superior parts are formed 

 by the choroid plexuses of the organ and by a 

 horny medullary lamella (valve of Tarin), the 

 extent of which is variable, and which adheres 

 to the prominent border of the fourth ventricle. 

 The form and dimensions of the opening vary 

 with the individual, and with the quantity of 

 cerebro-spinal fluid, so that when the latter 

 exists in considerable quantity the opening can 

 admit the extremity of a finger. Most fre- 

 quently, when the quantity of the liquid is 

 normal, the orifice does not exceed two or 

 three lines in diameter in every direction, but 

 it is frequently subdivided by vessels which 

 pass from the medulla oblongata to the cerebel- 

 lum. Sometimes the orifice is restricted by 

 one or by both of the posterior cerebellar arte- 

 ries which pass across it." 



Such is the description of the orifice to 

 which Majendie has given the high-sounding 

 title " Orifice des cavit'cs encephaliquts." He 

 states that when fluid is injected into the spinal 

 sub-arachnoid cavity, it makes its way into the 

 ventricles of the brain through this orifice, a 

 statement sufficiently difficult to prove. Cru- 

 veilhier, who seems to lean towards Majendie's 

 opinion, admits nevertheless several weighty 

 objections to it. The most important of these 

 appears to me to be that the margins of the 

 orifice which is brought into view by the me- 

 thod directed by Majendie, are irregular, and 

 have the appearance of lacerated membrane. 

 And it is recorded by M. Martin St. Angp, on 



