NEKVOUS CENTRES. (HUMAN ANATOMY. THE MENINGES.) 



(537 



culs-de-sac at the orifices through which the 

 nerves escape. 



Fig. 367. 



Traitsverse section of the same on a level with the fifth 

 cervical nerves. (After Arnold.) 



The same parts arc displayed as in the last figure, 

 and the reflection of the arachnoid at the exit of 

 the nerves is seen. 



af, anterior fissure ; n, n, spinal nerves. 



In the interval between each pair of nerves, 

 we find a triangular process of fibrous mem- 

 brane which is inserted by its apex into the 

 dura mater. This process lies in the sub- 

 arachnoid cavity and adheres by its base to the 

 pia mater. It seems to pierce both layers of 

 the arachnoid, or to pin them down, as it were, 

 to the dura mater. 



At the foramen magnum the spinal arach- 

 noid may be seen to be continuous with that 

 of the brain, and here its visceral layer invests 

 the medulla oblongata loosely. Interiorly we 

 trace the membrane down quite to the lowest 

 extremity of the dura mater, and in this region 

 the visceral layer is particularly loose and free, 

 as it lies over the cauda equina. 



When the dura mater is carefully slit up 

 along either the anterior or the posterior surface, 

 the arachnoid sac is laid open. It does not 

 always happen that the parietal layer separates 

 very readily from the visceral : frequently the 

 two layers adhere firmly at j-everal minute 

 points, yet this adhesion is effected without any 

 connecting membrane, and appears to arise from 

 the two layers becoming dried at several corres- 

 ponding points, and thus being, as it were, glued 

 together. We may frequently observe this in 

 specimens that have been some time kept in 

 spirits. This point is deserving of notice, as 

 these adhesions might be (and indeed they have 

 been) noted as of a morbid nature. 



The visceral layer of the spinal arachnoid is 

 connected to the pia mater by means of a num- 

 ber of loivj; filaments of fibrous tissue which in- 

 terlace slightly, and in the areolae thus formed 

 the fluid is contained. This tissue is most dis- 

 tinct and abundant in the cervical region, and 

 exists in very small quantity in the dorsal. It 

 ceases nearly altogether over the cauda equina. 

 Numerous minute bloodvessels are also to be 

 found in it passing from the pia mater to the 

 arachnoid. Majendie gives to this tissue the 

 name " tissu ceUulo-va&culuire. sub-arachnoide." 

 In general the adhesion of the visceral layer 

 of the arachnoid to the subjacent pia mater is 

 closer along the posterior than along the ante- 

 rior surface of the cord. 



Along the posterior surface of the cord on 

 the median line, the sub-arachnoid space is 

 divided by means of a septum, which is most 



pectiniform, as may be shown by pouring 

 quicksilver on either side of it, which will be 

 retained in the dorsal region, but will readily 

 pass from right to left in the other situations. 

 It is highly probable that this septum is a mo- 

 dified portion of the sub-arachnoid tissue. 



The existence of this septum (erroneously 

 described as complete) dividing the posterior 

 part of the sub-arachnoid space into a right 

 and a left portion, appears to have led to the 

 opinion that this space is lined by another 

 serous membrane, which has been called the 

 internal arachnoid, by which the fluid is sup- 

 posed to be secreted, and that the septum is 

 formed by the reflection of its visceral into its 

 parietal layer along the median plane. But there 

 are many objections to this hypothesis. In the 

 first place, if the septum were formed by the 

 reflection of a serous membrane, it would be 

 complete, and not a very imperfect one such as 

 it is ; it ought to resemble the mediastinum in 

 the chest, or one of the processes of the perito- 

 neum in the abdomen. Secondly, it is quite 

 contrary to all experience to find the cavity of a 

 serous membrane in the normal state traversed 

 by a quantity of filamentous tissue, as the sub- 

 arachnoid space is throughout a great part of its 

 extent. Thirdly, were there a serous membrane 

 in this space, the microscope ought to detect an 

 epithelium on its inner surface, but such a struc- 

 ture does not exist here. Lastly, such a serous 

 membrane must necessarily be continued into the 

 encephalic sub-arachnoid space. But the close 

 adhesion of the visceral layer of the arachnoid 

 to the pia mater, opposite to the prominent 

 parts of the cerebral convolutions, seems quite 

 incompatible with such an arrangement. 



Cerebral arachnoid. The cerebral portion 

 of the arachnoid exhibits essentially the same 

 general arrangement as the spinal portion. Its 

 parietal layer adheres very intimately to the pia 

 mater at certain points, leaving in the intervals 

 a considerable space for the accumulation of 

 liquid. If we trace it over the surface of the 

 hemispheres, it will be found to give them that 

 smooth and uniform character which is always 

 distinct on the recent healthy brain. The 

 arachnoid passes from convolution to convolu- 

 tion, adhering closely to the pia mater over the 

 most convex portions of those convolutions, 

 but allowing that membrane to separate from 

 it in the intervals between them, and to dip 

 down to the bottom of the sulci. Hence liquid 

 accumulated in the cerebral sub-arachnoid space 

 will be found to take the direction of the inter- 

 gyral sulci, and to cause the membrane to 

 bulge opposite to them ; and if air be blown 

 underneath the arachnoid, it will be found to 

 take the tortuous course of these sulci. 



The arachnoid sinks into the great longitu- 

 dinal fissure of the brain, lining the surfaces 

 which bound it on each side, and passing 

 across from right to left beneath the inferior 

 margin of the falx, and above the corpus cal- 

 losum. 



On the base of the brain, the arachnoid has 

 the same arrangement on those parts where 



perfect in the dorsal region, but which in the there are convolutions, as on the superior and 

 lumbar and cervical regions is cribriform or lateral surfaces of the hemispheres. It passes 



