NERVOUS SYSTEM. (NERVOUS CENTRES. THE MEXINCES.) 



638 



over the fissure of Sylvius from the anterior to 

 the middle lobe, and here its distinctness from 

 the pia mater may be clearly demonstrated; 

 here too it appears much stronger and more 

 opaque than elsewhere, which is probably 

 due to the existence of an increased quantity 

 of fibrous tissue beneath it. 



In that space on the base of the brain which 

 is bounded on each side by the middle lobes, 

 and which is limited in front by the optic 

 nerves and behind by the pons Varolii, the 

 arachnoid membrane stretches across from one 

 middle lobe to the other, leaving a considerable 

 space between the tuber cinereum and the pons, 

 in which it is connected to the pia mater by 

 several long filaments similar to those which 

 are met with on the surface of the spinal cord. 

 This space is favourable for the accumulation 

 of fluid, and it communicates in front with the 

 fissures of Sylvius and other deep fissures into 

 which fluid might make its way. Cruveilhier 

 calls it the anterior sub-arachnoid space, and 

 regards it as the principal reservoir of the cra- 

 nial serosity. Immediately in front of it we 

 observe that the arachnoid membrane is conti- 

 nued around the infundibulum to the pituitary 

 body. 



In tracing the arachnoid backwards from the 

 great longitudinal fissure of the brain, we ob- 

 serve that it stretches down from the posterior 

 edge of the corpus callosum to the superior 

 surface of the cerebellum, crossing over the 

 tubercula quadrigemina. At this situation the 

 arachnoid is reflected upon the venae magnae 

 Galeni as they pass to the straight sinus. It 

 was at this place that Bichat described the 

 canal which goes by his name, through which, 

 as he thought, a process of the arachnoid mem- 

 brane was carrried in to line the interior of the 

 ventricles. 



The arachnoid covers the superior surface of 

 the cerebellum and also its inferior surface, 

 stretching across the longitudinal fissure from 

 one hemisphere to the other, and it is also ex- 

 tended downwards, and a little forwards from 

 the superior surface of the cerebellum to the 

 posterior surface of the medulla oblongata, 

 below the inferior extremity of the fourth ven- 

 tricle. A considerable space is thus left, situate 

 posteriorly between the cerebellar hemispheres, 

 and bounded in front and inferiorly by the 

 medulla oblongata, which also forms a conside- 

 rably reservoir for cerebral fluid, and communi- 

 cates freely with the sub-arachnoid space of the 

 spinal canal; but as the arachnoid is tied down 

 somewhat more closely over the posterior sur- 

 face of the spinal cord, there is an appearance 

 of constriction where the cerebral passes into 

 the spinal arachnoid. This space is called by 

 Cruveilhier the posterior sub-arachnoid space 

 (posterior conflux of Majendie). It commu- 

 nicates with the anterior sub-arachnoid space 

 through the furrows around the crura cerebelli. 



Of the cerebro-spinul Jiuid. In examining 

 such a dissection of the membranes of the 

 spinal cord as that above described, we shall 

 find that at various points the visceral layer 

 of the arachnoid membrane appears raised up 

 by fluid or by a bubble or two of air from 



the subjacent viscus. If a small portion of 

 this layer be taken up in a forceps, and a 

 blow-pipe be introduced into it, air may be 

 blown underneath it, raising it up all around 

 the spinal cord to a considerable distance from 

 that organ. The inflation is more easily ef- 

 fected in the cervical and in the lumbar regions 

 than in the dorsal, and the air will pass down 

 quite to the lowest part of the canal of the dura 

 mater, where the connexion of the arachnoid 

 membrane to the cauda equina is particularly 

 loose. In the same way coloured fluid, or 

 some material which may assume the solid form, 

 as size, tallow, &c. may be injected to demon- 

 strate this anatomical arrangement. If now we 

 examine a transverse section, it will be observed 

 that a considerable interval exists between the 

 visceral layer of the arachnoid and the pia 

 mater of the cord, and that this interval is 

 much greater in the neck and in the loins than 

 in the back. We observe too that the spinal 

 cord is by no means of sufficient size to fill the 

 spinal canal, and that as a considerable interval 

 exists between its surface and the visceral layer 

 of the arachnoid, so also a still greater one is 

 found between it and the inner surface of the 

 dura mater. Now as it is of the very nature 

 of a serous membrane that its free and smooth 

 surfaces should always be in contact (for it is 

 in that way that it favours the movements of 

 the viscus with which it is connected), it is 

 plain that the sub-arachnoid space in the spine 

 must, during life, be kept in a state of distension, 

 otherwise the object of a serous membrane 

 would not be attained. 



Moreover, in tracing the arachnoid mem- 

 brane upwards over the medulla oblongata and 

 the other parts of the encephalon, we observe 

 an evident continuity between the spinal and 

 the cranial sub-arachnoid cavity, which is most 

 evident at the base of the brain, where the 

 latter possesses the greatest dimensions, so that 

 air or fluid may be readily made to pass from 

 one to the other. This is most conspicuous in 

 old subjects, in which the brain being small 

 and more or less shrunken, leaves a consider- 

 able interval between its surface and the vis- 

 ceral layer of the arachnoid. 



On opening the spinal canal in a body re- 

 cently dead, the visceral layer of the arachnoid 

 will almost always be found raised by fluid. 

 When a portion of the posterior wall of the 

 spinal canal is removed in a living animal, or 

 in one just killed, the dura mater is found to 

 be quite tense from the fluid which is accumu- 

 lated within it. In a horse, whose spinal canal 

 I opened in the dorsal region immediately after 

 he had been knocked down in the knacker's 

 yard, I found the dura mater perfectly tense, 

 and semi-transparent from being stretched so 

 firmly over fluid. Upon making a minute 

 puncture in it, a fine stream of clear fluid was 

 ejected with much force to a considerable dis- 

 tance, and immediately the dura mater became 

 quite flaccid. By a little careful dissection 

 through the dura mater and parietal layer of the 

 arachnoid, it may be shewn that this fluid is 

 not contained in the arachnoid sac, but in the 

 sub-arachnoid cavity. 



