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 venw magnm 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 carmed 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 forsvards 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 rable 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-spinal fluid.—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 org-an. The inflation is more easily ef fected in the cervical and in the lurnbar 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 membmne to the cauda equina is particularly loose. In the same way coloured fluid, or some material which may assume the solid forin, as size tallow, &e. may be injected to demon
strate this anatomical arrangement. If now we examine a tmnsverse section, it will be observed that a considerable interval exists between the visceral layer of the amchnoid 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 considemble interval exists between its surface and the visceral layer of the amchnoid, 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 niembmne would not be attained.
Moreover in tracing the arachnoid mem brane upwar'ds 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 bmin being small and more or less shrunken, leaves a consider able interval between its surface and the vi ccml layer of the arachnoid.
On opening the spinal canal in a body cently dead, the visceral layer of the arachno will almost always be found raised by flui When a portion of the posterior wall of t spinal canal is removed in a living animal, in one just killed, the dura mater is found be quite tense from the fluid which is accuil lated within it. In a horse, whose spinal I opened in the dorsal region immediate! he had been knocked down in the knacke yard, I found the dum 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 dum 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.