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 perfect in the dorsal region, but which in the lumbar and cervical regions is cribriform pectiniform, as may be shown by pouring, quicksilver on either side of it, which will be retained in the dorsal region, but will teadily 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 naembrane, 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 m ust '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 accu mutated 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 ! there are convolutions, as on the superior and lateral surfaces of the hemispheres. It passes uls-de-sac at the orifices through which the herves escape.
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 svhich 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.