Of the arachnoid membrane.—This mem brane is intermediate to those already described. NVe have preferred giving the description of it last, because to understand it demands an acquaintance with the dettils of both those membranes.
The arachnoid is a great serous membrane pervading the entire cranio-spinal cavity. Its parietal layer adheres intimately and insepara bly to the inner surface of the dura mater both cranial and spinal, and its visceral layer is attached to the outer surface of the pia mater.
In point of structure and general disposition the arachnoid membrane resembles other se rous membranes, so much as to render it inex pedient to enter into any minute comparison of them. It will only be necessary to refer to such peculiarities of arrangement as may arise from the anatomical characters of the nervous centres with which it is connected.
Spinal araehnoid.—The serous character of the spinal arachnoid is best seen by examining a transverse section of the spinal cord and its membranes. If the section be made across the interval between two sets of spinal nerves, the visceral and parietal layers of the mem bmne may be seen in contact with each other ; the parietal layer closely attached to the dura mater, the visceral layer adherent to the pia mater of the spinal cord so loosely as to leave a considerable space between it and the outer surface of that membrane.
We may here notice an important distinction which the student of this poition of anatomy will do well to note particularly, namely, that the space between the two layers of arachnoid membrane is the araehnoid bag or sac, in which it is very rare for any fluid to accumu late; and that that between the visceral layer of the arachnoid and the pia rnater is the sub arachnoid cavity, in which, as will be shown by-and-bye, a considerable quantity of fluid exists in the natural state.
When the section is made on a level with the nerves as they emerge through the dura mater, Ive may notice the manner in which the arach noid membrane is prolonged upon the nerves in the form of a loose sheath, forming little 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 water. 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 tnater.
At the foramen magnum the spinal arach noid may be seen to be continuous with that of the brain, and here its visceml layer invests the medulla oblongata loosely. Inferiorly we trace the membrane down quite to the lowest extremity of the dura mater, and in this reg,ion 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 arachnoicl 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 s: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. NVe 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 long filaments of fibrous tissue which in terlace slightly, and in the areoke 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 " assu cellalo-vasculaire 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.