ANATOMY MEMBRANES OF BRAIN AND SPINAL CORD These nerve centres are invested by three membranes or meninges, which lie between them and the bones that form the walls of the cranial cavity and spinal canal. The membranes are named clura mater, araehnoid mater, and pia mater.
Duna mater. - The most external membrane, named dura from its firmness, consists o; a cranial and a spinal subdivision. The cranial part is in contact with the inner table of the cranial bones, and is adherent along the lines of the sutures and to the margins of the foramina, which transmit the nerves, more especially to the foramen magnum. It forms, therefore, for these bones an internal periosteum, and the meningeal arteries which ramify in it are the nutrient arteries of the inner table. As the growth of bone is more active in infancy and youth than in the adult, the adhesion between the dura mater and the cranial bones is greater in early life than at maturity. From the inner surface of the dura mater strong bands pass into the cranial cavity, and form partitions between certain of the subdivisions of the brain. A vertical longitudinal mesial band, named, from its sickle shape, falx cerebri, dips between the two hemispheres of the cerebrum. A smaller sickle-shaped vertical mesial band, the falx cerebelli, attached to the internal occipital crest, passes between the two hemispheres of the cerebellum. A large band arches forward in the horizontal plane of the cavity, from the transverse groove in the occipital bone to the elinoid processes of the sphenoid, and is attached laterally to the upper border of the petrous part of each temporal bone. It separates the cerebrum from the cerebellum, and, as it forms a tent-like covering for the latter, is named tentorium cerebelli. Along certain lines the cranial clura mater splits into two layers, to form tubular passages for the transmission of venous blood. These passages are named the venous blood sinuses of the dura mater, and they are lodged in the grooves on the inner surface of the skull referred to in the description of the cranial bones, Opening into these sinuses are
numerous veins, which convey from the brain the blood that has been circulating through it ; and two of these sinuses, called cavernous, which lie at the sides of the body of the sphenoid bone, receive the ophthalmic veins from the eyeballs situated in the orbital cavities. These blood sinuses pass usually from before backwards : a superior longitudinal along the upper border of the falx cerebri as far as the internal occipital protuberance; an inferior longitudinal along its lower border as far as the tentorium, where it joins the straight sinus, which passes back as far as the same protuberance. One or two small occipital sinuses, which lie in the falx cerebelli, also pass to join tho straight and longitudinal sinuses opposite this protuberance; several currents of blood meet, therefore, at this spot, and as Herophilus supposed that a sort of whirlpool was formed in the blood, the name torcular IIerophili has been used to express the meeting of these sinuses. From the torcular the blood is drained away by two largo sinuses, named lateral, which curve forwards and downwards to the jugular foramina to terminate in the internal jugular veins. In its course each lateral sinus receives two petrosal sinuses, which pass from the cavernous sinus backwards along the upper and lower borders of the petrous part of the temporal bone.
The spinal part of the dura mater hangs loosely in the spinal canal. It does not form a periosteum for the vertebrm, but is separated from their bony rings by loose fat and a plexus of veins. It gives off no bands from its inner surface, and it does not split into two layers for the lodgment of venous blood sinuses. The spinal dura mater forms a tubular envelope for the spinal cord and the origins of the spinal nerves. It extends from the foramen magnum, where it is continuous with the cranial dura mater, to the lower end of the sacral canal, ends below in a funnel-shaped prolongation, and is pierced laterally by the roots of the several spinal nerves in their passage outwards to the intervertebral foramina.
Both the cranial and the spinal parts of the dura mater consist of a tough, fibrous membrane; somewhat flocculent externally, but smooth, glistening, and free on its inner surface. The inner surface has the appearance of a serous membrane, and when examined microscopically is seen to consist of a layer of squamous endothelial cells, similar to those drawn in fig. 34. Hence the dura materis sometimes called a fibro-serous membrane. The dura mater is well provided with lymph vessels, which in all probability open by stomata on the free inner surface. Between the dura mater and the subjacent arachnoid membrane is a fine space containing a minute quantity of limpid serum, which moistens the smooth inner surface of the dura and the corresponding smooth outer surface of the arachnoid. It is regarded as equivalent to the cavity of a serous membrane, and is named the arachnoid cavity, or, more appropriately, the sub-dural space.
Araclazoid mater. - The arachnoid is a membrane of • great delicacy and transparency, which loosely envelopes both the brain and spinal cord. It is separated from these organs by the pia mater ; but between it and the latter membrane is a distinct space, called sub-arachnoid. The sub-arachnoid space is more distinctly marked beneath the spinal than beneath the cerebral parts of the membrane, which forms a looser investment for the cord than for the brain. At the base of the brain, and opposite the fissures between the convolutions of the cerebrum, the interval between the arachnoid and the pia matter can, however, always be seen, for the arachnoid does not, like the pia mater, clothe the sides of the fissures, but passes directly across between the summits of adjacent convolutions. The sub-arachnoid space is subdivided into numerous freely-communicating loculi by bundles of delicate areolar tissue, which bundles are invested, as Key and Retzius have shown, by a layer of squamous endothelium. The space contains a limpid cerebro-spinal fluid, which varies in quantity from 2 drachms to 2 ounces. The fluid is alkaline, of sp. gr. 1.005, contains a little albumen, and a substance which, as Turner pointed out, reduces blue oxide of copper to the state of yellow sub-oxide. The arachnoid membrane is made up of delicate connective tissue. The free surface next the sub-dural space is smooth, like a serous membrane, and covered by a layer of squamous endothelium. This layer is reflected on to the roots of the spinal and cranial nerves, and, when they pierce the dura mater, it becomes continuous with the endothelial lining of that membrane. As the arrangement and structure so closely correspond with what is seen in the serous membranes, many anatomists regard the arachnoid as the visceral layer of a serous membrane, and the endothelial lining of the dura mater as the parietal layer, whilst the sub-dural space is the intermediate cavity.
When the skull cap is removed, clusters of granular bodies are usually to be seen imbedded in the dura mater on each side of the superior longitudinal sinus; these arc named the Pacchionian bodies. When traced through the (Tura mater they are found to spring from the visceral or proper cerebral arachnoid. The observations of Luschka and Cleland have proved that villous processes invariably grow from the free surface of that membrane, and that when these villi greatly increase in size they form the bodies in question. Sometimes the Pacchionian bodies greatly hypertrophy, occasion absorption of the bones of the cranial vault, and depressions on the upper surface of the brain.
Pia mater. - This membrane closely invests the whole : outer surface of the brain. It dips into the fissures between the convolutions, and a wide prolongation, named velum interpositum, lies in the interior of the cerebrum. With a little care it can be stripped off the brain without causing injury to its substance. The pia mater invests the spinal cord, and is more intimately attached to it than to the brain, for not only does it send prolongations into the anterior and posterior fissures of the cord, but slender bands pass repeatedly from its inner surface into the columns of the cord. Hence it cannot be stripped off the cord without causing injury to its substance. The pia matter is prolonged on to the roots both of the cranial and spinal nerves, and on to the filum terminale. This membrane consists of a delicate connective tissue, in which the arteries of the brain and spinal cord ramify and subdivide into small branches before they penetrate the nervous substance, and in which the veins conveying the blood from the nerve centres lie before they open into the blood sinuses of the cranial dura mater and the extradural venous plexus of the spinal canal. The arteries which pass from the pia mater into the brain and spinal cord are invested by a loose sheath, which has been described as forming the wall of a peri-vascular lymphatic vessel ; but Key and Retzius have shown that the space between the blood-vessel and the sheath opens into the subarachnoid space, and contains cerebro-spinal fluid. A network of lymph vessels ramifies freely in the pia mater. It is also well provided with nerves, which arise from the posterior roots of the spinal nerves, from some of the cranial nerves, and from the carotid and vertebral plexuses of the sympathetic. The epi-cerebral and epi-spinal spaces described by His as existing ;between this membrane and the brain and spinal cord are in all probability artificial productions.
In the spinal canal a slender fibrous band projects from the pia mater covering the side of the cord, and, pushing the arachnoid membrane in front of it, is attached by about twenty-two pairs of denticulated processes to the inner surface of the dura mater. It is named ligamentun denticulatum, and its teeth alternate with the successive pairs of spinal nerves.