SYSTEM.] ANATOMY 865 sinuses, named lateral, which curve forwards and down wards 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 back wards 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 vertebra, 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 tubu lar envelope for the spinal cord and the origins of the spinal nerves. It extends from the foramen magmim, 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 mater, is some times called a fibro-serous membrane. The dura mater is well provided with lymph vessels, which in all probability open by stornata on the free inner surfaca 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 mem brane, and is named the arachnoid cavity, or, more appropriately, the sub-dural space.
- hnoi<l Arachnoid mater. The arachnoid is a membrane of
ibnmc. 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 convolu tions. 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 quan tity from 2 drachms to 2 ounces. The fluid is alkaline, of sp. gr. I OOS, 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 squam ous 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 struc ture 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 laj er, whilst the Bub-dural space is the intermediate cavity. When the skull cap is removed, clusters of granular Pacdiio bodies are usually to be seen imbedded in the dura mater uian on each side of the superior longitudinal sinus; these arc bodies. named the Pacchionian bodies. When traced through the dura mater they arc 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 Pia mater outer surface of the brain. It dips into the fissures between the convolutions, and a wide prolongation, named vehim 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 sub stance. 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 extra- dural 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 de scribed 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 sub- arachnoid space, and contains cerebro-spinal fluid. A net work 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 Ligamen- the pia mater covering the side of the cord, and, pushing turn dent the arachnoid membrane in front of it, is attached by culatum - about twenty-two pairs of denticulated processes to the inner surface of the dura mater. It is named Kgatnentun denticulatum, and its teeth alternate with the successive pairs of spinal nerves. SPINAL COED. The MEDULLA SPINALIS, or SPINAL Spinal COED, occupies the spinal canal, and extends from the conl foramen magnum to opposite the body of the first lumbar vertebra. In the early foetus it equals in length the canal itself ; but as the spinal column grows at a greater pro portional rate than the cord, the latter, when growth has ceased, is several inches shorter than the column. The cord is continuous above with the medulla oblongata, whilst it tapers off below into a slender thread, the filum terminale, which lies in the axis of the sacral canal, and is attached below to the back of the coccyx, or to the fibrous mem brane which closes in below the sacral canal. The length of the cord is from 15 to 18 inches. It approaches a cylinder in shape, but is flattened on its anterior and pos terior surfaces, and presents two enlargements which have a greater girth than the rest of the cord. The upper,
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