The dura niater is perforated by numerous orifices fur the transmission of the encephalic nerves. It adheres firmly to the border of each of the foramina in the cranial bones, and is partly continued in the shape of neurileinma over the nerve that escapes through it. In the case of the optic nerve a strong fibrous sheath is prolonged from the dura mater, and at the same time that membrane appears to become continuous with the periosteum of the orbit, as if it had, opposite the optic foramen, split into two layers, one of which formed the sheath of the optic nerve, and the other applied itself to the interior of the orbit, forming a periosteum to the walls of that cavity.
0 f the arteries and veins of the dura mater. —The disposition of the bloodvessels of the dura rnater, both of the spine and of the cra nium, deserves a special description. The former membrane derives its arteries from the numerous vessels which take their rise close to the spinal column in its various regions. These are ramifications of the abdominal and thoracic aorta or of their large primary branches. In the neck the deep cervical, the occipital, and the vertebral arteries send in numerous branches, in the back the intercostal arteries, and in the loins the lumbar arteries. These vessels pass in at the vertebral foramina, and send bmnches to the spinal membranes as well as to the bones themselves.
The blood which is retumed from the spinal cord and its membranes, as well as from the vertebr-m, is poured into a very intricate plexus of veins which surrounds the dura muter on its lateral and posterior surfaces, ramifying among the lobules of soft fa.t by which the exterior of that membrane is invested. This plexus is less intricate in the dorsal than in the cervical or lumbar regions; it communi cates very freely with the plexus of veins which lies on the exterior of the vertebral laminm and processes (the dorsi-spinal veins of Dupuytren).
I n front of the dura mater and situate between the outer edge of the posterior common ment of the vertebrx and the pedicles, we find o remarkable venous sinuses which extend the who'e lenp,th of the vertebral column, from the occipital foramen to the sacral region (fig.360).
terior common ligament. These cross branches present the same characters as the sinuses them selves, being of variable calibre, and presenting the greatest degree of dilatation at their middle.
At this point these branches receive veins which emerge from the spongy texture of the bodies of the vertebne ( basi-vertebral veins of Breschet) (fig. 361, d). The vertebral sinuses diminish in These veins are loosely covered by a thin pro cess, which is prolonged from each margin of the posterior common ligatnent, and which is sufficiently transparent to allow them to be seen through it without removing it. They have been known since the time of Fallopius, and were described by Willis as the longitudinal spinal sinuses. In calibre they present many inequalities, being dilated at one part and con stricted at another, according to the number and size of the vessels which communicate with them. The sinuses of opposite sides run parallel to each other and communicate by cross branches, which pass between the posterior sur face of the body of each vertebra and the pos size at the highest part of the vertebral canal, and passing through the anterior condyloid foramina, communicate with the internal jug,ular reins. In the sacral region they diminish considerably likewise, and are lost in becoming continuous with the lateral sacral veins and other small veins in that region; and they communicate with the deep and superficial vertebral veins in the neck, with the intercostal veins in the back, and with the lumbar ones in the loins. They evidently differ from the sinuses of the cranial dura mater in not being enclosed between two layers of fibrous membrane as those vessels are.
Bloodvessels of the cranial dura mater.— The bloodvessels of the cranial dura mater are much more numerous than those of the spinal, in consequence, no doubt, of that mem brane performing the office of a periosteum to the cranial bones. The arteries are derived from numerous sources ; in front from the ophthalmic and ethmoidal arteries, in the middle from the internal maxillary artery by the middle ineningeal, which enters the cranium at the foramen spinosum, and by small branches from the internal carotid which have been called inferior meningeal arteries. Posteriorly the vertebml, the occipital, and the ascending pharyngeal supply branches which go by the name of posterior meningeal arteries.