Arachnid Granulations (Fig. 3o).—Along and within the superior sagittal, the straight, the transverse, the petrosal and the cavernous sinuses are the granulationes arachnoideales (Pac chionian bodies). These granulations are enlarged villi of the arachnid (Fig. 4) and seem to afford an outlet for the sub arachnoid fluid into the sinuses. They are said to make their appearance between the third and seventh years and to be more numerous and larger in the male sex. Some of them by absorption produce depressions in the cranial bones called foveolce granulares.
The arteries which supply the dura are: (1) The anterior meningeal from the anterior ethmoidal branch of the ophthal mic. (2) The middle meningeals, viz., the great and the small middle meningeal from the internal maxillary, the meningeal branch of the lacrimal and of the internal carotid, and the meningeal branch of the ascending pharyngeal which enters the middle fossa of the cranium through the lacerate foramen. (3) The posterior meningeal arteries, which rise from the ascending pharyngeal, the occipital and the vertebral and are distributed to the dura over the posterior cranial fossa.
The great middle meningeal artery (arteria meningea media) is much the largest and is most important (Fig. 5). It overlies the motor and somasthetic areas of the brain and is the most frequent seat of meningeal hemorrhage. Like the other menin
geal arteries it is usually accompanied by two veins. Ascend ing from the foramen spinosum it divides near the anterior border of the squama into two large branches, the anterior and posterior. The posterior runs horizontally backward across the middle of the squama temporalis, a half inch above the zygo matic arch and then ascends over the posterior half of the parietal bone: The anterior branch runs upward a half-inch behind the coronal suture. It may be located, according to Quain, at one inch, at one inch and a half, and at two inches behind the zygomatic process of the frontal bone and above the zygomatic process of the temporal bone.
The sharp superior border of the squama temporalis fre quently cuts the branches of the middle meningeal at their points of crossing, thus producing meningeal hemorrhage.
The following nerves give branches to the dura: Trochlear ophthalmic, semilunar ganglion, vagus and hypoglossal of the cerebral group; and the sympathetic. The motor fibers supply the meningeal arteries.
Six Points of Difference in the Dura of the Cord.—Absence of processes, of sinuses, of arachnoid granulations, and of periosteal function. It is covered on both surfaces by endothelium and is separated from the vertebrae by areolar tissue, fat and the plexus of internal vertebral veins.