THE DURA MATER OF THE BRAIN Structure and Relations.—It is a very dense and inelastic membrane (pachymeninx) composed of white fibrous and yellow elastic tissue lined with flat endothelial cells, which constitute its internal surface. The dura of the brain is made up of two layers which are separable up to the eighth or tenth year, viz., an outer endosteal layer and an inner meningeal layer. The external layer constitutes the endosteum of the cranial bones. It is their nutrient membrane. In children it is closely adherent to the cranial bones of which it forms the real periosteum; but it is attached chiefly at the foramina and along the sutures in adults. Through the cranial foramina and sutures it is continuous with the external periosteum. The meningeal layer of the dura is the more extensive as it is folded into the great fissures of the brain, forming the processes of the dura and greatly increasing its protective function. It fuses pretty closely with the external layer after the tenth year. In the adult the internal layer of the dura separates from the outer layer only over the apex of the petrous bone, to form Meckel's space for the semilunar ganglion (Gasseri); at the foramina, to form sheaths for the nerves; and, along the sinuses, to form their internal boundary and to pro duce the great incomplete partitions, called processes.
Processes (Processes dune matris).—From the inner layer of the dura the great processes are formed. The falx cerebri and falx cerebelli hang vertically in the longitudinal fissure of the cerebrum and the posterior notch of the cerebellum; and, into the transverse fissure of the cerebrum, extends horizontally the tentorium cerebelli. The falx cerebri (Figs. 1, and 4) is attached in front to the crista galli and behind to the crucial eminence and superior surface of the tentorium; the falx cere belli (Fig. r), which is absent in our domesticated animals and small in man, continues from the inferior surface of the ten torium, along the occipital crest, to the posterior border of the foramen magnum. The bony attachment of the tentorium cerebelli (Fig. 2) is to the center of the crucial eminence and its horizontal arms forward to the petrous bone; and, then, it is along the superior border of the petrous bone to the clinoid processes of the sphenoid. Between its clinoid attachments there is a deep bay, the incisura tentorii, which transmits the mid-brain.
The horizontal arms of the crucial eminence feebly represent the osseous part of the tentorium which forms a prominent shelf in the horse. The diaphragma sellm is a small centrally perforated sheet of meningeal dura which covers the hypophy seal fossa.
Sinuses (Sinus dune matris).—Large venous passages lined with endothelial cells, and called sinuses, are situated between the layers of the dura (Figs. 1, 2, 3 and 4). In the convex and in the free borders of the falx cerebri are, respectively, the superior sagittal sinus (s. sagittalis superior) and the inferior sagittal sinus (s. sagittalis inferior). The superior (Fig. 1) extends from the foramen cmcum back to the confiuens sinuum (torcular Herophili), located at the internal occipital protuber ance. Having run through the posterior two-thirds of the concave border of the falx cerebri, the inferior sagittal sinus joins the great cerebral vein at the margin of the tentorium and forms the straight sinus (s. rectos). The latter runs through the middle of the tentorium to the confluens (Fig. 2). The occipital sinus (s. occipitalis) traverses the falx cerebelli from the foramen magnum upward to the same point. In the confluens sinuum the transverse sinuses (s. transversi) rise (Fig. 2). Grooving the horizontal arms of the crucial ridge, each runs outward in the tentorium to the base of the petrous bone, where it receives the superior petrosal sinus; it then turns downward through the sigmoid fossa, communicates with the occipital sinus and unites with the inferior petrosal sinus in the jugular foramen. Situated on either side of the sella turcica is a continuation of the ophthalmic vein, the large cavernous sinus (s. cavernosus) (Fig. 3), which receives at the superior orbital fissure the spheno-parietal sinus (s. alce parvce), the course of which is along the posterior border of the lesser wing of the sphenoid bone. At the posterior clinoid process the cavernous sinus divides into the superior petrosal sinus (s. petrosus superior) and the inferior petrosal sinus (s. petrosus inferior). The sinus intercavernosus anterior and sinus intercavernosus posterior extend across the hypophyseal fossa, and join the two cavernous sinuses together, and these four communicating sinuses consti tute the circular sinus (s. circularis) (Fig. 3). From the bifurca tion of the cavernous sinus at the apex of the petrous bone, the petrosal sinuses run outward along the corresponding superior and inferior borders of that bone. The superior petrosal sinus (Figs. r and 3) empties into the transverse sinus at the base of the petrous bone; the inferior petrosal sinus, in its course to the jugular foramen, is joined to its fellow, across the basilar process of the occipital bone, by the basilar plexus (p. basilaris) and, in the jugular foramen, unites with the transverse sinus in forming the internal jugular vein.