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III./1.2.: Dura mater
III./1.2.1.: Layers of the dura mater
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The term dura mater (meninx fibrosa) is not referring to a single membrane, but to two laminated layers adhered together. The layers however are separable by careful dissection. The outer layer is not only the fibrous and rough covering of the brain, but also an internal periosteum to the bones (endosteum, endocranium), therefore outer cranial dura mater or endocranium and inner cerebral dura mater (dura mater encephali) are distinguished. In childhood the dura is thick and it adheres closely to the inner surfaces of the bones, while in advanced life the dura mater may get more attenuated and rupturable.
III./1.2.2.: Surroundings of the dura mater
Besides padding the cavity of the skull the dura mater also sends inward processes which disintegrates it. The cerebral hemispheres are separated by a double fold of the inner dural layer forming the falx cerebri. In front it is attached to the crista galli and crista frontalis and descends posteriorly to the straight sinus (sinus rectus), where the two layers separate and bending laterally they run toward the upper edge of the left and right petrosal bone (superior angle petrosal part of the temporal bone). Together the layers give a tentlike appearance, they form the tentorium cerebelli, which partially separates the middle and posterior cranial fossa. It bounds a large opening, the tentorial incisure, for the transmission of the upper part of the brainstem. It’s sharp, free margin extends anteriorly to the anterior clinoid process as the anterior petroclinoid ligament.
While the posterior petroclinoid ligament arises from the posterior clinoid process as a posteromedial extensions of the tentorial incisure. (The trochlear nerve commonly exits the midbrain at the juncture of the ligaments.) Similarly to the falx cerebri in the posterior cranial fossa a duplication of inner dura mater forms the falx cerebelli separating the two cerebellar hemispheres. Note that neither the falx cerebri nor the tentorial incisure is adhered to the surface of the separated structures, therefore under pathologic conditions (increase intracranial pressure caused by mass effect of any origin) those are predilection places for herination.
III./1.2.3.: Further processes of the dura mater
III./1.2.3.1.: Dural poruses
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The anterior, middle and posterior cranial fossae are leaklessly padded with the dural layers and the dura mater is also prolonged to form hornlike sheats for the nerves and vessels passing through to the neighboring structures of the skull, head and neck. Those „dural openings” are called the dural poruses.
III./1.2.3.2.: Dural venous sinuses
In certain places the dural layers separate to form the dural venous sinuses, which are lined with endothelium. They receive blood from the deep and superficial veins of the brain (bridge veins) passing through the space between the pia and dura mater. Ultimately the sinuses are passing the received intracranial venous blood to the extracranial veins.
(i) The superior sagittal sinus courses along the sagittal suture of the vertex occuping the falx cerebri and drains into the (ii) confluence of sinuses of posterior cranial fossa [4]. From it the (iii) transverse sinus is derived, which at the mastoid process and petrous part of the temporal bone is continued as (iv) sigmoid sinus. The sigmoid sinus drains the blood to the jugular venous bulb. The (v) occipital sinus runs along the internal occipital crest and ends in the confluence of the sinuses giving it a cross-shaped appearance, therefore its impression on the bones (internal occipital protuberance or „torcular Herophili”) was called cruciform eminence.
The (vi) sraight sinus also terminates in the confluence of sinuses, which recives blood from the (vii) inferior sagittal sinus coursing along the inferior, open border of the falx cerebri and from the internal cerebral veins and the basal vein (of Rosenthal) through the great cerebral vein (of Galeni). The (viii-ix) anterior and inferior petrosal sinuses are situated along the upper and lower margin of the posterior surface of the petrous bone, the anterior sinus courses in line with the insertion of the tentorium cerebelli. They receives blood from the (x) cavernous sinus, via the cavernous sinus from the (xi) sphenoparietal sinus coursing along the small wing of the sphenoid bone and from the (xii) basilar plexus lying in the double fold of dura over the clivus, and they drain into the sigmoid and transverse sinuses as well as into the confluence of the sigmoid sinus and the bulb of the jugular vein.
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Out of the mentioned sinuses the cavernous sinus has the most complex anatomical structure. Similarly to them it is composed of a double folded dura mater lined with endothelium. There are two cavernous sinuses on either side of the sphenoid bone and sella turcica (Turkish Chair). The roof of cavernous sinus is limited laterally and posteriorly by the petroclinoid ligaments, superiorly by the diaphragma sellae and medially the sides are connected through the intercavernous sinuses. The intracavernous segment of the internal carotid artery (ICA) with the it’s arising branches and the sympathetic plexus are passing through in the lumen of the cavernous sinus. The lumen is transverse by numerous interlacing filaments (trabeculae) and occasionally filled with adipose tissue.
The abducent nerve (n.V.) enters the sinus under the rearmost and also the strongest trabecula (Dorellos’s canal) on the side of the ICA. The medial wall of the sinus is also borders laterally the pituatory gland. The oculomotor nerve (n.III) entering the sinus through its roof and the trochlear nerve (n.IV) entering through the posterior wall at the angle of the petroclinoid ligaments are coursing forward in the dura of the lateral wall, than they cross each other’s path.
Inferior these nerves the first and under it for a short distance the second branch of the trigeminal nerve (n. V.), the ophthalmic and maxillar nerves posteriorly enter the lateral wall of cavernous sinus from the trigeminal cave (Meckel's Cave). It’s a dural cave is formed by two layers of dura mater neighboring the posterior wall of the cavernous sinus and between the layers in a narrow space it accommodates the trigeminal (Gasserian or semilunar) ganglion. The trigeminal nerve passes from the posterior to the middle cranial fossa in the trigeminal cave which opens posteriorly as the porus trigeminus. These beformentioned nerves exit the cavernous sinus through the superior orbital fissure, with the exception of the maxillar nerve which exits through the foramen rotondum. Through the superior orbital fissure it receives blood from the orbit through superior ophtalmic vein.
III./1.2.3.3.: Bridge veins
Finally we give you a few, neurosurgically relevant bridge veins. The superior petrosal sinus drains blood from the cerebellopontine angle and from parts of the mesencephalon into the superior petrosal vein, it enters the sinus inferior to the trigeminal nerve and it is known as the Vein of Dandy. The superior anastomotic vein, also known as the vein of Trolard, passes through blood from the from the convexity of the brain above the lateral cerebral fissure to the superior sagittal sinus. The inferior anastomotic vein, also known as the vein of Labbé drains from the occipital and temporal veins under the lateral cerebral fissure into the transverse sinus or into the confluence of the transverse and sigmoid sinuses.
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