Additionally, a meningeal branch of the ophthalmic nerve [V1] turns and runs posteriorly, supplying the tentorium cerebelli and the posterior part of the falx cerebri.
The middle cranial fossa is supplied medially by meningeal branches from the maxillary nerve [V2] and laterally, along the distribution of the middle meningeal artery, by meningeal branches from the mandibular nerve [V3].
The posterior cranial fossa is supplied by meningeal branches from the first, second, and, sometimes, third cervical nerves, which enter the fossa through the foramen magnum, the hypoglossal canal, and the jugular foramen. Meningeal branches of the vagus nerve [X] have also been described. (Possible contributions from the glossopharyngeal [IX] and hypoglossal [XII] nerves have also been reported.)
The arachnoid mater is a thin, avascular membrane that lines, but is not adherent to, the inner surface of the dura mater (Fig. 8.35). From its inner surface thin processes or trabeculae extend downward, cross the subarachnoid space, and become continuous with the pia mater.
Unlike the pia, the arachnoid does not enter the grooves or fissures of the brain, except for the longitudinal fissure between the two cerebral hemispheres.
The pia mater is a thin, delicate membrane that closely invests the surface of the brain (Fig. 8.35). It follows the contours of the brain, entering the grooves and fissures on its surface, and is closely applied to the roots of the cranial nerves at their origins.
Arrangement of meninges and spaces
There is a unique arrangement of meninges coupled with real and potential spaces within the cranial cavity (Fig. 8.35).
A potential space is related to the dura mater, while a real space exists between the arachnoid mater and the pia mater.
The potential space between dura mater and bone is the extradural space (Fig. 8.35). Normally, the outer or periosteal layer of dura mater is firmly attached to the bones surrounding the cranial cavity.
This potential space between dura and bone can become a fluid-filled actual space when a traumatic event results in a vascular hemorrhage. Bleeding into the extradural space primarily due to rupture of a meningeal artery or less often from a torn dural venous sinus results in an extradural hematoma.
Anatomically, a true subdural space does not exist. Blood collecting in this region (subdural hematoma) due to injury represents a dissection of the dural border cell layer, which is the innermost lining of the meningeal dura. Dural border cells are flattened cells surrounded by extracellular spaces filled with amorphous material. While very infrequent, an occasional cell junction may be seen between these cells and the underlying arachnoid layer. Bleeding due to the tearing of a cerebral vein as it crosses through the dura to enter a dural venous sinus can result in a subdural hematoma.
Deep to the arachnoid mater is the only normally occurring fluid-filled space associated with the meninges, the subarachnoid space (Fig. 8.35). It occurs because the arachnoid mater clings to the inner surface of the dura mater and does not follow the contour of the brain, while the pia mater, being against the surface of the brain, closely follows the grooves and fissures on the surface of the brain. The narrow subarachnoid space is therefore created between these two membranes (Fig. 8.35).
The subarachnoid space surrounds the brain and spinal cord and in certain locations it enlarges into expanded areas (subarachnoid cisterns). It contains cerebrospinal fluid (CSF) and blood vessels.
Cerebrospinal fluid is produced by the choroid plexus, primarily in the ventricles of the brain. It is a clear, colorless, cell-free fluid that circulates through the subarachnoid space surrounding the brain and spinal cord.