Within the bony encasement of the skull and vertebral column, the CNS is surrounded by three concentric, connective tissue coverings called meninges (from Greek word meninx for “membrane”), which act to support and stabilize the brain and spinal cord. The focus of this section will be on the cranial meninges. The spinal meninges, which have a slightly different configuration, will be discussed in the “Spinal Cord” section.
The outermost covering is the dura (Latin for “hard”) mater (Latin for “mother”), a tough, fibrous sheet composed of two layers. The outer periosteal layer is adherent to the skull, and the inner meningeal layer lies against the underlying arachnoid mater (eFig. 9.17). Although these two layers are closely adherent to one another, they do separate in some regions to form dural venous sinuses, which receive cerebral venous drainage (eFig. 9.18). The anatomy of the venous sinuses will be discussed in the “Cerebral Vasculature” section. Two potential spaces exist as the epidural (extradural) space, superficial to the periosteal layer, and subdural space, deep to the meningeal dural layer. These spaces can become filled with blood during vascular trauma (eTable 9.3). Within the cranial cavity, the meningeal layer of dura mater folds in on itself in several areas to form dural reflections, or septa. These reflections are known as the falx (Latin for “sickle”) cerebri between the cerebral hemispheres, as the tentorium cerebelli between the cerebral hemispheres and cerebellum, and as the falx cerebelli between the cerebellar hemispheres (eFig. 9.17). A smaller reflection, the diaphragm sellae, covers the pituitary fossa and underlying pituitary gland.
Deep to the dura mater is the arachnoid (from the Greek word arachne meaning “spider’s web”) mater. The outer layer of the arachnoid mater is composed of several layers of flattened cells that lie adjacent to the meningeal layer of the dura mater (eFig. 9.19). Strands of connective tissue extend from this outer layer to form arachnoid trabeculae, which connect internally to the pia mater (eFig. 9.19). The pia mater forms a thin, veil-like layer that closely follows the gyri and sulci on the surface of the brain. The pia and arachnoid matter are separated by a subarachnoid space that contains CSF and the major blood vessels supplying the brain.
Vascular supply to the brain is divided into the anterior circulation arising from the internal carotid arteries and posterior circulation from the vertebral arteries (eFig. 9.20). The internal carotid arteries arise from the branching of the common carotid arteries at the level of the fourth cervical vertebra. Bilaterally the arteries course through the neck to enter the middle cranial fossa through the carotid canal. The arteries then make a series of turns to pass through the petrous portion of the temporal bone and cavernous sinus before entering the subarachnoid space just lateral to the optic chiasm. Upon exiting the cavernous sinus, the internal carotid artery gives rise to the ophthalmic artery and then continues superiorly to give off the posterior communicating artery and anterior choroidal arteries before terminating as the anterior and middle cerebral arteries (eFig. 9.21).