Meninges

The meninges are the layers of connective tissue the cover the central nervous system, which includes the brain and the spinal cord. Both the brain and spinal cord are wrapped in three layers of meninges: the dura mater, arachnoid mater, and pia mater, from outer to inner. These layers continue from the brain through the foramen magnum and also cover the spinal cord.

The dura mater is the outermost tough layer of connective tissue. It consists of two sublayers: an outer periosteal layer that covers the inner surface of the skull, and an inner meningeal layer that surrounds the brain and spinal cord. Inside the dura mater is the delicate arachnoid mater, and within this is the innermost layer called the pia mater, which is the vascular meningeal layer.

Although the two layers of dura mater adhere tightly to each other, there is a potential space between them. Any abnormal fluid buildup, like bleeding or a tumor, can separate these fused layers. It's important to note that in certain natural spots, these two layers are separated. These include areas where dural partitions are formed or where the venous sinuses are located. The dura contains a network of venous sinuses between its two layers, such as the superior sagittal sinus, inferior sagittal sinus, straight sinus, and sigmoid sinus, among others. These sinuses carry venous blood and also allow cerebrospinal fluid (CSF) to drain into them via the subarachnoid villi/granulations.

In certain areas, the inner layer of the dural mater separates from the periosteal layer and extends into the brain, attaching to certain bony areas. This creates partitions or septa that divide brain regions and provide support for the brain's structure. These dural partitions include the falx cerebri, falx cerebelli, tentorium cerebelli, and diaphragma sellae.

Understanding the spaces that exist between the different meningeal layers is important. Most of these spaces are potential spaces, except for the subarachnoid space, which is a real space filled with cerebrospinal fluid between the arachnoid and pia mater. The subarachnoid space also contains cerebral arteries and veins, including the circle of Willis. If little berry aneurysms on the circle of Willis burst, it can lead to life-threatening subarachnoid hemorrhage. The space between the periosteal dural layer and the bony cranial cavity above it is called the epidural (or extradural) space. Epidural (arterial) hemorrhages can occur due to the rupture of the middle meningeal arteries and require immediate neurosurgical intervention. The subdural space, located between the dura and arachnoid mater, can have venous subdural hemorrhages due to the traction and rupture of bridging veins.

The meninges receive their nerve supply from the vagus and trigeminal nerves, as well as some cervical spinal nerves.