Anatomy of the Brain: The Meninges
The meninges are the coverings of the brain. They protect the brain by housing a fluid-filled space, and they function as a framework for blood vessels. The meninges have three layers: the dura mater, the arachnoid mater, and the pia mater. The arachnoid mater is attached to the pia mater by arachnoid trabeculae, which is a weblike matrix of connective tissue. The space between the two layers, the subarachnoid space, is filled with cerebrospinal fluid (CSF). The subarachnoid space includes several cisterns, areas where the space between the pia and arachnoid mater is widened due to an accumulation of CSF, that function as reservoirs.
Pressure from the cerebrospinal fluid presses the arachnoid mater against the dura mater. The pia mater adheres to the surface of the brain.
The dura mater is the most superficial layer of the meninges and contains folds and sinuses. The dura mater contacts the endosteum that lines bones of the cranial cavity. The dura mater is a strong fibrous membrane that surrounds the brain and is continuous with the dura that covers the spinal cord.
Meningitis is an infection of the meninges caused by viruses or bacteria. Generally, the bacterial forms are much more serious. Patients with meningitis usually are sensitive to light and have a fever, severe headache, stiff neck, mental disturbances, nausea, and vomiting. Meningitis can be diagnosed by blood tests and by performing a lumbar puncture, in which cerebrospinal fluid is collected and analyzed for cell counts, glucose, and protein. Treatment requires antibiotics and may require hospitalization.
The dural infoldings
The meningeal layer folds up to form dural infoldings that divide the cranial cavity into different compartments:
Falx cerebri: The largest infolding, the falx cerebri is found in the longitudinal cerebral fissure, which divides the two hemispheres of the cerebrum.
Tentorium cerebelli: This infolding separates the occipital lobes of the cerebrum from the cerebellum. The falx cerebri connects to the tentorium cerebelli at the midline and helps to hold it in place, except for the anterior part, which is left free, making a gap called the tentorial notch.
Falx cerebelli: This vertical infolding is below the tentorium cerebelli. It helps to separate the cerebellar hemispheres.
Diaphragma sellae: This smaller infolding forms a roof over the pituitary gland.
The dural infoldings form spaces between the two layers of the dura mater. These spaces are called dural venous sinuses, and they collect blood from veins on the surface of the brain. Blood from the sinuses empties into the internal jugular veins. The sinuses include the following:
Superior sagittal sinus: Formed in the superior part of the falx cerebri, this sinus runs from the crista galli of the ethmoid bone.
Inferior sagittal sinus: This sinus runs along the bottom of the falx cerebri to the straight sinus.
Straight sinus: This dural venous sinus is formed where the inferior sagittal sinus merges with the great cerebral vein. It follows the place where the tentorium cerebelli attaches to the falx cerebri and joins the confluence of sinuses.
Transverse sinuses: The transverse sinuses run laterally from the confluence of sinuses where the tentorium cerebelli attaches to the occipital bones. Where they reach the temporal bone they become the sigmoid sinuses.
Sigmoid sinuses: These sinuses run along an S-shaped course and pass through the jugular foramen, which is located at the base of the occipital and temporal bones. From there, they become the internal jugular veins.
Occipital sinus: The occipital sinus runs along the falx cerebelli up to the confluence of sinuses.
Cavernous sinuses: Found on each side of the sella turcica of the sphenoid bone, the cavernous sinuses drain into the superior and inferior petrosal sinuses.
Superior petrosal sinuses: These sinuses continue from the cavernous sinuses to the transverse sinuses.
Inferior petrosal sinuses: Running from the posterior part of the cavernous sinuses, these sinuses empty into the internal jugular veins.
Arachnoid granulations protrude from the arachnoid mater into the dural venous sinuses. They move cerebrospinal fluid from the subarachnoid space to the venous system. Emissary veins connect the dural venous sinuses with veins that run outside of the cranium.
The dura mater receives arterial blood from the middle meningeal artery. The frontal branch runs deep to the pterion and then moves posteriorly toward the top of the cranium. The parietal branch runs posteriorly and superiorly along the cranium. The meningeal arteries are accompanied by veins of the same name. The dura mater is innervated by the cranial nerves.