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2.4. The Meninges
Overview of the Meninges
The meninges are three protective membranes that line the brain and spinal cord. The meninges, along with cerebrospinal fluid (CSF), act as shock absorbers in scenarios when the head is moved suddenly in a particular direction. They also provide a safe environment for the passage of blood vessels and space for CSF to move through.
The Meningeal Layers
There are three main meningeal layers, which are described below and shown in Figure 2.4.1.
1. Dura mater
The dura mater is the outermost and toughest layer of the meninges. ‘Dura mater’ is actually a Latin phrase that translates as ‘hard mother of the brain’. The dura mater, which is sometimes just called the dura, is divided into two sub-layers: the outer endosteal layer and the inner meningeal layer. The endosteal layer is firmly attached to the cranium (skull). However, the meningeal layer drops downwards in the midline, forming a meningeal fold called the falx cerebri. The falx cerebri separates the left and right cerebral hemispheres. Another similar fold of the meningeal layer lies between the cerebrum and the cerebellum, and this is called the tentorium cerebelli. Both of these dural folds provide room for the passage of particular veins in the brain, called the dural venous sinuses, which help to drain blood from the brain into the internal jugular veins.
2. Arachnoid mater
The arachnoid mater is the thin middle layer of the meninges. It is attached to the meningeal layer of the dura mater and follows its folds. In Latin, the arachnoid and pia mater are known as the ‘leptomeninges’, which translates as ‘thin membranes’.
3. Pia mater
The pia mater is the thin innermost layer of the meninges that is intimately adhered to the folds of the brain matter.
It’s important to note that the meninges extend to cover the spinal cord too. In fact, the meninges of the brain are continuous with the meninges of the spinal cord through the foramen magnum of the skull. Although the spinal cord proper ends at L1/L2, the meninges, in one form or another, extend to the level of the S2 vertebra.
1. Dura mater
The dura mater is the outermost and toughest layer of the meninges. ‘Dura mater’ is actually a Latin phrase that translates as ‘hard mother of the brain’. The dura mater, which is sometimes just called the dura, is divided into two sub-layers: the outer endosteal layer and the inner meningeal layer. The endosteal layer is firmly attached to the cranium (skull). However, the meningeal layer drops downwards in the midline, forming a meningeal fold called the falx cerebri. The falx cerebri separates the left and right cerebral hemispheres. Another similar fold of the meningeal layer lies between the cerebrum and the cerebellum, and this is called the tentorium cerebelli. Both of these dural folds provide room for the passage of particular veins in the brain, called the dural venous sinuses, which help to drain blood from the brain into the internal jugular veins.
2. Arachnoid mater
The arachnoid mater is the thin middle layer of the meninges. It is attached to the meningeal layer of the dura mater and follows its folds. In Latin, the arachnoid and pia mater are known as the ‘leptomeninges’, which translates as ‘thin membranes’.
3. Pia mater
The pia mater is the thin innermost layer of the meninges that is intimately adhered to the folds of the brain matter.
It’s important to note that the meninges extend to cover the spinal cord too. In fact, the meninges of the brain are continuous with the meninges of the spinal cord through the foramen magnum of the skull. Although the spinal cord proper ends at L1/L2, the meninges, in one form or another, extend to the level of the S2 vertebra.
The Meningeal Spaces
As you might expect, there are a number of spaces associated with the three main meningeal layers. These are detailed below and in Figure 2.4.2.
1. Extradural (also called epidural) space
This space is found above the endosteal dura layer, i.e. it is found between the endosteal dura layer and the skull. The extradural space is a potential space - this means that it normally isn't occupied by anything at all but in the event of, for example, a bleed from the arteries supplying the meninges, blood can accumulate in it – this is called an extradural haematoma.
2. Subdural space
The subdural space lies between the arachnoid mater and dura mater. This is another potential space that has little ‘give’ if blood leaks into it – this would be called a subdural haematoma.
3. Subarachnoid space
The subarachnoid space is located between the arachnoid space and the pia mater. It is an actual space that is filled with moving CSF, as well as a variety of blood vessels that will eventually pierce the pia mater and supply the brain matter.
Note that the pia mater is so intimately adhered to the brain matter and all of ridges and dips that there is no subpial space.
1. Extradural (also called epidural) space
This space is found above the endosteal dura layer, i.e. it is found between the endosteal dura layer and the skull. The extradural space is a potential space - this means that it normally isn't occupied by anything at all but in the event of, for example, a bleed from the arteries supplying the meninges, blood can accumulate in it – this is called an extradural haematoma.
2. Subdural space
The subdural space lies between the arachnoid mater and dura mater. This is another potential space that has little ‘give’ if blood leaks into it – this would be called a subdural haematoma.
3. Subarachnoid space
The subarachnoid space is located between the arachnoid space and the pia mater. It is an actual space that is filled with moving CSF, as well as a variety of blood vessels that will eventually pierce the pia mater and supply the brain matter.
Note that the pia mater is so intimately adhered to the brain matter and all of ridges and dips that there is no subpial space.
Clinical Top Tip:
Herniation
The skull is a rigid box with the meninges inside. If there is a severe, ongoing bleed into one of the meningeal spaces, it could lead to a life-threatening condition called herniation. This occurs when a haematoma (collection of blood) gets so large that it pushes the rest of the brain through one of the spaces in the skull. Sometimes, this is tonsillar herniation, where the cerebellar 'tonsils' are forced downwards and through the foramen magnum. This has profound effects on an individual’s basic survival functions. It can also lead to very abnormal movements in what is called posturing, specifically, decorticate and decerebrate posturing.
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