Meninges

= **__MENINGES__** =

The brain and the spinal cord are covered by three layers of membraneous coatings called meninges. They protect and enclose its contents. The three layers from external to internal are named as follows:
 * 1) Dura mater
 * 2) Arachnoid mater
 * 3) Pia mater



1. **Cranial dura mater**

The cranial dura mater is the __outermost layer__ of the cranial meninges. It is tough and durable and contains two layers:
 * An external **Periosteal layer** - is normally fused with the skull. It houses the meningeal arteries. Fluid can accumulate between this layer and the skull, creating an extradural space, following trauma. It is called an **extradural haematoma** if the fluid in this potential space is blood.
 * An internal **meningeal layer**- the spinal dura mater joins this layer at the foramen magnum. It lies on top of the arachnoid mater.

The external and internal layers are normally in contact with each other, however at specific areas they do separate in order to form **dural sinuses** and **dural partitions**.

__Arterial supply of dura mater__

The **meningeal arteries** (anterior, middle, accessory and posterior) supply the dura mater. The most important clinically is the **middle meningeal artery**. It is by far the largest. It is a branch of the **maxillary artery** and it enters the cranial cavity through the **foramen spinosum** where it divides into anterior and posterior branches. The anterior branch passes underneath the __**pterion**__ (This is a very thin part of the skull where the parietal, frontal, sphenoid and temporal bones all lie in close by to each other).



If there is a fracture here and the anterior devision of the middle meningeal artery is severed, this could cause an extradural haematoma) as it makes it way to the vertex of the skull. The posterior branch travels in a posterosuperior direction and supplies the middle cranial fossa.

The **anterior me ningeal arteries ** arise from the ethmoidal arteries (anterior and posterior).

The **accessory meningeal artery** enters the cranial cavity through the **foramen ovale** supplies its surrounding area.

The ** posterior meningeal artery ** is the terminal branch of the ascending paryngeal artery and it enters the cranial cavity through the **jugular foramen**.

There are several **meningeal branches** from other sources that also supply the dura mater:
 * **ascending pharyngeal artery** which enters the cranial cavity through the **hypoglassal canal**
 * **occipital artery** which enters the cranial cavity via the **jugular and mastoid foramina**.
 * **vertebral artery** which enters the cranial cavity through the **foramen magnum**.



__Innervation of dura mater__

Meningeal branches from the V1, V2 and V3 divisions of the trigeminal nerve and cervical nerves 1 to 3:


 * The **anterior cranial fossa** is supplied by **meningeal branches from ethmoidal nerves** which arise from the ophthalmic nerve (V1).


 * The **medial aspect** of the **middle cranial fossa** is supplied by **meningeal branches of the maxiillary nerve** and the **lateral aspect** is innervated by **meningeal branches of the mandibular nerve (V3)**.


 * The **posterior cranial fossa** is innervated by **meningeal branches of cervical nerves 1 and 2 (occasionally cervical nerve 3 also)**.

2. **Arachnoid mater**

The subarachnoid space lies between the arachnoid and pia mater. This gap is maintained by the presence of cerebrospinal fluid and arachonoid trabeculae (finger- like processes that project through the subarachnoid space, connecting the arachnoid and pia mater).

The arachnoid mater is __thin and avascular__.

3. **Pia mater**

This is the __innermost layer__ of the cranial meninges. It is fused to the surface of the brain and gives it it's glossy appearance.

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