The Meninges

Learning Objectives

Name the three meningeal layers.
Describe the function of the meninges.
List and describe the location of the dural septa.
List and describe the dural sinuses.
Explain the blood supply to the meninges.
Describe the innervation of the dura mater.

Overview of the Meninges

The brain and spinal cord reside in bony structures (skull and vertebral column), which provide protection from external forces. Additionally, the central nervous system (CNS) is encased in membranous coverings called the meninges, which help to stabilize the structures particularly during body movements (). The three meningeal layers, from the outermost to the innermost are: the dura mater, the arachnoid mater, and the pia mater. The space between the arachnoid and pial layer is called the subarachnoid space. The subarachnoid space is filled with cerebrospinal fluid (CSF), which is a colorless liquid that is somewhat similar to plasma. All three meningeal layers are continuous around the spinal cord. The dura mater consists of two parts referred to as periosteal and meningeal layers. The meningeal dural layer creates septi that protrude into cranial cavities and separate different aspects of the brain. At several points along the dural reflections, the two layers separate to form venous channels called sinuses that drain the cerebral veins. The dura mater has its own blood supply from meningeal arteries, the largest of which is the middle meningeal artery. Unlike the brain and other meninges, the dura mater is sensitive to pain. It is largely innervated by the trigeminal nerve (CN V) with contributions from the vagus and upper cervical nerves.

Fig. 8.1 Brain in situ with the dura partially dissected from the arachnoid layer. The dura has been opened and reflected upward, leaving the underlying arachnoid and pia mater on the brain. Because the arachnoid is so thin, the underlying subarachnoid space and the vessels that lie within it can be seen. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)

Meningeal Layers

There are three distinct meningeal layers that make up the coverings of the brain and spinal cord. From superficial to deep direction, the layers are called the dura mater, the arachnoid mater, and the pia mater (a–c).

Fig. 8.2 (a) Brain and meninges in situ. Superior view of the cranial cavity with the calvarium removed.

(b) The calvarium has been removed, and the superior sagittal sinus and its lateral lacunae have been opened. (c) From superficial to deep, the layers are: dura, arachnoid, and pia mater. (d) The dura has two components, a periosteal layer and a meningeal layer. The arachnoid layer is attached to the inner aspect of the meningeal dura. It is separated from the pia by the subarachnoid space. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)

All three connective tissue layers are derived from the menix-primitiva, a meningeal mesenchyme originating from embryonic mesoderm and neural crest.
The tough, fibrous dura is the most appreciable of the layers and is often called the pachymenix, referring to its “thickness.” The word pachy is derived from the Greek word pachy, meaning “tough or thick.” Mater is the Latin word for “mother.”
The arachnoid and pial layers are significantly thinner and more delicate than the dura. These inner layers are continuous with each other and are often called the leptomeninges. Lepto is the Greek word meaning “fine.” Arachnoid is the Greek word for “spider” and pia is the Latin term for “tender or loving.”
The dural mater has two components: a periosteal layer that is attached to the inner aspect of the skull and a meningeal layer that covers the brain.
The arachnoid layer is attached to the inner surface of the meningeal dura. It is separated from the pial layer by the subarachnoid space, which contains CSF and blood vessels (d).
- The arachnoid layer characteristically has small projections called arachnoid villi that are important for the recirculation of CSF.
- Aggregates of villi that project into dural sinuses and facilitate the transfer of CSF are called arachnoid granulations. They are most commonly associated with the transverse and superior sagittal sinuses.
The pial layer is intimately associated with the surface of the brain and extends down into the sulci, which are grooves that cover the surface of the brain.
The meningeal dura, arachnoid, and pia layers surrounding the brain are continuous on the spinal cord.

Function of the Meninges

The meninges, in particular the dura, provide protection from mechanical damage due to trauma and also serve as a framework for the venous drainage of the brain.

Under normal conditions, there is no actual space on the superior or inferior surface of the dura. That is because the dura is tightly adherent to the inner surface of the skull and the arachnoid layer is adherent to the inner surface of the dura.
- In pathological conditions, such as hemorrhages, the epidural and subdural potential spaces can be involved (a, b).

Fig. 8.3 (a, b) After trauma, hemorrhages can develop in the potential spaces. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)