16

Meninges And Cerebrospinal Fluid

Meninges

The brain and spinal cord are enclosed in three protective membranes called meninges. From without inwards these are: (a) dura mater, (b) arachnoid mater, and (c) pia mater.

The dura mater is mesodermal in origin while arachnoid and pia mater are ectodermal in origin (derived from neural crests).

The dura mater is the thick outermost covering of the brain and spinal cord. The part enclosing the brain is called cranial/cerebral dura, and the part around the spinal cord, the spinal dura. It is very tough opaque inelastic membrane of fibrous tissue (Gk. dura = tough, mater = mother). It is also called pachymeninx (pachy = thick).

The arachnoid mater (Gk. arachnoid = cobweb like, mater = mother) is a delicate avascular membrane deep to dura mater. Many thread-like trabeculae extend from it to the pia mater.

The pia mater (Gk. pia = tender, mater = mother) is a thin transparent vascular membrane closely adherent to the surface of the brain and spinal cord.

The arachnoid mater and pia mater together are termed leptomeninges (Gk. lepto = thin).

The intracranial arrangement of these membranes differs from that in the vertebral canal and are therefore described separately.

The spinal meninges are described in Chapter 7).

Intracranial Meninges

Dura mater

The dura mater in the cranium (cranial dura) consists of two layers: an outer endosteal layer and an inner meningeal layer. These two layers are firmly adherent to each other everywhere except, (a) where they split to enclose the venous sinuses, and (b) where the inner layer is folded to form the dural septa.

The endosteal layer is attached to the inner surfaces of the cranial bones and is continuous through the sutural ligaments, and around the margins of the foramina with the periosteum on the external surface of the cranium.

Clinical Correlation

The cranial dura is more firmly adherent to the base of skull than on the vault, hence, usually torn in fractures of the skull base. It also forms the part of the wall of the basal venous sinuses, hence the fractures of skull base are often associated with bleeding from ear, nose, or into the pharynx. Cerebral dura is usually stripped off from the cranial vault, when an extradural haematoma is formed between the bone and dura.

The meningeal layer is a strong fibrous membrane and becomes continuous with the spinal dura at the foramen magnum. It ensheathes the cranial nerves in their osseous foramina and fuses externally with epineurium; the sheaths of the optic nerves fuse with the ocular sclera.

Dura! septa or folds (Figs 16.1, 16.2)

The meningeal layer gets reduplicated (infolded) along certain lines and forms septa or folds between the parts of the brain.

Fig. 16.1 Dural septa and dural venous sinuses viewed from superolateral aspect.

Fig. 16.2 Schematic coronal section of skull showing division of cranial cavity into three compartments by falx cerebri and tentorium cerebelli. Each half of the supratentorial compartment contains the cerebral hemisphere, whereas the infratentorial compartment contains the cerebellum and brainstem.

The four important dural septa are:

1. Falx cerebri

2. Falx cerebelli

3. Tentorium cerebelli

4. Diaphragma sellae.

Functions of dural septa

1. Dural septa divide the cranial cavity into compartments to separate the different parts of the brain and thus restrict their movements within the cranial cavity (Fig. 16.1).

2. They enclose intracranial dural venous sinuses.

Falx Cerebri

Falx cerebri is a large sickle-shaped vertical fold of dura mater which dips into the longitudinal fissure, between the two cerebral hemispheres.

Its narrow anterior end is attached to the crista galli and its broad posterior end to the upper surface of the tentorium cerebelli. Its convex upper border is attached to the vault of skull along the sagittal suture and its lower border is free and concave downwards.

The falx cerebri encloses three dural venous sinuses, viz.

• Superior sagittal sinus, along its upper attached border.

• Inferior sagittal sinus, along its lower free border.

• Straight sinus, along its line of attachment with the tentorium cerebelli.

Tentorium cerebelli

Tentorium cerebelli is tent-like semilunar fold of dura mater which forms the sloping roof of the posterior cranial fossa, between the cerebellum below and the occipital lobes of cerebral hemispheres above. It prevents the cerebellum from being compressed by the heavy cerebrum.

It has two borders: (a) an outer convex attached border, and (b) an inner concave free border. The inner border bounds an oval space, the tentorial notch or the door of tentorium through which passes the midbrain to connect the hindbrain with the forebrain (Fig. 16.2).

On each side, the anterior half of the outer border is attached anterolaterally to the superior border of the petrous temporal bone and posterior clinoid process, and posterior half is attached posterolaterally to the lips of the transverse sulcus.

The inner free border is ‘U’-shaped and its anterior ends are attached to the anterior clinoid processes.

Tentorium cerebelli contains four dural venous sinuses, two on either side:

• Superior petrosal sinus, along the anterior half of its attached border.

• Transverse sinus, along the posterior half of its attached border.

Falx cerebelli

Falx cerebelli is a small sickle-shaped fold of dura mater which intervenes between the two cerebellar hemispheres posteriorly. It is attached to the internal occipital crest by its outer border, and encloses the occipital venous sinus.

Diaphragma sellae (or tentorium hypophysii)

Diaphragma sellae is a small circular fold of dura mater which roofs the pituitary fossa/sella turcica. It has an aperture in its centre to provide passage for the stalk of the pituitary gland.

Arterial supply of dura mater

The dura is supplied by numerous branches of the internal carotid, ascending pharyngeal, maxillary, occipital and vertebral arteries.

N.B. Middle meningeal artery is the largest of the meningeal arteries and from clinical point of view is the most important for it is often damaged in head injuries.

Clinical Correlation

The middle meningeal artery, a branch of maxillary artery enters the cranial cavity through the foramen spinosum to lie between the endosteal and meningeal layers of dura mater. Its anterior and posterior branches, along with accompanying meningeal veins (between the arteries and bone) stand out prominently as if in relief on the external surface of the dura mater to groove and supply bones of the cranial vault. The anterior (frontal) branch crosses the pterion, on its inner aspect and the posterior (parietal) branch ascends backwards towards the lambda. A fracture of thin squamous temporal bone may cause a middle meningeal haemorrhage from the artery or vein, producing an extradural haematoma.

Nerve supply of dura mater

The nerve supply of dura mater is derived mainly from three sources:

1. Three divisions of trigeminal nerve

2. First three cervical spinal nerves

3. Cervical sympathetic trunk.

• The supratentorial dura is supplied by the meningeal branches from the three divisions of the trigeminal nerve:

1. In the anterior cranial fossa by meningeal branches of the anterior and posterior ethmoidal nerves.

2. In the middle cranial fossa by meningeal branches of maxillary and mandibular nerves.

• The infratentorial dura is supplied by ascending meningeal branches of upper three cervical nerves.

Clinical Correlation

The stimulation of sensory nerve endings in the dura mater due to stretching causes pain and is the basis of certain forms of headache. Pain arising from supratentorial dura is referred to the forehead while pain arising from infratentorial dura is referred to the back of the neck and occiput. The role of the autonomic supply of the cranial dura mater is uncertain.

N.B. The brain itself, the arachnoid mater, and the pia mater do not have sensory nerve endings. These are restricted only to the dura mater and cerebral vessels.

Dural venous sinuses

The dural venous sinuses are formed in following two ways: (a) by separation of the two layers of cerebral dura, and (b) by reduplication of the meningeal layer of dura (Fig. 16.2).

The dural venous sinuses are lined by endothelium which becomes continuous with the endothelial lining of the veins.

Characteristic features

• Have no valves, hence the blood can flow in either direction in the sinuses,

• Are devoid of smooth muscle fibres in their walls,

• Drain finally into the internal jugular veins,

• Have cerebral, diploic and some meningeal veins as their tributaries,

• Communicate via valveless emissary veins with the extracranial veins through skull foramina.

Classification

The dural venous sinuses are classified into two types, unpaired and paired (Table 16.1).

Table 16.1

Classification of dural venous sinuses

Unpaired	Paired
• Superior sagittal sinus	• Sphenoparietal sinuses
• Inferior sagittal sinus	• Cavernous sinuses
• Straight sinus	• Superior petrosal sinuses
• Occipital sinus	• Inferior petrosal sinuses
• Anterior intercavernous sinus	• Transverse sinuses
• Posterior intercavernous sinus	• Sigmoid sinuses

Unpaired sinuses

Superior sagittal sinus (SSS)

Superior sagittal sinus lies in the attached border of the falx cerebri and extends anteroposteriorly from the foramen caecum where it communicates with the nasal veins to the internal occipital protuberance, where it usually turns to the right side to continue as the right transverse sinus.

Large clusters of arachnoid villi (arachnoid granulations), concerned with the absorption of CSF project into the venous lacunae of the SSS (the clefts between the two layers of dura mater along side the SSS).

The 8 to 12 superior cerebral veins ascend with slight anterior inclination, and traverse the subdural space to drain into the superior sagittal sinus and its lacunae.

Clinical Correlation

The superior sagittal sinus communicates with the veins of scalp, diploic veins and sometimes with the veins of the nose. As a result the infection from these areas can spread to the sinus producing thrombosis of superior sagittal sinus.

Inferior sagittal sinus

Inferior sagittal sinus lies in the posterior two-third of the lower free border of falx cerebri and receives the veins from the medial surfaces of the cerebral hemispheres. At the junction of falx cerebri with the tentorium cerebelli, it is joined by the great cerebral vein to form the straight sinus.

Straight sinus

Straight sinus runs along the line of attachment of falx cerebri with the tentorium cerebelli. At the external occipital protuberance it usually turns to the left to continue as the left transverse sinus.

Occipital sinus

Occipital sinus is the smallest sinus and situated along the attached margin of the falx cerebelli. It extends downwards from the confluence of the sinuses to the foramen magnum, where it communicates with the internal vertebral venous plexus.

N.B. At the external occipital protuberance the superior sagittal sinus communicates with the left transverse, occipital and straight sinuses to form, what is termed confluence of sinuses.

The shapes of various dural folds, and sinuses enclosed in them are summarized in Table 16.2.

Table 16.2

Shapes of dural folds and enclosed venous sinuses

Fold	Shape	Venous sinuses enclosed
Falx cerebri	Sickle-shaped	Superior sagittal, inferior sagittal and straight sinuses
Tentorium cerebelli	Tent-shaped (semilunar)	Transverse and superior petrosal sinuses
Falx cerebelli	Sickle-shaped	Occipital sinus
Diaphragma sellae	Horizontal fold	Anterior and posterior intercavernous sinuses

Paired sinuses

Sphenoparietal sinuses (Fig. 16.4)

These small sinuses lie along the lesser wing of sphenoid and drain into the cavernous sinus.

Cavernous sinuses (Figs 16.3, 16.4)

Cavernous sinuses are situated one on either side of the sella turcica and, the body of sphenoid. Each sinus is a short wide venous channel measuring 2 cm anteroposteriorly and 1 cm transversely. It extends from superior orbital fissure anteriorly to the apex of petrous temporal bone posteriorly. The cavernous sinus is so named because it is traversed by a network of trabeculae which subdivide the cavity into numerous communicating caverns lined by endothelium. The two sinuses communicate with each other through anterior and posterior inter-cavernous sinuses which lie in the anterior and posterior margins of the diaphragma sellae, and pass anterior and posterior to the stalk of the pituitary gland respectively.

Fig. 16.3 Coronal section of cavernous sinus showing its relations and contents.

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