Neuroanatomy An Illustrated Colour Text, 4 ed.

Chapter 5. Coverings of the central nervous system

 

The CNS is supported and protected by bone and membranous coverings. The brain is located within the cranial cavity of the skull and the spinal cord lies in the vertebral, or spinal, canal within the vertebral column, or spine. Within their bony coverings, the brain and spinal cord are invested by three concentric membranous envelopes. The outermost membrane is the dura mater, the middle layer is the arachnoid mater and the innermost layer is the pia mater. The vertebral column and spinal meninges are described in Chapter 8; consequently, only the skull and cranial meninges are considered here.

Skull

The brain lies on the floor of the cranial cavity, which, together with the bones of the cranial vault, provides support and protection from physical injury. The floor of the cranial cavity consists of three fossae. Each of these accommodates particular parts of the brain and possesses foramina through which cranial nerves and blood vessels enter and leave the cranial cavity (Fig. 5.1).

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Figure 5.1 Floor of the skull showing the three cranial fossae and principal foramina.

imageRaised intracranial pressure

space-occupying lesion is an expanding focal lesion such as a tumour, haematoma or abscess. Since the cranial cavity is closed and unyielding, the brain is distorted and displaced downwards, towards the foramen magnum, as the intracranial pressure rises. The patient complains of headache, vomiting, blurring of vision and drowsiness. The optic discs are swollen (papilloedema), signs of brain stem dysfunction are found, and coma and death supervene if the pressure is not relieved by neurosurgery (craniotomy). Benign intracranial hypertension is caused by generalised swelling of the brain in the absence of a focal space-occupying lesion. It often occurs in obese young women; the syndrome of raised intracranial pressure mimics a brain tumour, hence the old designation, ‘pseudo tumour cerebri’.

Anterior cranial fossa

The anterior cranial fossa is formed by the frontal, ethmoid and sphenoid bones. It contains the frontal lobes of the brain. The greater part of the floor of the anterior cranial fossa consists of the frontal bone and it also forms the roof of the orbit. The part of the frontal bone that forms the anterior wall of the fossa contains the frontal air sinus. In the midline of the floor of the anterior cranial fossa is the ethmoid bone. In the midline a sharp ridge, the crista galli, provides attachment for the falx cerebri. In a depression on either side of the crista galli lie the cribriform plates of the ethmoid. These accommodate the olfactory bulbs. The bone of the cribriform plate is peppered with small perforations through which the fascicles of the olfactory nerve enter the cranial cavity from the nasal cavity to attach to the olfactory bulb.

Middle cranial fossa

The middle cranial fossa is formed by the sphenoid and temporal bones. In the midline, the body of the sphenoid forms a deep depression, the hypophyseal fossa, encompassed by four spurs of bone, the anterior and posterior clinoid processes. In the hypophyseal fossa lies the hypophysis, or pituitary gland. Lateral to the body of the sphenoid, the rest of the middle cranial fossa contains the temporal lobes of the cerebral hemisphere. The middle cranial fossa contains numerous points of entry and exit from the cranial cavity for cranial nerves and blood vessels. In particular:

image The optic canal is located medial to the anterior clinoid process and communicates with the orbit. Through it pass the optic (II) nerve and the ophthalmic artery (a branch of the internal carotid artery).

image The superior orbital fissure lies between the greater and lesser wings of the sphenoid bone and also communicates with the orbit. It carries the oculomotor (III), trochlear (IV) and abducens (VI) nerves and the ophthalmic division of the trigeminal (V) nerve.

image The foramen rotundum opens into the pterygopalatine fossa and carries the maxillary division of the trigeminal nerve.

image The foramen ovale carries the large mandibular division of the trigeminal nerve.

image The foramen spinosum is the point of entry of the middle meningeal artery.

imageForaminal syndromes

The exit foramina of the skull represent sites of potential extrinsic compression of structures running through them, by disorders such as bony deformity and tumours of bone, meninges or blood vessels. The particular cranial nerves damaged at the exit site lead to ‘foraminal syndromes’, e.g. of the superior orbital fissure (see p. 102) or jugular foramen (see p. 110). The spinal cord, lower brain stem and the tonsils of the cerebellum are compromised in the foramen magnum syndrome.

Posterior cranial fossa

The posterior cranial fossa is formed by the occipital and petrous temporal bones. Anteriorly, in the midline, it forms a steep, smooth slope (the clivus) that is continuous with the body of the sphenoid bone, posterior to the hypophyseal fossa. The brain stem rests upon the clivus, the medulla passing through the foramen magnum to become continuous with the spinal cord. The foramen magnum also admits the vertebral arteries and the spinal root of the accessory (XI) nerve. In the lateral wall of the foramen magnum lies the hypoglossal canal through which the hypoglossal (XII) nerve leaves the cranial cavity. Between the occipital and petrous temporal bones lies the large jugular foramen through which pass the internal jugular vein, and the glossopharyngeal (IX), vagus (X) and accessory (XI) nerves. In the vertical wall of the petrous temporal bone is located the internal auditory (acoustic) meatus, which transmits the vestibulocochlear (VIII) and facial (VII) nerves. The cerebellum rests on the floor of the posterior cranial fossa.

Skull

image The bones of the skull and the meninges provide protection for the brain.

image The brain lies on the floor of the cranial cavity, which consists of three fossae.

image The anterior cranial fossa contains the frontal lobes of the cerebral hemispheres.

image It forms the roof of the orbit and is closely associated with the frontal air sinus.

image The cribriform plate admits the olfactory nerves to the cranial cavity and accommodates the olfactory bulb.

image The middle cranial fossa contains the temporal lobe. In the midline, the hypophyseal fossa holds the pituitary gland.

image A number of foramina provide entry and exit for important blood vessels and cranial nerves (indicated in parentheses):

image Optic canal (optic nerve, ophthalmic artery)

image Superior orbital fissure (oculomotor, trochlear, abducens and ophthalmic division of trigeminal nerves)

image Foramen rotundum (maxillary division of trigeminal nerve)

image Foramen ovale (mandibular division of trigeminal nerve)

image Foramen spinosum (middle meningeal artery).

image The posterior cranial fossa accommodates the brain stem and cerebellum.

image A number of important structures pass through the foramina of the posterior fossa:

image Foramen magnum (medulla oblongata, vertebral arteries, spinal root of the accessory nerve)

image Hypoglossal canal (hypoglossal nerve)

image Jugular foramen (internal jugular vein, glossopharyngeal, vagus and accessory nerves)

image Internal auditory meatus (facial and vestibulocochlear nerves).

Cranial meninges

Dura mater

The cranial dura is a tough, fibrous membrane that ensheathes the brain like a loose-fitting bag. In some regions, such as the floor of the cranial cavity and the midline of the cranial roof, the dura is tightly adherent to the interior surface of the skull, while elsewhere, such as the frontoparietal area, the two are separated by a narrow extradural space. Two large reflections of dura extend into the cranial cavity and occupy the fissures between major components of the brain (Figs 5.25.3). In the midline, a vertical sheet of dura, the falx cerebri, extends from the cranial roof into the great longitudinal fissure between the cerebral hemispheres. The falx, therefore, has an attached border that adheres to the inner surface of the skull and a free border that lies above the corpus callosum. A horizontal shelf of dura, the tentorium cerebelli, extends inwards from the occipitotemporal region of the skull to lie in the transverse cerebral fissure, between the posterior part of the cerebral hemispheres and the cerebellum. The tentorium has a free border that encircles the midbrain, as the brain stem communicates between the posterior and middle cranial fossae. In the midline the tentorium becomes continuous superiorly with the falx cerebri.

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Figure 5.2 Cranial cavity showing the arrangement of the dura mater.

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Figure 5.3 Paramedian sagittal section of the head, showing the disposition of the brain and meninges.

imageHead trauma

Head trauma, especially resulting from road traffic accidents, is the most common cause of death and disability in youth. The injury may be blunt (‘closed’) or caused by a penetrating missile. The skull may be fractured and depressed, tearing brain coverings and the brain itself. Displacement and torsion of the brain lead to contusion, tearing of white matter and bleeding into the brain (intracerebral haematoma), causing unconsciousness (concussion), neurological and psychological deficits, and post-traumatic epilepsy.

Tearing of the middle meningeal artery causes bleeding into the extradural space (extradural haematoma). As the blood clot expands, the brain is compressed; as a result, coma supervenes a delayed period of hours after the blow. Without neurosurgical evacuation, the rising intracranial pressure causes brain displacement and death.

Tearing of the veins stretching across the subdural space causes gradual seepage of blood, collecting to form a chronic subdural haematoma with eventual coma. The delay between the blow and the development of symptoms may be of weeks or months. The elderly are particularly vulnerable and the head injury may be slight and forgotten. Again, surgical removal of the clot is life-saving.

Arachnoid mater and pia mater

The arachnoid mater is a soft, translucent membrane that, like the dura mater, loosely envelops the brain (Figs 1.136.10). It is separated from the dura by a narrow subdural space, through which pass veins en route to the dural venous sinuses (Ch. 7).

The pia mater is a microscopically thin, delicate and highly vascular membrane that is closely adherent to the surface of the brain, following all its concavities and convexities. Between the pia and arachnoid mater lies the subarachnoid space. This contains a filamentous network of connective tissue strands (trabeculae) and is traversed by numerous arteries and veins. It also contains CSF, which is produced by the choroid plexus within the cerebral ventricles (Ch. 6). Since the arachnoid mater fits loosely round the brain while the pia closely follows its surface contours, the subarachnoid space is of greatly varying depth in different regions. Where significant depressions or fissures in the brain are spanned by the arachnoid mater, subarachnoid cisterns are formed. Two of these are particularly large:

image The cisterna magna lies between the cerebellum and the dorsal surface of the medulla (Figs 5.35.4). Into this cistern flows CSF from the fourth ventricle.

image The interpeduncular cistern is located at the base of the brain (Fig. 5.3), where the arachnoid spans the space between the two temporal lobes. This cistern contains the optic chiasma. The cistern is deepest between the two cerebral peduncles of the midbrain.

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Figure 5.4 Cisterna magna.

Dural venous sinuses

The dura mater is considered to be comprised of two layers. These are normally closely adherent to one another but, in certain locations, become separated to enclose blood-filled spaces, the dural venous sinuses. Major venous sinuses lie in the attached borders of the falx cerebri and tentorium cerebelli and also on the floor of the cranial cavity. Venous blood from the brain flows into the sinuses through a series of venous channels and in turn the sinuses drain principally into the internal jugular vein, through which blood is returned to the general extracranial circulation. The dural venous sinuses are described further in Chapter 7, which deals with the blood supply to the CNS.

Cranial meninges

image The dura mater is the outermost meningeal membrane. Two dural sheets or reflections extend into the cranial cavity:

image The falx cerebri, lying between the two cerebral hemispheres

image The tentorium cerebelli, lying between the cerebellum and the occipital lobes of the cerebrum and encircling the midbrain.

image The dura contains a number of venous sinuses, which are important in the venous drainage of the brain.

image Important sinuses lie within the falx cerebri, tentorium cerebelli and on the floor of the cranial cavity.

image The middle meningeal layer is the arachnoid mater. Both the dura and arachnoid surround the brain loosely.

image The innermost meningeal layer is the pia mater, which adheres to the surface of the brain, closely following its contours. This creates a subarachnoid space of variable depth.

image The subarachnoid space contains CSF, which is secreted by the choroid plexus within the cerebral ventricles.

image Meningitis

Inflammation of the meninges may result from infection with viruses (e.g. lymphocytic choriomeningitis), bacteria (meningococcal and tuberculous meningitis) or other organisms, or from chemical reaction to injected contrast medium during neuroradiological procedures. The patient complains of headache, photophobia and vomiting, is febrile and has neck stiffness on attempting to move the head. Viral and chemical meningitis are normally mild and self-limiting. Bacterial or fungal meningitis, however, leads to damage to cranial nerves and the brain itself; if untreated, it proceeds to raised intracranial pressure, brain displacement and death.