Cranial Cavity

INTRODUCTION

– The cranial cavity is enclosed by the bones of the skull and communicates with adjoining regions through several foramina and canals.

Contents of Cranial Cavity

The cranial cavity contains the following structures:

1. Brain.
2. Meninges:
   • Outer dura mater.
   • Middle arachnoid mater.
   • Inner pia mater.
3. Hypophysis cerebri (pituitary gland).
4. Intracranial parts of cranial nerves.
5. Dural venous sinuses, including the cavernous sinuses.
6. Arteries:
   • Intracranial parts of internal carotid arteries.
   • Fourth parts of vertebral arteries.
   • Cerebral arterial circle (circle of Willis).
   • Middle meningeal arteries.

Meningeal Coverings

• Dura mater: It is the outermost, tough fibrous covering that lines the inner surface of the skull.
• Arachnoid mater: It is a thin, avascular membrane situated between the dura mater and pia mater.
• Pia mater: It is a delicate, vascular membrane closely adherent to the surface of the brain, following its gyri and sulci.

Meningeal Spaces

1. Subdural Space

• It is a potential space between the dura mater and arachnoid mater.
• Under normal conditions, these membranes remain closely apposed.

2. Subarachnoid Space

• It is a real space between the arachnoid mater and pia mater.
• It contains cerebrospinal fluid (CSF), blood vessels, and arachnoid trabeculae.

CEREBRAL DURA MATER

• The cerebral dura mater is the outermost, thickest, and toughest meningeal covering of the brain.
• It is a dense fibrous membrane that protects and supports the intracranial structures.

Layers of Dura Mater

The cranial dura mater consists of two layers:

1. Outer endosteal (periosteal) layer.
2. Inner meningeal layer.

– These layers are closely adherent except where they separate to form the dural venous sinuses.

Endosteal layer (endocranium)

– It forms the outer layer of cranial dura mater and acts as the periosteal lining of the internal surface of the skull.

Features

1. Continuity: It is continuous with the pericranium through cranial sutures and foramina. It also continues with the periosteum of the orbit through the superior orbital fissure.
2. Dural sheaths: It contributes to the dural coverings of cranial nerves as they pass through skull foramina. These coverings merge with the epineurium outside the skull.
3. Meningeal vessels: It is closely associated with the meningeal vessels that supply the dura and adjacent cranial bones.
4. Adherence: It is relatively loosely attached to the cranial vault but firmly adherent at the skull base, sutures, and margins of foramina.

Note: The dural sheath surrounding the optic nerve extends into the orbit and blends with the sclera of the eyeball.

Meningeal layer

• It is the inner, strong fibrous layer of the dura mater.
• It encloses the brain and continues through the foramen magnum as the spinal dura mater.
• It forms inward projections that separate and support different parts of the brain.

Folding of Meningeal Dura Mater

– The meningeal layer forms four major dural folds that partially divide the cranial cavity into compartments:

1. Falx cerebri.
2. Tentorium cerebelli.
3. Falx cerebelli.
4. Diaphragma sellae.

ARACHNOID MATER

• The arachnoid mater is a thin, avascular membrane situated between the dura mater and pia mater.
• Delicate arachnoid trabeculae extend from its inner surface towards the pia mater.

Meningeal Spaces

1. Subdural Space

• It is a potential space between the dura mater and arachnoid mater.
• Normally, these membranes remain closely apposed.
• Bridging veins traverse this region before draining into the dural venous sinuses.

2. Subarachnoid Space

• It is a real space between the arachnoid mater and pia mater.
• It contains cerebrospinal fluid (CSF), cerebral blood vessels, and arachnoid trabeculae.

Arachnoid Granulations

• Arachnoid granulations, also called Pacchionian granulations, are projections of the arachnoid mater into dural venous sinuses, particularly the superior sagittal sinus.
• They provide an important pathway for the absorption of CSF into venous blood.

PIA MATER

• The pia mater is a delicate, highly vascular membrane closely adherent to the surface of the brain and spinal cord.
• It follows the contours of cerebral gyri and sulci.

Leptomeninges: The arachnoid mater and pia mater are collectively called leptomeninges.

Falx cerebri

– The falx cerebri is a large, sickle-shaped fold of meningeal dura mater projecting into the longitudinal fissure between the two cerebral hemispheres.

Parts and Attachments

• Anterior end: It is narrow and attached to the crista galli of the ethmoid bone.
• Posterior end: It is broad and attached to the superior surface of the tentorium cerebelli in the median plane.
• Superior margin: It is convex and attached along the midline of the cranial vault.
• Inferior margin: It is concave and free.
• Right and left surfaces: They are related to the medial surfaces of the corresponding cerebral hemispheres.

Related Dural Venous Sinuses

1. Superior sagittal sinus: It lies along the attached superior margin.
2. Inferior sagittal sinus: It lies along the free inferior margin.
3. Straight sinus: It lies at the junction of the falx cerebri and tentorium cerebelli.

Tentorium cerebelli

• The tentorium cerebelli is a tent-shaped fold of meningeal dura mater forming the roof of the posterior cranial fossa.
• It separates the cerebellum below from the occipital lobes of the cerebral hemispheres above.

Margins and Attachments

1. Anterior (Free) Margin

• It is U-shaped and surrounds the tentorial notch (tentorial incisura).
• The tentorial notch provides passage for the midbrain.
• Its anterior ends attach to the anterior clinoid processes.

2. Posterior (Attached) Margin

– It is convex and firmly attached to:

1. Posterior clinoid processes.
2. Superior borders of the petrous temporal bones.
3. Posterolateral portions of the parietal bones.
4. Margins of the grooves for transverse sinuses on the occipital bone.

Surfaces

• Superior surface: It is convex and slopes downwards on both sides. The falx cerebri attaches to its midline.
• Inferior surface: It is concave and related to the cerebellum. The falx cerebelli attaches to its posterior median part.

Related Dural Venous Sinuses

1. Transverse sinuses: They lie along the posterior attached margins.
2. Superior petrosal sinuses: They lie along the attached margins over the superior borders of the petrous temporal bones.
3. Straight sinus: It lies at the junction of the tentorium cerebelli and falx cerebri.

TRIGEMINAL CAVE (MECKEL’S CAVE)

– Meckel’s cave is a dural recess situated near the apex of the petrous temporal bone, beneath the adjoining tentorium cerebelli.

Formation

• It is formed by a dural recess containing an extension of the subarachnoid space.
• It occupies the trigeminal impression on the petrous temporal bone.

Contents

1. Trigeminal (semilunar) ganglion.
2. Sensory root of the trigeminal nerve and proximal portions of its divisions.
3. Cerebrospinal fluid within the trigeminal cistern.

Falx cerebelli

– The falx cerebelli is a small, sickle-shaped fold of dura mater projecting between the two cerebellar hemispheres posteriorly.

Parts and Attachments

• Base: It is attached to the inferior surface of the posterior part of the tentorium cerebelli.
• Apex: It extends towards the foramen magnum and may divide into two small folds.
• Posterior margin: It is convex and attached to the internal occipital crest.
• Anterior margin: It is concave and free.

Related Dural Venous Sinus

– The occipital sinus lies along the attached posterior margin of the falx cerebelli.

Diaphragma sellae

– The diaphragma sellae is a small, circular, horizontal fold of dura mater forming the roof of the hypophyseal fossa.

Attachments

• Anteriorly: Tuberculum sellae.
• Posteriorly: Dorsum sellae.
• Laterally: It is continuous with the adjoining dura mater of the middle cranial fossa and cavernous sinus.

Central Aperture

– A small opening in the centre allows passage of the infundibulum (pituitary stalk) connecting the hypothalamus to the pituitary gland.

Blood Supply of Dura Mater

• The cranial dura mater receives blood from meningeal branches of several arteries.
• The endosteal layer is richly vascular, whereas the meningeal layer is relatively less vascular.

Anterior Cranial Fossa

• Anterior ethmoidal artery.
• Posterior ethmoidal artery.
• Meningeal branches of the ophthalmic artery.

Middle Cranial Fossa

• Middle meningeal artery.
• Accessory meningeal artery.
• Meningeal branches of the internal carotid and ascending pharyngeal arteries.

Posterior Cranial Fossa

• Meningeal branches of vertebral arteries.
• Meningeal branches of occipital arteries.
• Contributions from the ascending pharyngeal arteries.

Important Features

• The middle meningeal artery is the principal arterial supply to much of the supratentorial dura mater.
• Meningeal vessels contribute significantly to the blood supply of the internal surfaces and diploë of cranial bones.

The external surfaces also receive blood through periosteal vessels associated with the scalp.

Nerve Supply of Dura Mater

• The dura mater is sensitive to pain, particularly near the dural venous sinuses and meningeal arteries.
• Its sensory innervation is derived mainly from the trigeminal nerve, vagus nerve, and upper cervical spinal nerves.

Anterior Cranial Fossa

• It is supplied mainly by meningeal branches of the ophthalmic nerve (V1).
• Important branches include the anterior and posterior ethmoidal nerves.

Middle Cranial Fossa

• Maxillary nerve (V2): It supplies portions of the anterior and medial middle cranial fossa.
• Mandibular nerve (V3): Its meningeal branch supplies much of the lateral middle cranial fossa.
• Ophthalmic nerve (V1): Its recurrent meningeal (tentorial) branch supplies the tentorium cerebelli and posterior part of the falx cerebri.

Posterior Cranial Fossa

– It receives sensory fibres from:

1. Upper cervical spinal nerves, mainly C1–C3.
2. Meningeal branches of the vagus nerve.
3. Additional contributions that may arise from the glossopharyngeal and hypoglossal nerves.

INTRACRANIAL DURAL VENOUS SINUSES

• The dural venous sinuses are endothelium-lined venous channels situated within the cranial dura mater.
• They are formed by separation of the endosteal and meningeal layers of dura mater or between folds of the meningeal layer.

Features of dural venous

1. They lie between dural layers and are lined by endothelium.
2. Their walls consist of fibrous tissue and lack a muscular coat.
3. They are devoid of valves.
4. They receive venous blood from the brain, meninges, and cranial bones.
5. They receive cerebrospinal fluid (CSF) through arachnoid granulations.
6. They communicate with extracranial veins through emissary veins, providing alternative pathways for venous drainage.

Classification of dural venous sinuses

The dural venous sinuses are classified into paired and unpaired groups.

A. Paired Sinuses

1. Cavernous sinuses.
2. Superior petrosal sinuses.
3. Inferior petrosal sinuses.
4. Transverse sinuses.
5. Sigmoid sinuses.
6. Sphenoparietal sinuses.
7. Petrosquamous sinuses, when present.

B. Unpaired Sinuses and Venous Channels

1. Superior sagittal sinus.
2. Inferior sagittal sinus.
3. Straight sinus.
4. Occipital sinus.
5. Anterior intercavernous sinus.
6. Posterior intercavernous sinus.
7. Basilar venous plexus.

Important Features

• Lateral sinus: The term traditionally refers to the transverse and sigmoid sinuses collectively.
• Middle meningeal venous channels: These accompany the middle meningeal artery and may form prominent dural venous channels.

CAVERNOUS SINUS

• The cavernous sinuses are paired dural venous spaces situated on either side of the body of the sphenoid bone.
• They receive venous blood from the orbit, brain, and meninges.
• They are closely related to the internal carotid artery and several cranial nerves.

  Location and Dimensions

• Location: Each sinus lies in the middle cranial fossa, lateral to the sella turcica.
• Length: Approximately 2 cm.
• Width: Approximately 1 cm.

Parts (Structure)

• Each cavernous sinus is enclosed by dural layers and presents a roof, floor, medial wall, and lateral wall.
• The roof and lateral wall are formed mainly by meningeal dural layers.
• The medial wall separates the sinus from the pituitary gland, while the floor overlies the sphenoid bone.
• The sinus contains a network of venous channels separated by fine connective tissue trabeculae, giving it a cavernous appearance.

Extent

• Anteriorly: It extends to the medial end of the superior orbital fissure.
• Posteriorly: It reaches the apex of the petrous part of the temporal bone.

Relations

Superiorly

1. Optic chiasma and adjoining optic pathways.
2. Supraclinoid part of the internal carotid artery.
3. Adjacent basal structures of the cerebral hemispheres.

Inferiorly

1. Body and greater wing of the sphenoid bone.
2. Region of the foramen lacerum.

Medially

1. Pituitary gland.
2. Sphenoidal air sinus.

Laterally

– Temporal lobe of the cerebral hemisphere.

Anteriorly

– Superior orbital fissure.

Posteriorly

– Apex of the petrous temporal bone.

Structures in the Walls of Cavernous Sinus

Structures in the Lateral Wall

The following nerves are arranged from above downwards:

1. Oculomotor nerve (CN III).
2. Trochlear nerve (CN IV).
3. Ophthalmic nerve (V1).

Maxillary nerve (V2): It passes along the inferior boundary of the cavernous sinus region towards the foramen rotundum. Some anatomical descriptions include it in the inferolateral dural wall, whereas others place it below the cavernous sinus proper.

Structures Passing Through the Sinus

1. Cavernous part of the internal carotid artery, surrounded by its sympathetic plexus.
2. Abducent nerve (CN VI), which passes close to the internal carotid artery.

Important Anatomical Features

• The oculomotor nerve divides into superior and inferior divisions before or near the superior orbital fissure.
• The trochlear nerve passes through the superior orbital fissure to supply the superior oblique muscle.
• The ophthalmic nerve divides into the lacrimal, frontal, and nasociliary nerves before entering the orbit.
• The maxillary nerve passes through the foramen rotundum to enter the pterygopalatine fossa.

Dorello’s Canal

• Dorello’s canal is a short osteofibrous passage near the petrous apex.
• The abducent nerve traverses it beneath the petrosphenoidal ligament before entering the cavernous sinus.

Infratrochlear Triangle (Parkinson’s Triangle)

• It is an anatomical space in the lateral wall of the cavernous sinus.
• Superior boundary: Trochlear nerve.
• Inferior boundary: Ophthalmic nerve.
• Deep to this triangle lie:
  1. Cavernous segment of the internal carotid artery.
  2. Abducent nerve.

– This triangle is an important landmark in surgical approaches to the cavernous sinus.

Tributaries of Cavernous Sinus

The cavernous sinus receives venous blood from the orbit, brain, and meninges.

A. Tributaries from Orbit

1. Superior ophthalmic vein.
2. Inferior ophthalmic vein.
3. Central retinal vein, occasionally.

B. Tributaries from Brain

1. Superficial middle cerebral vein.
2. Small inferior cerebral veins, particularly from the temporal lobe.

C. Tributaries from Meninges

1. Sphenoparietal sinus or adjoining sphenoidal venous channels.
2. Anterior division of the middle meningeal veins.

Important Features

• The central retinal vein may drain into the superior ophthalmic vein or directly into the cavernous sinus.
• The anterior middle meningeal veins show variable drainage into sphenoidal venous channels, cavernous sinus, or pterygoid venous plexus.

Communication of Cavernous Sinus

The cavernous sinus communicates with the following venous channels:

1. Superior petrosal sinus: It connects the cavernous sinus with the transverse-sigmoid junction.
2. Inferior petrosal sinus: It drains the cavernous sinus towards the internal jugular vein.
3. Pterygoid venous plexus: Communication occurs through emissary veins passing through foramina at the skull base.
4. Facial vein: Communication occurs through the angular and superior ophthalmic veins or through the deep facial vein and pterygoid venous plexus.
5. Superior sagittal sinus: Indirect communication occurs through cerebral veins and the superior anastomotic vein.
6. Internal vertebral venous plexus: Communication occurs through the basilar venous plexus.
7. Opposite cavernous sinus: The two cavernous sinuses communicate through anterior and posterior intercavernous sinuses.

Note: These venous pathways are largely valveless, allowing blood flow in either direction depending on pressure differences.

Factors Influencing Venous Drainage

• Pulsations of the internal carotid artery.
• Venous pressure gradients.
• Gravity and changes in head position.

Development Of Cavernous Sinus

• The cavernous sinus develops from embryonic venous channels associated with the primitive head veins.
• Incorporation and remodelling of small venous channels produce its plexiform arrangement.
• The resulting venous spaces are separated by delicate connective tissue trabeculae.

SUPERIOR SAGITTAL SINUS

– The superior sagittal sinus lies along the attached superior margin of the falx cerebri.

Beginning

– It begins anteriorly near the crista galli, receiving small meningeal veins.

Course and Termination

• It runs posteriorly along the midline of the cranial vault.
• It reaches the confluence of sinuses near the internal occipital protuberance.
• Its drainage commonly continues predominantly into the right transverse sinus.

Features

• It is triangular in cross-section.
• Numerous arachnoid granulations project into its lumen.
• Fibrous bands called chordae Willisii may cross the sinus.
• It receives blood from superficial cerebral veins.

Tributaries

1. Superior cerebral veins.
2. Parietal emissary veins.
3. Small veins from the nasal and frontal regions.
4. Diploic and meningeal veins.

INFERIOR SAGITTAL SINUS

• The inferior sagittal sinus is a small, unpaired venous channel situated within the free inferior margin of the falx cerebri.
• It runs posteriorly and joins the great cerebral vein to form the straight sinus.

STRAIGHT SINUS

– The straight sinus lies along the junction between the falx cerebri and tentorium cerebelli.

Beginning

– It is formed by the union of:

1. Inferior sagittal sinus.
2. Great cerebral vein (vein of Galen).

Termination

• It passes posteriorly towards the confluence of sinuses.
• Its drainage commonly continues predominantly into the left transverse sinus.

Tributaries

– Superior cerebellar veins and small veins from adjoining structures.

GREAT CEREBRAL VEIN (VEIN OF GALEN)

• The great cerebral vein is a short, unpaired deep cerebral vein.
• It is formed by the union of the right and left internal cerebral veins.
• It lies beneath the splenium of the corpus callosum, posterior to the midbrain.
• It receives tributaries, including the basal veins of Rosenthal, and joins the inferior sagittal sinus to form the straight sinus.

TRANSVERSE SINUSES

– The transverse sinuses are paired venous channels situated along the posterior attached margins of the tentorium cerebelli.

Course

• Each sinus extends laterally from the confluence of sinuses.
• It follows the transverse groove of the occipital bone towards the mastoid angle of the parietal bone.
• It continues as the sigmoid sinus.

Formation

• The right transverse sinus commonly receives most of the blood from the superior sagittal sinus.
• The left transverse sinus commonly receives most of the blood from the straight sinus.
• The size and drainage patterns of the two sinuses show considerable variation.

CONFLUENCE OF SINUSES

• The confluence of sinuses, also called torcular Herophili, is a venous junction near the internal occipital protuberance.
• It connects the following sinuses:
  1. Superior sagittal sinus.
  2. Straight sinus.
  3. Occipital sinus.
  4. Right and left transverse sinuses.

– It distributes venous blood towards the transverse sinuses.

SIGMOID SINUSES

– The sigmoid sinuses are paired, S-shaped venous channels that continue from the transverse sinuses.

Beginning

– Each sigmoid sinus begins as a continuation of the transverse sinus near the mastoid angle of the parietal bone.

Course

• It descends through the sigmoid groove of the temporal and occipital bones.
• It curves towards the jugular foramen.

Termination

– It continues as the internal jugular vein at the jugular foramen, forming its superior bulb.

Tributaries

1. Mastoid emissary vein.
2. Condylar emissary vein.
3. Cerebellar veins.
4. Small diploic and meningeal veins.

OCCIPITAL SINUS

• The occipital sinus is a small venous channel situated along the attached posterior margin of the falx cerebelli.
• It extends from the region of the foramen magnum towards the internal occipital protuberance.
• It usually drains into the confluence of sinuses.
• Inferiorly, it communicates with the internal vertebral venous plexus and marginal sinus.

BASILAR VENOUS PLEXUS

• The basilar venous plexus is a network of interconnected veins situated on the clivus, anterior to the pons and medulla.
• It communicates with:
  1. Right and left inferior petrosal sinuses.
  2. Internal vertebral venous plexus.
  3. Marginal sinus around the foramen magnum.

– It provides an important venous communication between the intracranial and vertebral venous systems.

SPHENOPARIETAL SINUSES

• The sphenoparietal sinuses are traditionally described as paired venous channels situated along the posterior margins of the lesser wings of the sphenoid bone.
• They drain medially towards the anterior portions of the cavernous sinuses.
• Their anatomy and communications with the middle meningeal veins and superficial middle cerebral veins are variable.

SUPERIOR PETROSAL SINUSES

• The superior petrosal sinuses are paired venous channels situated along the superior borders of the petrous temporal bones.
• They lie within the attached anterolateral margins of the tentorium cerebelli.

Tributaries

1. Inferior cerebral veins.
2. Cerebellar veins.
3. Small veins from adjoining dural structures.

Communications

– They connect the cavernous sinuses with the transverse-sigmoid junctions.

INFERIOR PETROSAL SINUSES

• The inferior petrosal sinuses are paired venous channels situated along the petro-occipital fissures.
• Each sinus begins near the posterior part of the cavernous sinus.
• It descends towards the jugular foramen and usually drains into the superior bulb of the internal jugular vein.

PETROSQUAMOUS SINUS

• The petrosquamous sinus is a variable venous channel associated with the petrosquamous suture of the temporal bone.
• When persistent, it may communicate with the transverse or sigmoid sinus and extracranial venous channels.
• It is clinically important during temporal bone and mastoid surgery.

INTERCAVERNOUS SINUSES

– The anterior and posterior intercavernous sinuses connect the right and left cavernous sinuses.

Anterior Intercavernous Sinus

– It passes anterior to the pituitary stalk near the diaphragma sellae.

Posterior Intercavernous Sinus

– It passes posterior to the pituitary stalk.

Circular Sinus

– The anterior and posterior intercavernous sinuses, together with the cavernous sinuses, form a venous ring around the pituitary gland called the circular sinus.

MIDDLE MENINGEAL VEINS

• The middle meningeal veins accompany the branches of the middle meningeal artery.
• They may form prominent venous channels within the dura mater.

Trunks

1. Anterior (Frontal) Trunk

• It accompanies the frontal branch of the middle meningeal artery.
• It may communicate with the sphenoidal venous channels, cavernous sinus, or pterygoid venous plexus.

2. Posterior (Parietal) Trunk

• It accompanies the parietal branch of the middle meningeal artery.
• It commonly drains towards the pterygoid venous plexus through the region of the foramen spinosum.

SUPERFICIAL MIDDLE CEREBRAL VEIN

• The superficial middle cerebral vein runs along the lateral sulcus of the cerebral hemisphere.
• It receives venous blood from the adjoining frontal, parietal, and temporal cortices.

Course and Termination

• It passes anteriorly along the lateral sulcus towards the sphenoidal region.
• It may drain into:
  1. Cavernous sinus.
  2. Sphenoparietal or adjoining laterocavernous venous channels.
  3. Pterygoid venous plexus through emissary veins.
  4. Superior petrosal or transverse sinus through alternative pathways.

Communications

1. Superior Anastomotic Vein (Vein of Trolard)

– It connects the superficial middle cerebral venous system with the superior sagittal sinus.

2. Inferior Anastomotic Vein (Vein of Labbé)

– It connects the superficial middle cerebral venous system with the transverse sinus.

– These connections provide important collateral pathways for cerebral venous drainage.

TRIGEMINAL GANGLION

• The trigeminal ganglion, also called the Gasserian or semilunar ganglion, is the sensory ganglion of the trigeminal nerve (CN V).
• It is homologous to the dorsal root ganglion of a spinal nerve.

Structure

• The ganglion contains the cell bodies of pseudounipolar sensory neurons.
• Each neuron has a central process directed towards the brainstem and a peripheral process that carries sensory impulses from its area of distribution.

Shape

– It is crescent-shaped or semilunar, with its convex border directed anterolaterally.

Situation

– The ganglion lies in the trigeminal impression near the apex of the petrous temporal bone, in the middle cranial fossa.

Meningeal Relations

• It is enclosed within a dural recess called Meckel’s cave (cavum trigeminale).
• This recess contains the trigeminal cistern, an extension of the subarachnoid space containing cerebrospinal fluid (CSF).
• The motor root of the trigeminal nerve passes inferior to the ganglion without synapsing.

Relations

Superiorly

– Temporal lobe of the cerebral hemisphere.

Inferiorly

1. Motor root of the trigeminal nerve.
2. Greater petrosal nerve.
3. Apex of the petrous temporal bone.
4. Region of the foramen lacerum.

Medially

1. Cavernous sinus.
2. Internal carotid artery.

Laterally

– Temporal lobe, including the adjoining uncus.

Branches

A. Central Processes

– The central processes of ganglion cells form the large sensory root of the trigeminal nerve, which enters the pons.

B. Peripheral Processes

– They form three major divisions:

1. Ophthalmic nerve (V1).
2. Maxillary nerve (V2).
3. Mandibular nerve (V3).

Important Features

• The three sensory divisions emerge from the anterior aspect of the ganglion and proceed towards their respective routes of exit from the cranial cavity.
• The small motor root passes below the ganglion and joins the mandibular nerve near the foramen ovale.
• Proprioceptive neurons of the muscles of mastication are exceptional because their cell bodies lie mainly in the mesencephalic nucleus of the trigeminal nerve.

Blood Supply

The trigeminal ganglion receives arterial branches from:

1. Internal carotid artery.
2. Middle meningeal artery.
3. Accessory meningeal artery.
4. Meningeal branches of the ascending pharyngeal artery.

INTRACRANIAL STRUCTURES

The following important structures can be identified in the cranial fossae after removal of the brain:

1. Intracranial portions of the twelve pairs of cranial nerves.
2. Petrosal nerves.
3. Middle meningeal artery.
4. Intracranial part of the internal carotid artery.
5. Fourth part of the vertebral artery.
6. Pituitary gland.

Petrosal Nerves

– Four petrosal nerves are traditionally described on each side:

1. Greater petrosal nerve.
2. Deep petrosal nerve.
3. Lesser petrosal nerve.
4. External petrosal nerve.

Greater petrosal nerve

– Fibres: It carries preganglionic parasympathetic fibres of the facial nerve.

Origin

• It arises from the facial nerve at the region of the geniculate ganglion.
• Its preganglionic parasympathetic fibres originate from the superior salivatory nucleus and pass through the geniculate ganglion without synapsing.

Course

• It leaves the facial canal through the hiatus for the greater petrosal nerve.
• It runs forwards and medially along the floor of the middle cranial fossa.
• Near the foramen lacerum, it joins the deep petrosal nerve to form the nerve of the pterygoid canal (Vidian nerve).

Termination

– Its preganglionic parasympathetic fibres synapse in the pterygopalatine ganglion.

Distribution

– Postganglionic fibres provide secretomotor innervation to:

1. Lacrimal gland.
2. Mucous glands of the nasal cavity.
3. Palatine glands.
4. Glands of the nasopharynx.

Deep petrosal nerve

– Fibres: It carries postganglionic sympathetic fibres originating from the superior cervical ganglion.

Origin

– It arises from the internal carotid sympathetic plexus near the foramen lacerum.

Course

• It joins the greater petrosal nerve near the foramen lacerum to form the nerve of the pterygoid canal.
• It reaches the pterygopalatine ganglion, through which its sympathetic fibres pass without synapsing.

Distribution

– Its fibres provide mainly vasomotor innervation to blood vessels supplying the lacrimal gland and mucous membranes of the nose, palate, and pharynx.

Lesser petrosal nerve

– Fibres: It carries preganglionic parasympathetic fibres derived mainly from the glossopharyngeal nerve.

Origin

– It arises from the tympanic plexus in the middle ear, which receives parasympathetic fibres through the tympanic branch of the glossopharyngeal nerve.

Course

• It enters the middle cranial fossa through the hiatus for the lesser petrosal nerve.
• It runs forwards along the floor of the middle cranial fossa.
• It usually exits the skull through the foramen ovale, although its route may vary.

Termination

– It terminates in the otic ganglion, where its preganglionic parasympathetic fibres synapse.

Distribution

– Postganglionic fibres reach the parotid gland through the auriculotemporal nerve and provide secretomotor innervation.

External petrosal nerve

– The external petrosal nerve is a small, inconstant sympathetic communicating nerve associated with the geniculate ganglion.

Fibres

– It carries postganglionic sympathetic fibres associated with the plexus surrounding the middle meningeal artery.

Course

• It establishes a connection between the sympathetic plexus of the middle meningeal artery and the region of the geniculate ganglion.
• It traverses the floor of the middle cranial fossa and enters the facial canal through a small opening.

Note: The external petrosal nerve is variable and may not be identifiable in routine dissections.

Middle Meningeal Artery

• The middle meningeal artery is the largest meningeal branch of the maxillary artery.
• It supplies the dura mater and adjoining cranial bones.

Origin

– It arises from the first part of the maxillary artery in the infratemporal fossa.

Course

• It ascends in the infratemporal fossa, deep to the lateral pterygoid muscle.
• It usually passes between the two roots of the auriculotemporal nerve.
• It enters the middle cranial fossa through the foramen spinosum.
• Within the cranial cavity, it follows an extradural course along grooves on the internal surface of the skull.
• It divides into two principal terminal branches:
  1. Anterior (frontal) branch.
  2. Posterior (parietal) branch.

Branches

1. Anterior (Frontal) Branch

• It passes forwards and upwards, crossing the region of the pterion.
• It supplies the dura mater of the frontal and anterior parietal regions.

2. Posterior (Parietal) Branch

• It passes backwards and upwards.
• It supplies the dura mater of the posterior parietal and adjoining temporal regions.

3. Ganglionic Branches

– They provide arterial supply to the trigeminal ganglion and adjoining structures.

4. Petrosal Branch

– It supplies the dura mater near the petrous temporal bone and contributes to the vascular supply of the facial nerve and geniculate ganglion.

5. Superior Tympanic Artery

• It usually arises from the petrosal branch.
• It enters the facial canal through the hiatus for the greater petrosal nerve and supplies structures of the middle ear and facial canal.

6. Temporal Branches

– They supply the dura mater and adjoining bones of the temporal region.