Nerves and Vessels of Orbit

INTRODUCTION

The orbit contains important nerves and blood vessels responsible for vision, ocular movements, sensation, and autonomic functions.

Nerves

1. Optic nerve (CN II).
2. Oculomotor nerve (CN III).
3. Trochlear nerve (CN IV).
4. Abducent nerve (CN VI).
5. Ophthalmic division of trigeminal nerve (V1).
6. Ciliary ganglion and its branches.

Blood Vessels

1. Ophthalmic artery.
2. Superior ophthalmic vein.
3. Inferior ophthalmic vein.

Optic Nerve

• The optic nerve is the second cranial nerve (CN II) and carries sensory impulses concerned with vision.
• It consists of axons arising from retinal ganglion cells.
• It emerges from the posterior aspect of the eyeball, approximately 3–4 mm medial to its posterior pole.
• It passes backwards and medially through the optic canal into the cranial cavity.
• The right and left optic nerves meet at the optic chiasma, where fibres from the nasal halves of the retinae cross to the opposite side.

Length and Parts

– The total length of the optic nerve is approximately 4 cm.

Part	Approximate length
Intraorbital	25 mm
Intracanalicular	5 mm
Intracranial	10 mm

Peculiarities of Optic Nerve

1. Development: It develops as an extension of the diencephalon and is considered a tract of the central nervous system rather than a typical peripheral nerve.
2. Neuronal fibres: It contains axons of retinal ganglion cells.
3. Myelination: Its axons are myelinated by oligodendrocytes rather than Schwann cells.
4. Meningeal coverings: It is surrounded by dura mater, arachnoid mater, and pia mater.
5. Subarachnoid space: Its meningeal coverings enclose a CSF-containing subarachnoid space continuous with the intracranial subarachnoid space.
6. Regeneration: Its axons have very limited capacity for regeneration following injury.

OCULOMOTOR NERVE

• The oculomotor nerve is the third cranial nerve (CN III).
• It is predominantly motor and supplies most extraocular muscles.
• It also carries preganglionic parasympathetic fibres to the ciliary ganglion.

Motor Distribution

The oculomotor nerve supplies:

1. Superior rectus.
2. Inferior rectus.
3. Medial rectus.
4. Inferior oblique.
5. Levator palpebrae superioris.

Exceptions: The lateral rectus is supplied by CN VI, and the superior oblique by CN IV.

Mnemonic: LR6 SO4, all others III.

Parasympathetic Distribution

• Preganglionic parasympathetic fibres synapse in the ciliary ganglion.
• Postganglionic fibres pass through the short ciliary nerves to supply:
  1. Sphincter pupillae: Produces pupillary constriction.
  2. Ciliary muscle: Produces accommodation for near vision.

Functional Components and Nuclei

1. General Somatic Efferent (GSE)

• These fibres arise from the oculomotor nuclear complex in the midbrain.
• They supply the voluntary muscles innervated by the oculomotor nerve.

2. General Visceral Efferent (GVE)

• These are preganglionic parasympathetic fibres arising from the Edinger–Westphal nucleus.
• They relay in the ciliary ganglion and supply the sphincter pupillae and ciliary muscle.

Proprioceptive Fibres

• Proprioceptive information from extraocular muscles is conveyed through trigeminal sensory pathways and associated brainstem circuits.
• Such sensory fibres are not usually classified as a major functional component of the oculomotor nerve.

Oculomotor nuclear complex

• The oculomotor nuclear complex lies in the rostral midbrain at the level of the superior colliculus.
• It is situated ventral to the cerebral aqueduct, within the periaqueductal grey matter.
• It consists of somatic motor subnuclei and parasympathetic neurons.

Components

1. Inferior rectus subnucleus: Supplies the ipsilateral inferior rectus.
2. Inferior oblique subnucleus: Supplies the ipsilateral inferior oblique.
3. Medial rectus subnucleus: Supplies the ipsilateral medial rectus.
4. Superior rectus subnucleus: Supplies the contralateral superior rectus.
5. Central caudal nucleus: Supplies both levator palpebrae superioris muscles.
6. Edinger–Westphal nucleus: Provides preganglionic parasympathetic fibres for pupillary constriction and accommodation.

Important Features

• The superior rectus receives motor fibres from the contralateral oculomotor nucleus.
• The levator palpebrae superioris muscles receive bilateral innervation from the unpaired central caudal nucleus.
• These arrangements help explain the clinical manifestations of nuclear lesions.

Central Connections

The oculomotor nuclear complex receives inputs from:

1. Cortical motor centres: These influence voluntary eye movements through supranuclear pathways.
2. Medial longitudinal fasciculus (MLF): It coordinates the oculomotor, trochlear, abducent, and vestibular nuclei for conjugate eye movements.
3. Pretectal nuclei: They participate in the direct and consensual pupillary light reflexes.
4. Supranuclear near-response pathways: They coordinate convergence, accommodation, and pupillary constriction during near vision.

Origin of Nerve

• The oculomotor nerve emerges from the anterior surface of the midbrain through the oculomotor sulcus.
• This sulcus lies on the medial aspect of the cerebral peduncle, within the interpeduncular fossa.

Course and Relations

The course of the oculomotor nerve is divided into three parts:

1. Intramesencephalic course.
2. Intracranial course.
3. Intraorbital course.

Intramesencephalic course

• The nerve fibres arise from the oculomotor nuclear complex in the midbrain.
• They pass ventrally through the midbrain tegmentum, traversing the region of the red nucleus and medial part of the substantia nigra.
• They emerge from the oculomotor sulcus on the medial side of the cerebral peduncle.

Intracranial course

• The nerve emerges into the interpeduncular cistern.
• It runs forwards between the posterior cerebral artery above and superior cerebellar artery below.
• It passes close to the posterior communicating artery.
• It pierces the dura mater near the posterior clinoid process and enters the roof of the cavernous sinus.
• It continues forwards within the lateral wall of the cavernous sinus, superior to the trochlear nerve.
• Near the superior orbital fissure, it divides into superior and inferior divisions.
• Both divisions enter the orbit through the superior orbital fissure within the common tendinous ring.

Intraorbital course

A. Superior Division

• It passes forwards above and lateral to the optic nerve.
• It supplies:
  1. Superior rectus.
  2. Levator palpebrae superioris.

B. Inferior Division

– It passes forwards beneath the optic nerve and divides into branches supplying:

1. Medial rectus.
2. Inferior rectus.
3. Inferior oblique.

– The branch to the inferior oblique gives a parasympathetic root to the ciliary ganglion.

Branches and Distribution

Division	Branches	Distribution
Superior division	Muscular branches	Superior rectus and levator palpebrae superioris
Inferior division	Muscular branches	Medial rectus, inferior rectus, and inferior oblique
Inferior division	Parasympathetic root to ciliary ganglion	Sphincter pupillae and ciliary muscle through short ciliary nerves

TROCHLEAR NERVE

• The trochlear nerve is the fourth cranial nerve (CN IV) and is purely motor.
• It supplies only the superior oblique muscle of the eyeball.
• It is the thinnest cranial nerve and has the longest intracranial course among the cranial nerves.

Functional Components and Nuclei

1. General Somatic Efferent (GSE)

• Motor fibres arise from the trochlear nucleus in the midbrain at the level of the inferior colliculus.
• They supply the superior oblique muscle of the opposite eye because the fibres decussate before emerging from the brainstem.

2. Proprioceptive Fibres

– Proprioceptive information from the superior oblique muscle is conveyed through trigeminal sensory pathways, including connections with the mesencephalic nucleus of the trigeminal nerve.

Peculiarities of Trochlear Nerve

1. It is the thinnest cranial nerve.
2. It supplies only one muscle—the superior oblique.
3. It is the only cranial nerve emerging from the dorsal surface of the brainstem.
4. It is the only cranial nerve whose motor fibres completely decussate before emerging from the brainstem.
5. Each trochlear nucleus supplies the superior oblique muscle of the contralateral eye.
6. It has the longest intracranial course and is particularly vulnerable to traumatic injury.

Origin of Trochlear Nerve

• The trochlear nerve emerges from the dorsal surface of the midbrain, immediately below the inferior colliculus.
• It appears on either side of the frenulum of the superior medullary velum.

Course of Trochlear Nerve

The course of the trochlear nerve is divided into three parts:

1. Intramesencephalic course.
2. Intracranial course.
3. Intraorbital course.

Intramesencephallic course

• The trochlear nucleus lies in the ventral part of the periaqueductal grey matter at the level of the inferior colliculus.
• Motor fibres pass dorsally around the cerebral aqueduct.
• They decussate in the superior medullary velum before emerging from the dorsal surface of the midbrain.
• Consequently, each trochlear nerve supplies the superior oblique muscle of the opposite side to its nucleus.

Intracranial course

• The nerve emerges below the inferior colliculus, close to the frenulum of the superior medullary velum.
• It winds laterally and forwards around the midbrain, near the superior border of the pons.
• It passes between the posterior cerebral and superior cerebellar arteries.
• It pierces the dura mater near the free margin of the tentorium cerebelli.
• It enters the lateral wall of the cavernous sinus and runs forwards between the oculomotor nerve above and ophthalmic nerve below.

Intraorbital course

• The trochlear nerve enters the orbit through the superior orbital fissure, outside and above the common tendinous ring.
• It passes forwards and medially, above the levator palpebrae superioris.
• It enters the superior surface of the superior oblique muscle.

Branch and distribution

– The trochlear nerve supplies only the superior oblique muscle.

Actions of Superior Oblique Muscle

1. Intorsion: It rotates the superior pole of the eyeball medially.
2. Depression: It depresses the eyeball, particularly when the eye is adducted.
3. Abduction: It assists in lateral movement of the eyeball.

ABDUCENT NERVE

• The abducent nerve is the sixth cranial nerve (CN VI) and is purely motor.
• It supplies only the lateral rectus muscle of the eyeball.
• It produces abduction of the eyeball, hence its name.
• It has a long intracranial course and is particularly vulnerable to raised intracranial pressure.

Functional Components and Nuclei

1. General Somatic Efferent (GSE)

• Motor fibres arise from the abducent nucleus in the caudal pons.
• They supply the lateral rectus muscle, which abducts the eyeball.

2. Proprioceptive Fibres

– Proprioceptive information from the lateral rectus muscle is conveyed through trigeminal sensory pathways, including connections with the mesencephalic nucleus of the trigeminal nerve.

Origin of Abducent Nerve

• The abducent nerve emerges from the anterior surface of the brainstem at the pontomedullary junction.
• It appears immediately above the pyramid of the medulla, medial to the facial nerve.

Course and Relations

The course of the abducent nerve is divided into three parts:

1. Intrapontine course.
2. Intracranial course.
3. Intraorbital course.

Intrapontine course

• The abducent nucleus lies in the caudal pons, beneath the facial colliculus in the floor of the fourth ventricle.
• The internal genu of the facial nerve curves around the abducent nucleus, producing the elevation of the facial colliculus.
• Motor fibres from the abducent nucleus pass ventrally through the pontine tegmentum and basilar pons.
• They emerge at the pontomedullary junction.

Intracranial course

• The nerve emerges at the pontomedullary junction, immediately above the medullary pyramid.
• It passes forwards and upwards through the prepontine cistern, along the anterior surface of the pons.
• It pierces the dura mater over the clivus and ascends towards the apex of the petrous temporal bone.
• It passes through Dorello’s canal, beneath the petrosphenoidal ligament.
• It enters the cavernous sinus and runs forwards alongside the internal carotid artery.
• Within the cavernous sinus, it lies lateral or inferolateral to the internal carotid artery.
• It enters the orbit through the superior orbital fissure, within the common tendinous ring.

Intraorbital course

• The abducent nerve enters the orbit through the superior orbital fissure.
• It passes forwards and laterally towards the lateral rectus muscle.
• It enters the medial (deep) surface of the lateral rectus muscle and supplies it.

Branch and Distribution

• The abducent nerve supplies only one muscle—the lateral rectus.
• Action: Abduction of the eyeball.

DORELLO’S CANAL

• Dorello’s canal is a short osteofibrous passage located near the apex of the petrous temporal bone.
• It is bounded by the petrous apex, adjoining clival region, and the petrosphenoidal ligament (Gruber’s ligament).
• It transmits the abducent nerve towards the cavernous sinus.
• The relatively fixed position of the nerve within this canal makes it susceptible to stretching during displacement of the brainstem.

OPHTHALMIC NERVE

• The ophthalmic nerve (V1) is the smallest division of the trigeminal nerve.
• It is purely sensory and supplies the orbit, eyeball, forehead, upper eyelid, and adjoining regions of the nose.
• It arises from the anterolateral aspect of the trigeminal ganglion in the middle cranial fossa.
• It passes forwards in the lateral wall of the cavernous sinus, below the trochlear nerve.
• Before entering the orbit, it divides into three terminal branches:
  1. Lacrimal nerve.
  2. Frontal nerve.
  3. Nasociliary nerve.
• All three branches enter the orbit through the superior orbital fissure.

Mnemonic: LFN – Lacrimal, Frontal, Nasociliary.

Branches of Ophthalmic Nerve

Main branch	Sub-branches
Lacrimal nerve	Sensory branches to lacrimal gland, conjunctiva, and upper eyelid; communicating branch carrying secretomotor fibres
Frontal nerve	Supraorbital and supratrochlear nerves
Nasociliary nerve	Sensory root to ciliary ganglion, long ciliary nerves, posterior ethmoidal nerve, anterior ethmoidal nerve, and infratrochlear nerve

LACRIMAL NERVE

– The lacrimal nerve is the smallest terminal branch of the ophthalmic nerve.

Course

• It enters the orbit through the lateral part of the superior orbital fissure, outside the common tendinous ring.
• It passes forwards along the superior border of the lateral rectus muscle.
• It reaches the lacrimal gland and terminates in the lateral part of the upper eyelid.

Branches and Distribution

1. Sensory branches: They supply the lacrimal gland, adjoining conjunctiva, and lateral part of the upper eyelid.
2. Communicating branch: It receives postganglionic parasympathetic fibres through the zygomaticotemporal branch of the maxillary nerve (V2).
3. Secretomotor fibres: These fibres originate from the pterygopalatine ganglion and reach the lacrimal gland through the lacrimal nerve.

FRONTAL NERVE

– The frontal nerve is the largest branch of the ophthalmic nerve.

Course

• It enters the orbit through the superior orbital fissure, outside the common tendinous ring.
• It passes forwards above the levator palpebrae superioris muscle.
• It divides into two terminal branches:
  1. Supratrochlear nerve.
  2. Supraorbital nerve.

Branches

1. Supratrochlear Nerve

• It is the smaller, medial terminal branch.
• It passes forwards and medially above the trochlea of the superior oblique muscle.
• It supplies the skin of the medial forehead, medial part of the upper eyelid, and adjacent scalp.

2. Supraorbital Nerve

• It is the larger, lateral terminal branch.
• It passes forwards through the supraorbital notch or foramen.
• It supplies the upper eyelid, forehead, and scalp extending towards the vertex.

NASOCILIARY NERVE

– The nasociliary nerve is a major sensory branch of the ophthalmic nerve.

Course

• It enters the orbit through the superior orbital fissure, within the common tendinous ring.
• It passes between the superior and inferior divisions of the oculomotor nerve.
• It crosses superior to the optic nerve from lateral to medial, usually accompanied by the ophthalmic artery.
• It continues along the medial wall of the orbit and gives off several branches.

Branches

1. Posterior Ethmoidal Nerve

• It enters the posterior ethmoidal foramen.
• It supplies the posterior ethmoidal air cells and sphenoidal sinus.

2. Anterior Ethmoidal Nerve

• It enters the anterior cranial fossa through the anterior ethmoidal foramen.
• It runs forwards along the cribriform plate and enters the nasal cavity.
• It divides into:
  • Internal nasal branches: They supply the mucosa of the anterosuperior nasal cavity.
  • External nasal branch: It supplies the skin of the dorsum and tip of the nose.

3. Infratrochlear Nerve

• It passes forwards below the trochlea of the superior oblique muscle.
• It supplies the medial ends of both eyelids, conjunctiva, lacrimal sac, lacrimal caruncle, and adjoining upper part of the nose.

4. Long Ciliary Nerves

• Usually two or three long ciliary nerves arise from the nasociliary nerve.
• They pass forwards alongside the optic nerve and pierce the sclera.
• They carry sensory fibres from the cornea, iris, and ciliary body.
• They also carry postganglionic sympathetic fibres to the dilator pupillae and ocular blood vessels.

5. Sensory Root to Ciliary Ganglion

• It arises from the nasociliary nerve near the orbital apex.
• Its sensory fibres pass through the ciliary ganglion without synapsing.
• They convey general sensory impulses from the eyeball.

Ciliary ganglion

• The ciliary ganglion is a small parasympathetic ganglion situated near the apex of the orbit.
• It is approximately 1–2 mm in diameter.

Location

• It lies between the optic nerve medially and lateral rectus muscle laterally.
• It is connected anatomically to the nasociliary nerve.
• It lies medial to the ophthalmic artery.

Neuronal Relationships

• Topographical relationship: Nasociliary nerve (V1).
• Functional parasympathetic relationship: Oculomotor nerve (CN III).

Roots of Ciliary Ganglion

The ciliary ganglion receives three roots.

1. Parasympathetic Root

• Preganglionic parasympathetic fibres arise from the Edinger–Westphal nucleus in the midbrain.
• They travel through the oculomotor nerve and its inferior division.
• They reach the ciliary ganglion through a branch associated with the nerve to the inferior oblique muscle.
• These fibres synapse in the ciliary ganglion.
• Postganglionic fibres leave through the short ciliary nerves and supply:
  1. Sphincter pupillae.
  2. Ciliary muscle.

2. Sympathetic Root

• Preganglionic sympathetic fibres arise from the upper thoracic spinal cord, predominantly T1.
• They ascend through the sympathetic trunk and synapse in the superior cervical ganglion.
• Postganglionic fibres reach the orbit through sympathetic plexuses around the internal carotid and ophthalmic arteries.
• They pass through the ciliary ganglion without synapsing.
• They supply ocular blood vessels and contribute to the innervation of the dilator pupillae.

3. Sensory Root

• It arises from the nasociliary nerve.
• Its sensory fibres pass through the ciliary ganglion without synapsing.
• They carry general sensory impulses from the eyeball.

Branches

• The ciliary ganglion gives rise to approximately 8–10 short ciliary nerves.
• These nerves divide into smaller branches and pierce the sclera around the optic nerve.

Functions of Short Ciliary Nerves

1. Parasympathetic fibres: They produce pupillary constriction and accommodation.
2. Sympathetic fibres: They provide vasomotor innervation and contribute to pupillary dilatation.
3. Sensory fibres: They convey general sensation from intraocular structures.

OPHTHALMIC ARTERY

• The ophthalmic artery is the principal artery supplying the eyeball and orbital structures.
• It is a branch of the internal carotid artery.

Origin

– It usually arises from the cerebral part of the internal carotid artery near the anterior clinoid process and optic canal.

Course

• It enters the orbit through the optic canal, initially lying inferolateral to the optic nerve.
• Within the orbit, it usually crosses superior to the optic nerve from lateral to medial.
• It runs forwards along the medial wall of the orbit, between the superior oblique and medial rectus muscles.
• It accompanies the nasociliary nerve along part of its course.

Termination

– Near the medial angle of the eye, it divides into two terminal branches:

1. Supratrochlear artery.
2. Dorsal nasal artery.

Branches and Sub-branches

1. Central retinal artery.
2. Lacrimal artery.
3. Posterior ciliary arteries (long and short).
4. Muscular branches, giving rise to anterior ciliary arteries.
5. Supraorbital artery.
6. Posterior ethmoidal artery.
7. Anterior ethmoidal artery.
8. Medial palpebral arteries.
9. Supratrochlear artery.
10. Dorsal nasal artery.

Central artery of retina

• The central retinal artery is a branch of the ophthalmic artery.
• It is the principal arterial supply to the inner layers of the retina.

Origin and Course

• It usually arises from the ophthalmic artery below the optic nerve.
• It passes forwards within the dural sheath of the optic nerve.
• It pierces the optic nerve approximately 10–15 mm behind the eyeball.
• It continues within the optic nerve and enters the retina through the optic disc.

Termination

• At the optic disc, it divides into superior and inferior divisions.
• Each division gives rise to nasal and temporal branches.
• These branches divide progressively to supply the inner retinal layers.

Peculiarities

1. End artery: It behaves as a functional end artery with limited collateral circulation.
2. Distribution: It supplies predominantly the inner two-thirds of the retina.
3. Outer retinal layers: They receive nutrients mainly by diffusion from the choriocapillaris.
4. Ophthalmoscopic appearance: Its branches can be observed directly during fundus examination.

LACRIMAL ARTERY

• The lacrimal artery usually arises from the ophthalmic artery before it crosses the optic nerve.
• It passes forwards along the superior border of the lateral rectus muscle towards the lacrimal gland.

Branches

1. Glandular branches: They supply the lacrimal gland.
2. Lateral palpebral arteries: They supply the upper and lower eyelids and participate in the palpebral arterial arcades.
3. Zygomatic branches: They accompany the zygomaticofacial and zygomaticotemporal nerves.
4. Recurrent meningeal branches: They pass backwards through the superior orbital fissure and communicate with branches of the middle meningeal artery.
5. Muscular branches: They supply adjoining extraocular muscles.

POSTERIOR CILIARY ARTERIES

• The posterior ciliary arteries arise from the ophthalmic artery and pierce the sclera around the optic nerve.
• They are classified into long and short posterior ciliary arteries.

Long Posterior Ciliary Arteries

• There are usually two long posterior ciliary arteries.
• They pass forwards between the sclera and choroid towards the ciliary body and iris.
• They anastomose with the anterior ciliary arteries to contribute to the major arterial circle of the iris.

Short Posterior Ciliary Arteries

• They are numerous small arteries that pierce the sclera around the optic nerve.
• They supply the choroid, optic nerve head, and adjoining ocular structures.

OTHER BRANCHES OF OPHTHALMIC ARTERY

Supraorbital Artery

• It passes forwards with the supraorbital nerve through the supraorbital notch or foramen.
• It supplies the forehead, upper eyelid, and scalp.

Supratrochlear Artery

• It is a terminal branch of the ophthalmic artery.
• It accompanies the supratrochlear nerve and supplies the medial forehead and anterior scalp.

Posterior Ethmoidal Artery

• It enters the posterior ethmoidal foramen.
• It supplies the posterior ethmoidal air cells and sphenoidal sinus, with contributions to the adjoining nasal cavity and dura mater.

Anterior Ethmoidal Artery

• It passes through the anterior ethmoidal foramen into the anterior cranial fossa.
• It supplies the anterior ethmoidal air cells, nasal cavity, and adjoining dura mater.

Medial Palpebral Arteries

• They consist of superior and inferior branches.
• They supply the medial parts of the eyelids and participate in the palpebral arterial arcades.

Dorsal Nasal Artery

• It is a terminal branch of the ophthalmic artery.
• It supplies the medial canthal region and dorsum of the nose.
• It anastomoses with branches of the facial artery, establishing communication between the internal and external carotid arterial systems.

Muscular Branches

• They supply the extraocular muscles.
• Some give rise to anterior ciliary arteries supplying the anterior segment of the eyeball.

Note: The eyeball does not possess a conventional lymphatic vascular network, although lymphatic vessels are present in adjoining structures such as the conjunctiva.

OPHTHALMIC VEINS

• The orbit is drained mainly by two ophthalmic veins:
  1. Superior ophthalmic vein.
  2. Inferior ophthalmic vein.
• These veins are largely valveless and communicate with intracranial and extracranial venous channels.

Superior Ophthalmic Vein

Beginning

– It begins near the medial angle of the eye through communications involving the angular and supraorbital veins.

Course

• It runs backwards through the orbit, usually above the optic nerve.
• It is closely related to the ophthalmic artery.
• It leaves the orbit through the superior orbital fissure.

Termination

– It drains into the cavernous sinus.

Inferior Ophthalmic Vein

Beginning

– It arises from a venous network in the floor of the orbit.

Course

• It runs backwards beneath the optic nerve.
• It receives tributaries from the inferior orbital structures.

Termination

– It may:

1. Join the superior ophthalmic vein.
2. Drain directly into the cavernous sinus.
3. Communicate with the pterygoid venous plexus through the inferior orbital fissure.