Bony Orbit and Muscles of Orbit

INTRODUCTION

• The orbits are paired, pyramid-shaped bony cavities situated on either side of the nasal cavity.
• They contain the eyeballs and associated structures responsible for vision and ocular movements.

Contents of Orbit

1. Eyeball: The organ of vision.
2. Muscles: Extraocular muscles responsible for movements of the eyeball and upper eyelid. Intraocular muscles are located within the eyeball.
3. Fasciae: Orbital fascia and bulbar fascia (Tenon’s capsule).
4. Nerves:
   • Optic nerve (CN II).
   • Oculomotor nerve (CN III).
   • Trochlear nerve (CN IV).
   • Abducent nerve (CN VI).
   • Ophthalmic division of trigeminal nerve (V1).
   • Branches of maxillary nerve (V2).
5. Ganglion: Ciliary ganglion.
6. Blood vessels: Ophthalmic artery and ophthalmic veins.
7. Gland: Lacrimal gland.
8. Orbital fat: It surrounds and cushions the orbital structures.

Boundaries of Orbit

• Each bony orbit is approximately pyramidal, with its base directed forwards and apex directed backwards.
• It consists of:
  1. Apex.
  2. Four walls: medial, lateral, roof, and floor.
  3. Base (orbital margin).

Apex

• The apex is directed posteromedially towards the sphenoid bone.
• It is situated near the optic canal and the medial end of the superior orbital fissure.
• The optic canal and superior orbital fissure provide important communications between the orbit and cranial cavity.

Medial wall

– The medial wall is the thinnest wall of the orbit, particularly in its ethmoidal portion.

Formation

It is formed by four bones, arranged from anterior to posterior:

1. Frontal process of maxilla.
2. Lacrimal bone.
3. Orbital plate of ethmoid.
4. Body of sphenoid.

Features

1. Fossa for Lacrimal Sac

• It lies between the anterior and posterior lacrimal crests.
• It accommodates the lacrimal sac and continues inferiorly into the nasolacrimal canal.

2. Anterior Lacrimal Crest

– It is a vertical bony ridge on the frontal process of the maxilla.

3. Posterior Lacrimal Crest

– It is formed by a vertical ridge on the lacrimal bone.

4. Anterior and Posterior Ethmoidal Foramina

• They are located along the junction of the medial wall and roof of the orbit.
• They transmit the corresponding ethmoidal nerves and vessels.

Lateral wall

– The lateral wall is the strongest and thickest wall of the orbit.

Formation

It is formed by two bones:

1. Zygomatic bone: Anteriorly.
2. Greater wing of sphenoid: Posteriorly.

Features

1. Zygomatic Foramina

• Zygomaticofacial foramen: It transmits the zygomaticofacial nerve and vessels.
• Zygomaticotemporal foramen: It transmits the zygomaticotemporal nerve and vessels.

2. Whitnall’s Tubercle (Lateral Orbital Tubercle)

• It is a small bony projection situated on the orbital surface of the zygomatic bone.
• It lies immediately behind the lateral orbital margin, below the frontozygomatic suture.

Attachments to Whitnall’s Tubercle

1. Lateral palpebral ligament.
2. Lateral check ligament of the lateral rectus muscle.
3. Lateral attachment of the suspensory ligament of the eyeball (Lockwood’s ligament).
4. Lateral horn of the levator palpebrae superioris aponeurosis.

Roof

Formation

The roof is formed by:

1. Orbital plate of frontal bone: Anteriorly.
2. Lesser wing of sphenoid: Posteriorly.

Features

1. Lacrimal Fossa

• It is a shallow depression situated in the anterolateral part of the roof.
• It accommodates the orbital part of the lacrimal gland.

2. Trochlear Fovea or Spine

• It is situated near the anteromedial angle of the roof.
• It provides attachment to the fibrocartilaginous pulley (trochlea) of the superior oblique muscle.

3. Optic Canal

• It is situated near the posteromedial apex of the orbit.
• It transmits:
  1. Optic nerve with its meningeal coverings.
  2. Ophthalmic artery.
  3. Accompanying sympathetic fibres.

Floor

Formation

The floor is formed by three bones:

1. Maxilla: It forms the major portion.
2. Zygomatic bone: It forms the anterolateral portion.
3. Orbital process of palatine bone: It forms a small posteromedial portion.

Features

1. Infraorbital Groove and Canal

• The infraorbital groove begins near the inferior orbital fissure.
• It continues anteriorly as the infraorbital canal and opens onto the face through the infraorbital foramen.
• It transmits the infraorbital nerve and vessels.

2. Origin of Inferior Oblique Muscle

• The inferior oblique muscle arises from the anteromedial part of the orbital floor.
• Its origin lies just lateral to the entrance of the nasolacrimal canal.

Base (orbital margin)

• The base is the anterior, quadrangular opening of the orbit.
• It is bounded by the orbital margins formed by the frontal, zygomatic, and maxillary bones.

Boundaries

1. Superior (Supraorbital) Margin

• It is formed by the frontal bone.
• It presents the supraorbital notch or foramen near the junction of its medial one-third and lateral two-thirds.
• It transmits the supraorbital nerve and vessels.

2. Medial Orbital Margin

• It is relatively indistinct.
• It is formed mainly by the frontal bone superiorly and the frontal process of the maxilla inferiorly.
• It is continuous with the anterior lacrimal crest.

3. Inferior (Infraorbital) Margin

• It is formed by:
  • Maxilla medially.
  • Zygomatic bone laterally.
• Medially, it continues into the anterior lacrimal crest.

4. Lateral Orbital Margin

– It is formed by:

• Zygomatic process of the frontal bone superiorly.
• Frontal process of the zygomatic bone inferiorly.

Relations of Orbit

Direction	Related Structures
Superiorly	Anterior cranial fossa and frontal air sinus
Inferiorly	Maxillary air sinus
Medially	Ethmoidal air cells and sphenoidal sinus posteriorly
Laterally	Temporal fossa anteriorly and middle cranial fossa posteriorly

AXES OF ORBIT

Visual Axis

• The visual axis is an imaginary line extending from the object of fixation through the optical nodal points of the eye to the fovea centralis.
• In the primary position of gaze, the visual axes of the two eyes are approximately parallel.

Orbital Axis

• The orbital axis is an imaginary line passing through the centre of the orbital opening and the apex of the orbit.
• It is directed forwards and laterally.

Angle Between Axes

• Each orbital axis forms an angle of approximately 22–23° with the sagittal plane.
• In the primary position of gaze, the angle between the orbital axis and the visual axis is approximately 20–25°.

ORBITAL INDEX

– The orbital index expresses the relationship between the maximum height and maximum breadth of the bony orbit.

Applied Importance

• It is used in anthropometry to study variations in orbital shape.
• It is useful in forensic anthropology for assessing skeletal characteristics.
• Orbital measurements also assist in craniofacial reconstruction and orbital surgery.

ORBITAL FASCIA OR PERIORBITA

• The orbital fascia, or periorbita, is the periosteal lining of the bony orbit.
• It is loosely attached to most orbital bones and can be separated easily during dissection.
• Posteriorly, it is continuous with the cranial dura mater near the optic canal and superior orbital fissure.
• At the orbital openings, it becomes continuous with the periosteum of adjoining bones.

Lacrimal Fascia

• The lacrimal fascia is a fascial covering associated with the lacrimal sac and its fossa.
• It extends between the anterior and posterior lacrimal crests.
• It is related to the medial palpebral ligament and lacrimal part of the orbicularis oculi muscle.

Bulbar Fascia (Tenon’s Capsule)

• Tenon’s capsule is a thin fibrous sheath surrounding the eyeball.
• It extends from the region of the optic nerve posteriorly to the sclerocorneal junction (limbus) anteriorly.
• It separates the eyeball from the surrounding orbital fat and permits smooth ocular movements.

Structures Piercing Tenon’s Capsule

1. Tendons of extraocular muscles.
2. Ciliary nerves and vessels.
3. Optic nerve posteriorly.

Orbitalis Muscle

• The orbitalis muscle is a small, rudimentary smooth muscle associated with the inferior orbital fissure.
• It is closely related to the periorbita.
• Nerve supply: Postganglionic sympathetic fibres from the superior cervical ganglion.
• Action: Its functional significance in humans is uncertain.

Orbital Septum

• The orbital septum is a fibrous membrane extending from the orbital margins into the upper and lower eyelids.
• It blends with the tarsal plates and adjoining eyelid structures.
• It forms an important barrier between the superficial eyelid tissues and deeper orbital contents.

Episcleral Space

• The episcleral space lies between Tenon’s capsule and the sclera.
• It contains loose connective tissue that allows free movement of the eyeball within its fascial sheath.

FASCIAL SHEATHS OF EXTRAOCULAR MUSCLES

• The extraocular muscles are enclosed by tubular fascial sheaths continuous with Tenon’s capsule.
• These sheaths form specialised ligamentous expansions that support and stabilise the eyeball.

Medial Check Ligament

• It is a strong triangular expansion of the fascial sheath of the medial rectus muscle.
• It extends medially towards the lacrimal bone and adjacent orbital fascia.
• It restricts excessive lateral movement of the eyeball.

Lateral Check Ligament

• It is a thickened fascial expansion arising from the sheath of the lateral rectus muscle.
• It attaches laterally to the zygomatic bone near Whitnall’s tubercle.
• It limits excessive medial movement of the eyeball.

Suspensory Ligament of Eyeball (Lockwood’s Ligament)

• It is a hammock-shaped fascial support situated beneath the eyeball.
• It is formed mainly by thickening and fusion of fascial expansions from the inferior rectus and inferior oblique muscles.
• Medially: It is attached to the medial check ligament.
• Laterally: It is attached to the lateral check ligament.
• Function: It supports the eyeball and helps maintain its position within the orbit.

EXTRAOCULAR MUSCLES

– The extraocular muscles are classified into voluntary skeletal muscles and involuntary smooth muscles.

Voluntary muscles

There are seven voluntary skeletal muscles.

1. Four Recti Muscles

1. Superior rectus.
2. Inferior rectus.
3. Medial rectus.
4. Lateral rectus.

2. Two Oblique Muscles

1. Superior oblique.
2. Inferior oblique.

3. Muscle of Upper Eyelid

– Levator palpebrae superioris.

Involuntary muscles

Three smooth muscles are described in relation to the orbit:

1. Superior tarsal muscle.
2. Inferior tarsal muscle.
3. Orbitalis muscle.

Superior Tarsal Muscle (Müller’s Muscle)

• It is a thin sheet of smooth muscle arising from the deep surface of the levator palpebrae superioris.
• Insertion: Upper margin of the superior tarsal plate.
• Nerve supply: Postganglionic sympathetic fibres from the superior cervical ganglion.
• Action: It assists in elevating the upper eyelid and maintains its resting position.

Clinical Significance

– Interruption of sympathetic innervation produces mild ptosis as part of Horner’s syndrome.

Inferior Tarsal Muscle

• It is a small smooth muscle associated with the lower eyelid.
• Origin: Fascial tissues related to the inferior rectus and inferior oblique muscles.
• Insertion: Inferior tarsal plate.
• Nerve supply: Sympathetic fibres.
• Action: It assists in retracting the lower eyelid.

Orbitalis Muscle

• It is a rudimentary smooth muscle bridging the inferior orbital fissure.
• Nerve supply: Sympathetic fibres.
• Action: Its precise function in humans is not established.

Recti Muscles

– The four recti muscles extend forwards from the orbital apex towards the eyeball.

Origin

• All four recti muscles arise from the common tendinous ring (annulus of Zinn).
• The ring surrounds the optic canal and the medial part of the superior orbital fissure at the orbital apex.
• The lateral rectus has two heads of origin associated with the common tendinous ring and adjoining sphenoid bone.

Insertion

• All four recti muscles insert into the sclera anterior to the equator of the eyeball.
• Their tendons attach at varying distances behind the limbus.

Distance of Insertion from Limbus

Muscle	Distance from limbus
Medial rectus	5.5 mm
Inferior rectus	6.5 mm
Lateral rectus	7 mm
Superior rectus	8 mm

– The progressive arrangement of these insertions forms the spiral of Tillaux.

Oblique Muscles

• There are two oblique muscles: superior oblique and inferior oblique.
• Both insert posterior to the equator of the eyeball.

Superior oblique

Origin

– It arises from the body of the sphenoid bone, superomedial to the optic canal.

Course

• The muscle passes forwards along the superomedial part of the orbit.
• Its tendon passes through the fibrocartilaginous pulley (trochlea) attached to the frontal bone.
• After passing through the trochlea, the tendon turns backwards, laterally, and downwards beneath the superior rectus.

Insertion

– It inserts into the posterosuperolateral part of the sclera, behind the equator of the eyeball.

Inferior oblique

Origin

– It arises from the orbital surface of the maxilla, lateral to the opening of the nasolacrimal canal.

Course

• Its fibres pass backwards, laterally, and upwards beneath the inferior rectus muscle.
• The muscle continues towards the posterolateral aspect of the eyeball.

Insertion

– It inserts into the posteroinferolateral part of the sclera, behind the equator of the eyeball.

Levator Palpebrae Superioris

– The levator palpebrae superioris is a thin, triangular skeletal muscle responsible for elevation of the upper eyelid.

Origin

– It arises from the lesser wing of the sphenoid bone, immediately above the optic canal.

Course

• It passes forwards above the superior rectus muscle.
• Anteriorly, its tendon expands to form the levator aponeurosis.
• Smooth muscle fibres associated with its deep surface form the superior tarsal muscle.

Insertion

Levator Aponeurosis

– It attaches to the anterior surface of the superior tarsal plate and to the skin of the upper eyelid.

Superior Tarsal Muscle

– It inserts into the upper margin of the superior tarsal plate.

Action

• The levator palpebrae superioris elevates the upper eyelid.
• The superior tarsal muscle assists in maintaining eyelid elevation.

Innervation

– Most voluntary extraocular muscles are supplied by the oculomotor nerve (CN III).

Mnemonic: LR6 SO4, all others III

• LR6: Lateral rectus is supplied by the abducent nerve (CN VI).
• SO4: Superior oblique is supplied by the trochlear nerve (CN IV).
• All others III: Remaining voluntary extraocular muscles are supplied by the oculomotor nerve (CN III).

Actions

– The eyeball performs movements around three principal axes.

1. Movements Around Transverse Axis

• Elevation: Upward movement of the eyeball.
• Depression: Downward movement of the eyeball.

2. Movements Around Vertical Axis

• Adduction: Movement of the eyeball towards the nose.
• Abduction: Movement of the eyeball towards the temple.

3. Movements Around Anteroposterior Axis

• Intorsion (incycloduction): Rotation of the eyeball in which its superior pole turns medially towards the nose.
• Extorsion (excycloduction): Rotation of the eyeball in which its superior pole turns laterally towards the temple.

Important Anatomical Features

• The superior and inferior recti are most effective in producing elevation and depression when the eyeball is abducted.
• The superior and inferior oblique muscles are most effective in producing depression and elevation, respectively, when the eyeball is adducted.
• Coordinated contraction of extraocular muscles enables smooth, conjugate movements of both eyes.

MOVEMENTS OF EYEBALL

Conjugate and Disconjugate Movements

• Conjugate movements: Both eyes move simultaneously in the same direction, maintaining approximately parallel visual axes during distant fixation.
• Disconjugate (Vergence) movements: Both eyes move in opposite directions to adjust their visual axes.
  • Convergence: Both eyes turn medially to focus on a near object.
  • Divergence: Both eyes turn laterally to focus on a distant object.

Muscles Producing Movements of Eyeball

• Pure movements of the eyeball result from coordinated contraction of extraocular muscles.
• The combined actions reinforce the desired movement while neutralising unwanted movements.

Movement	Muscles involved	Important feature
Elevation	Superior rectus and inferior oblique	Superior rectus is most effective in abduction; inferior oblique in adduction
Depression	Inferior rectus and superior oblique	Inferior rectus is most effective in abduction; superior oblique in adduction
Adduction	Medial rectus	Superior and inferior recti assist in adduction
Abduction	Lateral rectus	Superior and inferior oblique muscles assist in abduction
Intorsion	Superior rectus and superior oblique	The superior pole of the eyeball rotates medially
Extorsion	Inferior rectus and inferior oblique	The superior pole of the eyeball rotates laterally