Orbit

BONY ORBITS

General Features Boundaries Roof Lateral Wall Floor Medial Wall Foramina Contents Orbital Fascia Bulbar Fascia Extraocular Muscles Actions Clinical Anatomy
01

General Features

  • The orbits are pyramidal bony cavities, situated one on each side of the root of the nose.
  • They:
    • Provide sockets for rotatory movements of the eyeballs.
    • Protect the eyeballs.
  • The long or orbital axis of the orbit passes backwards and medially.
  • The medial walls of the two orbits are parallel.
  • The lateral walls are set at right angles to each other.
  • The axis passing through the centres of the anterior and posterior poles of the eyeball is called the visual axis.
  • The visual axis makes an angle of 20–25° with the orbital axis.
02

Boundaries of Bony Orbit

  • Each orbit resembles a four-sided pyramid.
  • It has:
    1. Apex
    2. Base
    3. Four walls → roof, floor, lateral wall and medial wall.

A. Apex

  • Situated at the posterior end of the orbit.
  • Located at the medial end of the superior orbital fissure.

B. Base

  • Seen as the orbital opening on the face.
03

Roof

  • The roof is concave from side to side.
  • It is formed:
    1. Mainly by the orbital plate of the frontal bone.
    2. Completed posteriorly by the lesser wing of the sphenoid.

Features of Roof

1. Lacrimal Fossa

  • Placed anterolaterally.
  • Lodges the lacrimal gland.

2. Optic Canal

  • Lies posteriorly.
  • Situated at the junction of the roof and medial wall.

3. Trochlear Fossa

  • Lies anteromedially.
  • Provides attachment to the fibrous pulley or trochlea for the tendon of the superior oblique muscle.
04

Lateral Wall

  • It is the thickest and strongest of all the walls of the orbit.
  • It is formed:
    1. By the anterior surface of the greater wing of the sphenoid bone, posteriorly.
    2. By the orbital surface of the frontal process of the zygomatic bone, anteriorly.

Features

1. Superior Orbital Fissure

  • Occupies the posterior part of the junction between the roof and lateral wall.

2. Foramen for Zygomatic Nerve

  • Seen in the zygomatic bone.

3. Whitnall’s or Zygomatic Tubercle

  • A palpable elevation on the zygomatic bone just within the orbital margin.
  • Provides attachment to the lateral check ligament of eyeball.
05

Floor

  • The floor slopes upwards and medially to join the medial wall.
  • It is formed:
    1. Mainly by the orbital surface of the maxilla.
    2. By the lower part of the orbital surface of the zygomatic bone, anterolaterally.
    3. By the orbital process of the palatine bone, at the posterior angle.

Features

1. Inferior Orbital Fissure

  • Occupies the posterior part of the junction between the lateral wall and floor.
  • Through this fissure, the orbit communicates:
    • With the infratemporal fossa anteriorly.
    • With the pterygopalatine fossa posteriorly.

2. Infraorbital Groove

  • Runs forwards in relation to the floor.

3. Small Depression

  • Present on the anteromedial part of the floor.
  • Gives origin to the inferior oblique muscle.
06

Medial Wall

  • It is very thin.
  • From before backwards, it is formed by:
    1. Frontal process of the maxilla
    2. Lacrimal bone
    3. Orbital plate of the ethmoid
    4. Body of the sphenoid bone

Features

Lacrimal Groove

  • Lies anteriorly on the medial wall.
  • It is bounded:
    • Anteriorly → by the anterior lacrimal crest of the frontal process of the maxilla.
    • Posteriorly → by the posterior lacrimal crest of the lacrimal bone.
  • The floor of the groove is formed:
    • By the maxilla in front.
    • By the lacrimal bone behind.
  • The groove lodges the lacrimal sac.
  • The lacrimal sac lies deep to the lacrimal fascia bridging the lacrimal groove.
  • The groove leads inferiorly through the nasolacrimal duct to the inferior meatus of the nose.

Anterior and Posterior Ethmoidal Foramina

  • Lie on the frontoethmoidal suture.
  • Situated at the junction of the roof and medial wall.
07

Foramina in Relation to the Orbit

1. Inferior Orbital Fissure

Transmits:

  • Zygomatic nerve
  • Orbital branches of the pterygopalatine ganglion
  • Infraorbital nerve and vessels
  • Communication between the inferior ophthalmic vein and the pterygoid plexus of veins.

2. Infraorbital Groove and Canal

  • Transmit the corresponding nerve and vessels.

3. Zygomatic Foramen

  • Transmits the zygomatic nerve.

4. Anterior Ethmoidal Foramen

  • Transmits the corresponding nerve and vessels.

5. Posterior Ethmoidal Foramen

  • Transmits only the vessels of the same name.
08

Contents of Orbits

1. Eyeball

  • Occupies the anterior one-third of the orbit.

2. Fascia

  • Orbital
  • Bulbar

3. Muscles

  • Extraocular
  • Intraocular

4. Vessels

  • Ophthalmic artery
  • Superior ophthalmic vein
  • Inferior ophthalmic vein
  • Lymphatics

5. Nerves

  • Optic
  • Oculomotor
  • Trochlear
  • Abducent
  • Branches of ophthalmic and maxillary nerves
  • Sympathetic nerves

6. Lacrimal Gland

7. Orbital Fat

09

Orbital Fascia / Periorbita

It forms the periosteum of the bony orbit.

Features

1. Relation with Bone

  • Due to its loose connection to bone, it can be easily stripped.
  • Posteriorly, it is continuous with:
    • Dura mater
    • Sheath of the optic nerve
  • Anteriorly, it is continuous with the periosteum lining the bones around the orbital margin.

2. Gap over Inferior Orbital Fissure

  • There is a gap in the periorbita over the inferior orbital fissure.
  • The gap is bridged by connective tissue with some smooth muscle fibres.
  • These fibres constitute the orbitalis muscle.

3. Orbital Septum

  • At the upper and lower margins of the orbit, the orbital fascia sends flap-like continuations into the eyelids.
  • These extensions form the orbital septum.

4. Fibrous Pulley

  • A process of the fascia holds the fibrous pulley of the tendon of the superior oblique muscle in place.

5. Lacrimal Fascia

  • Another process forms the lacrimal fascia.
  • It bridges the lacrimal groove.

Orbitalis Muscle

  • Orbitalis is a smooth muscle that bridges the inferior orbital fissure.
  • It is considered a rudimentary muscle.
  • It is fused with the orbital fascia.
  • It receives sympathetic innervation.
  • Its function is not known.
10

Fascial Sheath of Eyeball / Bulbar Fascia

1. Tenon’s Capsule

  • Tenon’s capsule forms a thin, loose membranous sheath around the eyeball.
  • It extends from the optic nerve to the sclerocorneal junction or limbus.
  • It is separated from the sclera by the episcleral space.
  • The episcleral space is traversed by delicate fibrous bands.
  • The eyeball can freely move within this sheath.

2. Structures Piercing the Sheath

  1. Tendons of the various extraocular muscles
  2. Ciliary vessels and nerves around the entrance of the optic nerve

3. Expansions of the Sheath

a. Tubular Sheath

  • Covers each orbital muscle.

b. Medial Check Ligament

  • A strong triangular expansion from the sheath of the medial rectus muscle.
  • Attached to the lacrimal bone.

c. Lateral Check Ligament

  • A strong triangular expansion from the sheath of the lateral rectus muscle.
  • Attached to the zygomatic bone.

4. Suspensory Ligament of Lockwood

  • The lower part of Tenon’s capsule is thickened.
  • It is called:
    • Suspensory ligament of the eye
    • Suspensory ligament of Lockwood
  • It is:
    • Expanded in the centre.
    • Narrow at its extremities.
  • It is slung like a hammock below the eyeball.
  • It is formed by union of the margins of the sheaths of:
    • Inferior rectus
    • Inferior oblique
  • These unite with:
    • Medial check ligament
    • Lateral check ligament.
11

Extraocular Muscles

The extraocular muscles of the eyeball are classified into two groups:

  1. Involuntary
  2. Voluntary.

1. Involuntary Muscles

A. Superior Tarsal Muscle

  • It is the deeper portion of the levator palpebrae superioris.
  • It is inserted on the upper margin of the superior tarsus.
  • It elevates the upper eyelid.

B. Inferior Tarsal Muscle

  • Extends from the fascial sheath of:
    • Inferior rectus
    • Inferior oblique
  • Extends to the lower margin of the inferior tarsus.
  • It possibly depresses the lower eyelid.

C. Orbitalis

  • Bridges the inferior orbital fissure.
  • Its action is uncertain.

Nerve Supply of Involuntary Muscles

  • Involuntary muscles of the eye are supplied by postganglionic sympathetic fibres from the superior cervical ganglion.

2. Voluntary Extraocular Muscles

A. Four Recti

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

B. Two Obliques

  1. Superior oblique
  2. Inferior oblique

C. Levator

Levator palpebrae superioris.

12

Origin of Recti

  • All recti arise from a common annular tendon or tendinous ring of Zinn.
  • The ring is attached to:
    • Middle part of the superior orbital fissure
    • Around the optic canal.

Additional Origin of Lateral Rectus

  • The lateral rectus has an additional small tendinous head.
  • It arises from the orbital surface of the greater wing of the sphenoid bone, lateral to the tendinous ring.
  • The abducent nerve passes through the gap between the two heads.
13

Insertion of Recti

  • The recti are inserted into the sclera, a little posterior to the limbus (corneoscleral junction).

Average Distance of Insertions from Cornea

Muscle Distance
Superior rectus 7.7 mm
Inferior rectus 6.5 mm
Medial rectus 5.5 mm
Lateral rectus 6.9 mm
14

Oblique Muscles

1. Superior Oblique

Origin

  • Arises from the undersurface of the lesser wing of the sphenoid.
  • It is superomedial to the optic canal.

Insertion

  • Its tendon passes through a fibrocartilaginous pulley attached to the trochlear fossa of the frontal bone.
  • The tendon then passes:
    • Laterally
    • Downwards
    • Backwards
  • It passes below the superior rectus.
  • It is inserted into the sclera behind the equator of the eyeball in the posterolateral quadrant.

2. Inferior Oblique

Origin

  • Arises from the orbital surface of the maxilla, lateral to the lacrimal groove.
  • The muscle is situated near the anterior margin of the orbit.

Insertion

  • It is fleshy throughout.
  • It passes:
    • Laterally
    • Upwards
    • Backwards
  • It passes below the inferior rectus and then deep to the lateral rectus.
  • It is inserted close to the superior oblique, a little below and posterior to the latter.
15

Levator Palpebrae Superioris

Origin

  • Arises from the orbital surface of the lesser wing of the sphenoid bone.
  • It is anterosuperior to the optic canal.
  • It is also anterosuperior to the origin of the superior rectus.

Insertion

  • Its flat tendon splits into:
    1. Superior or voluntary lamella
    2. Inferior or involuntary lamella

Superior Lamella

Inserted into:

  • Anterior surface of the superior tarsus
  • Skin of the upper eyelid

Inferior Lamella

  • It is the smooth part.
  • Inserted into:
    • Upper margin of the superior tarsus
    • Superior conjunctival fornix.
16

Nerve Supply of Extraocular Muscles

1. Superior Oblique

Supplied by the IV cranial or trochlear nerve.

2. Lateral Rectus

Supplied by the VI cranial or abducent nerve.

3. Remaining Four and a Half Extraocular Muscles

Supplied by the III cranial or oculomotor nerve:

  • Superior rectus
  • Inferior rectus
  • Medial rectus
  • Inferior oblique
  • Part of levator palpebrae superioris.
17

Actions of Extraocular Muscles

Movements of the Eyeball

Axis Movement Range
Around a Transverse Axis Upward rotation or elevation 33°
Around a Transverse Axis Downwards rotation or depression 33°
Around a Vertical Axis Medial rotation or adduction 50°
Around a Vertical Axis Lateral rotation or abduction 50°
Around an Anteroposterior Axis Intorsion —
Around an Anteroposterior Axis Extorsion —

Definition of Movements

  • Rotatory movements of the eyeball:
    • Upwards
    • Downwards
    • Medially
    • Laterally
  • These are defined in terms of the direction of movement of the centre of the pupil.
  • Torsions are defined in terms of the direction of movement of the upper margin of the pupil at the 12 o’clock position.

Actions of Individual Muscles

  • Single or pure movements are produced by the actions of muscles.
  • Similar actions get added together.
  • Opposing actions cancel each other, enabling pure movements.

Combined Movements of the Eyes

  • The movements given above can take place in various combinations.
  • Normally, movements of the two eyes are harmoniously coordinated.
  • Such coordinated movements of both eyes are called conjugate ocular movements.
18

Clinical Anatomy

1. Squint / Strabismus

  • Weakness or paralysis of a muscle causes squint or strabismus.
  • It may be:
    1. Concomitant
    2. Paralytic

Concomitant Squint

  • It is congenital.
  • There is no limitation of movement.
  • There is no diplopia.

Paralytic Squint

  • Movements are limited.
  • Diplopia and vertigo are present.
  • The head is turned in the direction of the function of the paralysed muscle.
  • There is a false orientation of the field of vision.

2. Nystagmus

  • Nystagmus is characterized by involuntary, rhythmical oscillatory movements of the eyes.
  • It is due to incoordination of the ocular muscles.
  • It may be:
    • Vestibular
    • Cerebellar
    • Congenital

Note

  • The terms medial squint/internal strabismus and lateral squint/external strabismus are currently not in use.
Bony Orbits — Study Notes