The orbit is the cavity of the skull in which the eye and its appendages are situated. It is composed of seven bones and has a pyramidal shape, with its apex pointed posteromedially. The orbital contents comprise the eye; orbital and retrobulbar fascia; extraocular muscles; cranial nerves II, III, IV, V, and VI; blood vessels; fat; lacrimal gland with its sac and nasolacrimal duct; eyelids; palpebral and suspensory ligaments; ciliary ganglion; and short ciliary nerves.

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Eye orbit anatomy – anterior view

Image: “Eye orbit anatomy – anterior view” by Patrick J. Lynch, medical illustrator, License: CC BY 2.5


Bones and Structure of the orbit

  • Shaped like a quadrangular pyramidal cavern in the upper face, with a superficial base (orbital margin) and a deep, posteromedial apex. 
    • Orbital margin: 
      • Superior margin: frontal bone 
      • Medial margin: frontal process of the maxilla
      • Inferior margin: zygomatic process of the maxilla and zygomatic bone
      • Lateral margin: zygomatic process of the frontal bone and frontal process of the zygomatic bone
    • Apex: optic foramen
  • Walls: covered with periosteum (periorbita)
    • Superior (roof): orbital part of the frontal and lesser wing of the sphenoid
    • Medial: orbital plate of ethmoid bone, body of sphenoid bone, frontal bone, lacrimal bone, and maxilla
    • Inferior (floor): orbital surface of the maxilla, zygomatic bone, and palatine bone
    • Lateral: greater wing of sphenoid, orbital plate of frontal bone, and frontal process of zygomatic bone
  • Important openings:
    • Optic foramen or canal
    • Anterior and posterior ethmoidal foramina
    • Superior and inferior orbital fissures
    • Infraorbital groove
    • Supraorbital notch
Location Contents
Optic foramen or canal Apex, bordered by the body and lesser wing of the sphenoid Optic nerve (CN I)

Ophthalmic artery

Ethmoidal foramina Junction between the superior and medial orbital walls

In the ethmoid bone, lateral to olfactory groove

Anterior and posterior ethmoidal veins, arteries, and nerves
Superior orbital fissure Between the greater and lesser wings of the sphenoid bone Inside the common tendinous ring:

  • Nasociliary nerve
  • Superior and inferior branches of the oculomotor nerve (CN III)
  • Abducens (CN VI) nerve

Outside the common tendinous ring:

  • Frontal nerve
  • Lacrimalis nerve
  • Trochlear (CN IV) nerve
  • Superior ophthalmic vein
Inferior orbital fissure Lateral border of orbital floor

Formed by greater wing of sphenoid superiorly and palatine and maxillary bones inferiorly

Inferior ophthalmic vein

Infraorbital artery and vein

Branches of the maxillary division of the trigeminal nerve (CN V2) – zygomatic and infraorbital nerves

Orbital branches of the pterygopalatine ganglion

Infraorbital foramen Middle of the orbital floor (maxilla) Exit of the infraorbital vein, artery, and nerve
Supraorbital notch or foramen Superior margin of the orbit (frontal bone) Exit of the supraorbital vein, artery, and nerve

Mnemonic

To help memorize the bones that make up the orbit, remember:  

Many Friendly Zebras Enjoy Lazy Summer Picnics

  • Many: Maxilla
  • Friendly: Frontal bone
  • Zebras: Zygomatic bone
  • Enjoy: Ethmoid bone
  • Lazy: Lacrimal bone
  • Summer: Sphenoid bone
  • Picnics: Palatine bone

Extraocular muscles

Muscle Origin Insertion Irrigation Innervation Function
Medial rectus                                         Annulus of Zinn (common tendinous ring) Anterior, medial surface of the eye Inferior muscular branch of ophthalmic artery Inferior branch of oculomotor nerve (CN III) Adduction
Lateral rectus Anterior, lateral surface of the eye Lacrimal artery Abducens nerve (CN VI) Abduction
Inferior rectus Anterior, inferior surface of the eye Inferior muscular branch of ophthalmic artery and the infraorbital branch of the maxillary artery Inferior branch of oculomotor nerve (CN III) Depression, extorsion, and adduction.

In abduction: only depresses

Superior rectus Anterior, superior surface of the eye Superior muscular branch of ophthalmic artery Superior branch of the oculomotor nerve (CN III) Elevation, intorsion and adduction. 

In abduction: only elevates

Superior oblique Lesser wing of sphenoid, medial to optic canal Posterior, superior, lateral surface of the eye Superior muscular branch of the ophthalmic artery Trochlear nerve (CN IV) Intorsion, depression and abduction
Inferior oblique Lateral to the lacrimal groove (maxilla) Posterior, inferior, lateral surface of the eye Inferior branch of the ophthalmic artery and infraorbital artery Inferior branch of oculomotor nerve (CN III) Extorsion, elevation and abduction
Levator palpebrae superioris Lesser wing of the sphenoid,  above the optic canal Tarsal plate of upper eyelid Supraorbital branch of the ophthalmic artery Superior branch of oculomotor nerve (CN III)

Sympathetic fibers innervate the smooth muscle fibers on the inferior surface of this muscle

Retracting and elevating the eyelid
Extraocular muscles

Image: “Extraocular muscles” by OpenStax, License: CC BY 3.0

Nerve Palsy Causes Symptoms involving affected eye
Oculomotor (CN III)
  • Infarction of midbrain
  • Berry aneurysm at junction of posterior communicating and internal carotid arteries
  • Lesions of cavernous sinuses (neoplasm, vascular, or inflammatory)
  • Horizontal temporal deviation
  • Down and out gaze
  • Ptosis
  • Pupillary dilation
Trochlear (CN IV)
  • Head trauma
  • Tumor at base of skull
  • Microvasculopathy
  • Idiopathic
Eye is up and in
Abducens (CN VI)
  • Postviral syndrome
  • Ischemic mononeuropathy
Eye directed medially

Mnemonic

To help memorize the innervation of the extraocular muscles, remember:

LR6, SO4, 3

Lateral rectus innervated by the abducens nerve (CN VI)

Superior oblique innervated by the trochlear nerve (CN IV)

The remaining extraocular muscles are innervated by the oculomotor nerve (CN III

Video Gallery

Overview of the 7 Extraocular Muscles – Orbital Muscles and Innervation by Craig Canby, PhD

Functions of the Extraocular Muscles – Orbital Muscles and Innervation by Craig Canby, PhD

Innervation of Extraocular Muscles – Orbital Muscles and Innervation by Craig Canby, PhD

Lacrimal apparatus

  • Lacrimal gland:
    • Serous, releases aqueous layer of tear fluid directly onto the eyeball
    • Minimizes friction, protects, and cleans the eye (“tear film”)
    • Located on upper lateral aspect of orbit, drains via lacrimal ducts to the superior conjunctival fornix
    • Innervated by parasympathetic fibers of the facial nerve (VII) via the pterygopalatine ganglion
  • Tears:
    • isotonic solution
    • Contains bactericidal enzymes (lysozyme and lactoferrin), immunoglobulin A, and lipocalin
  • Tarsal or Meibomian glands:
    • Sebaceous glands, produce meibum, which prevents evaporation of the tear film
    • Located within tarsal plate of eyelids (within the superior eyelid) with orifices at the rim of the marginal zone of the conjunctiva
  • Tear drainage system: consists of
    • Lacrimal canaliculi
    • Lacrimal sac: dilated portion of the nasolacrimal duct
    • Nasolacrimal duct: drains into the inferior nasal meatus

Image: Schematic diagram of the location of the lacrimal gland and apparatus. By CNX OpenStax. License: CC BY 4.0

Differential Diagnosis

Orbital and preseptal cellulitis: Possible complications to paranasal sinusitis or trauma to the eye orbit. Patients may present with fever, malaise, proptosis ophthalmoplegia, toxic shock, and impaired vision.

Orbital fractures: Fractures to the eye sockets and are classified as orbital rim fractures, direct orbital floor fracture, and blowout fractures. These fractures can present with blurry, decreased or double vision, and black and blue bruising around the eyes.

Dacryoadenitis: An inflammation of the lacrimal glands commonly due to a bacterial or viral infection. May present as swelling of the outer portion of the upper lid, with redness and tenderness, pain in the area of swelling, excess tearing, and swelling of the preauricular lymph nodes.

Sjögren’s syndrome: An autoimmune disease that affects the moisture-producing glands of the body. Primary symptoms are dry mouth and dry eyes. 

Strabismus: A condition characterized by a misalignment of the eyes. If left untreated throughout childhood, it may result in amblyopia or loss of depth perception. 

Blepharitis: One of the most common ocular conditions characterized by inflammation, scaling, reddening, and crusting of the eyelid. May also present with a burning, itching, or grainy sensation. 

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One thought on “The Orbit and Extraocular muscles

  • Raihan Alheyali

    The diagram in the end makes no sense. I thought SR & IR move the eye sup-medially and inf-medially, respectively. Also SO moves it inf-laterally, IO moves it sup-laterally. This diagram does not match the text.