The Orbit and Extraocular Muscles

The orbit is the cavity of the skull in which the eye and its appendages are situated. The orbit is composed of 7 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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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, and supraorbital notch
LocationContents
Optic foramen or canalApex, 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 fissureBetween the greater and lesser wings of the sphenoid boneInside 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
  • Lacrimal 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 foramenMiddle of the orbital floor (maxilla)Exit of the infraorbital vein, artery, and nerve
Supraorbital notch or foramenSuperior margin of the orbit (frontal bone)Exit of the supraorbital vein, artery, and nerve

Mnemonic 1

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

MuscleOriginInsertionIrrigationInnervationFunction
Medial rectusAnnulus of Zinn (common tendinous ring)Anterior, medial surface of the eyeInferior muscular branch of ophthalmic arteryInferior branch of oculomotor nerve (CN III)Adduction
Lateral rectusAnterior, lateral surface of the eyeLacrimal arteryAbducens nerve (CN VI)Abduction
Inferior rectusAnterior, inferior surface of the eyeInferior muscular branch of ophthalmic artery and the infraorbital branch of the maxillary arteryInferior branch of oculomotor nerve (CN III)Depression, extorsion, and adduction. In abduction: only depresses
Superior rectusAnterior, superior surface of the eyeSuperior muscular branch of ophthalmic arterySuperior branch of the oculomotor nerve (CN III)Elevation, intorsion, and adduction. In abduction: only elevates
Superior obliqueLesser wing of sphenoid, medial to optic canalPosterior, superior, lateral surface of the eyeSuperior muscular branch of the ophthalmic arteryTrochlear nerve (CN IV)Intorsion, depression, and abduction
Inferior obliqueLateral to the lacrimal groove (maxilla)Posterior, inferior, lateral surface of the eyeInferior branch of the ophthalmic artery and infraorbital arteryInferior branch of oculomotor nerve (CN III)Extorsion, elevation, and abduction
Levator palpebrae superiorisLesser wing of the sphenoid, above the optic canalTarsal plate of upper eyelidSupraorbital branch of the ophthalmic arterySuperior 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

Mnemonic 2

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)

Mnemonic 3

To help memorize the actions of the muscles, remember: RAD

Recti are Adductors, except the lateral rectus.

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 decreases 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
Orbit lacrimal apparatus

Schematic diagram of the location of the lacrimal gland and apparatus

Image: “Microbiology” by CNX OpenStax. License: CC BY 4.0

Clinicopathologic Correlations

Nerve palsyCausesSymptoms 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
  • 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 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, strabismus 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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