The Protection of the Eye

Eyelid

  • Skin
    • Thinner and more elastic/mobile than elsewhere in the body
    • Little or no subcutaneous fat (good source of skin graft)
    • Extremely good blood supply (heals quickly)
  • Muscles
    • Orbicularis oculi- Eyelid closure (CNVII)
    • Levator Palpebrae- elevates eyelid (CNIII)
    • Muller’s muscle- helps to retract the upper lid (cervical sympathetic innervation)
  • Tarsal plates
    • Composed of dense fibrous tissue, keeping the eyelid rigid/firm
    • Contains meibomian glands, which open at the lid margin and makes oily secretion that forms part of the tear film

Conjunctiva

  • Thin mucous membrane which lines the inner surface of the eye lid and outer surface of the eyeball, protecting the eye ball/cornea
    • During blinking, it lubricates the cornea with tears
    • Protects other exposed parts of the eye from infection (contains lymphocytes and macrophages)
    • Mucin from goblet cells has wetting effect of tear film
  • 3 parts
    • Tarsal- Inner eyelid area- firmly attached to tarsal plate
    • Bulbar- Lining the eyeball- loosely attached to underlying sclera
    • Fornix- part where these meet
  • NB the conjunctival epithelium is continuous with the corneal epithelium at the limbus (margin of the cornea)

Lacrimal Apparatus

  • Consists of lacrimal gland, punctum, canaliculi, nasolacrimal sac and duct

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  • The tear forms a thin film of fluid on the surface of conjunctiva/cornea, vital for their health and the transparency of the cornea
  • 3 parts of the tear film
    • Outer (lipid) layer- oily secretion from Meibomian and Zeis gland
    • Middle (aqueous) layer- Water from lacrimal gland and accessory lacrimal glands of Krause and Wolfring
    • Inner (mucinous) layer- Mucus from goblet cells of conjunctiva
  • Provides a moist environment for surface epithelial cells; washes away debris, transports metabolic products (oxygen, CO2) to and from surface cells, antimicrobial, and provides a smooth refracting surface over the cornea

Orbit

  • Made up of 7 skull bones:
    • NB the maxillary bone is the most likely to fracture in a blowout fracture
    • Roof: Frontal and sphenoid
    • Floor: Zygomatic, maxillary and palatine
    • Medial: Ethmoid, frontal, lacrimal and sphenoid
    • Lateral (strongest): zygomatic and sphenoid
  • At the orbital apex, there is the superior orbital fissure and optic canal.
    • Through the superior fissure: Superior and inferior divisions of CNIII, CNIV, Lacrimal, frontal and nasocilliary branches of CNV1, CN VI, superior branche of the ophthalmic vein, sympathetic fibres and orbital branch of the middle meningeal artery.
    • Through the Optic foramen travels the ophthalmic artery, central retinal vein and optic nerve (CN II).

In the base of the orbit is the inferior fissure

  • the inferior branch of the ophtalmic vein, infraorbital artery and nerve, as well as parasympathetic nerves to the lacrimal glands enter the orbit through this.

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