Accessory structures of eye

Accessory structures of the eye (Latin: structurae bulbi accessoriae) are responsible for the movement of the eyeball and its protection. The accessory structures are:

  • Eyelids
  • Eyelashes
  • Eyebrows
  • Conjunctiva
  • Orbit
  • Lacrimal apparatus
  • Extraocular muscles

The eyelids are divided into an orbital and a tarsal part. The division is done by the superior palpebral sulcus. The eyelids have six layers: the skin, subcutaneous tissue, striated muscle fibers, orbital septum, smooth muscles, and conjunctiva. The skin is very thin and houses the eyelashes. The eyelashes block bright light and dust. The subcutaneous tissue is rich in elastic tissue. The orbicularis oculi surround the orbital margin of the eyelid. The orbital septum attaches to the orbital margin of the eyelids. The septum is the structure separating the eyelids from the content of the orbital cavity. The smooth muscle makes the tarsal muscles. The upper tarsal muscle raises the upper eyelid, while the lower tarsal muscle lowers the lower eyelid. The conjunctiva is a mucous membrane lining the eyelids. The eyelids are responsible for protecting the eyes from injuries and too much light. It is done by closing the eyelids. The eyelids distribute tears over the anterior surface of the eyeball.

The eyebrows protect the eyes from sweat. The eyebrows are found at the junction between the forehead and the upper eyelid. The movement of the eyebrows is permitted by three facial muscles: the frontalis muscle, the orbital part of the orbicularis oculi muscle, and the corrugator supercilii muscle.

The conjunctiva is a mucous membrane lining the eyelids. At the lid margin, the conjunctival epithelium turns into the epidermis of the skin and, at the limbus - the corneal epithelium. The conjunctiva has three parts or regions: the palpebral conjunctiva, conjunctival fornices, and bulbar conjunctiva. The palpebral conjunctiva is lining the eyelids. There are superior and inferior conjunctival fornices. The fornices are transitional areas between the palpebral and bulbar conjunctiva. The bulbar conjunctiva has contact with the eyeball.

The orbit is the cavity of the skull where the eyeball lies. The orbit contains not only the eyeball but also extraocular muscles, some of the cranial nerves, orbital and retrobulbar fascia, blood vessels, ligaments, and lacrimal gland. The orbit houses and protects the eyes.

The lacrimal apparatus is a set of connected anatomical structures located within the orbit that are responsible for the production and drainage of tears. The lacrimal apparatus include the following structures:

  • Lacrimal gland
  • Lacrimal canaliculi
  • Lacrimal sac
  • Nasolacrimal duct

The lacrimal gland is the tear-producing organ of the lacrimal apparatus. The tears wash over the eyeball and drain into the lacrimal canaliculi (superior and inferior) that open in the medial corner of the eye with puncta lacrimalia, the beginning of the tear drainage system. The lacrimal canaliculi convey the tears to the lacrimal sac, a dilated portion of the nasolacrimal duct lodged in a depression between the lacrimal bone and the frontal process of the maxilla. The fluid drains further into the nasolacrimal duct, which is the final part of the lacrimal apparatus.

The extraocular muscles are a set of seven muscles located within each orbit and connected with the eye. There are six extraocular muscles responsible for eye movements, and one providing the upper eyelid elevation.