Ocular Adnexa: Definition & Anatomy

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

In this lesson, we'll go over the definition of ocular adnexa, some of examples, as well as their functions. This will include the orbit, extraocular muscles, lacrimal system, and optic nerves.

Ocular Adnexa

Accessories. We all have and love them. Houses have things like pools; phones have all sort of cases; laptops have external mice. You get the picture. Accessories are things that aren't part of the main component they are attached to, next to, or surround.

Well, your eyes have accessories too. Maybe with not as much bling as a phone case made out of solid gold, but cool in their own way nonetheless. The term for accessory structures of the eye is ocular adnexa. 'Ocular' refers to eyes and 'adnexa' is a Latin term meaning 'fasten to' and in this case refers to accessory structures attached to the eye itself.

The ocular adnexa include the:

  • Orbits
  • Extraocular muscles
  • Eyelids
  • Lacrimal system
  • Optic nerves

Let's go into these terms in more detail.

The Orbit, Extraocular Muscles, & Eyelids

The bony cavity that contains your eyeball is called the orbit. Basically, it's the socket that holds the eye. When you see a human skull at a museum or in a Halloween decoration, it's the hole where the eye would have gone. The orbit itself is made up of many bones, including the maxilla (your upper jaw) and zygomatic bones (the cheekbones).

Inside of the orbit, but outside of the eyeball itself, are the extraocular muscles. These are the guys that help the eyeball move up, down, left, right, and rotate every which way. They also control the movement of the upper (superior) eyelid.

Extraocular muscles
Extraocular muscles

More familiar to you are the eyelids, the guys that help protect the eyeball against everything from dust to those pesky little flies trying to get into your eye. You have four eyelids, two for each eye (upper and lower), although other animals have six of them, three for each eye.

Have you ever noticed a dog halfway between asleep and awake, and see that thin, transparant film that's halfway over their eye? That's called the nictitating membrane, or third eyelid. Mammals tend to only have a vestigial one in the corner of their eye, but others have full nictitating membranes, like camels who need extra protection from the blowing sands in the desert.

The vestigial bit of nictitating membrane (plica semilunaris) in the corner of a human eye
Ocular Adnexa

The furry eyebrows over the top of your eyes and those beautiful long eyelashes protruding from the eyelids are considered an accessory of the eye as well.

Lacrimal System & Optic Nerves

Not as obvious as the eyelids or those long lashes, although arguably more important, are two other accessory structures of the eyes. These are the lacrimal system and the optics nerves.

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