What Is A Cat’s Third Eyelid And How Does It Work?

Such as you, your cat has two eyes—however not like you, your cat additionally has a 3rd eyelid!

This “third eyelid” is definitely referred to as the nictitating membrane, explains Amber Carter, board-certified feline licensed veterinarian and proprietor of CatVetLife. This skinny construction is positioned on the eye’s medial canthus—the internal half closest to the nostril.

In a wholesome cat, you must solely see the third eyelid as a tiny, crescent-shaped slit on the nook of the eyes, Dr. Carter says. Some cats, nonetheless, could expertise medical points that trigger the third eyelid to face out.

Under, get the main points on this third eyelid, together with the way it works, what it ought to seem like, and when to go to the vet.

Why Do Cats Have A Third Eyelid?

Cats aren’t the one animals which have nictitating membranes. Dogs, birds, and reptiles have a 3rd eyelid, too—however humans and other primates aren’t a part of the third eyelid membership.

Consultants imagine a cat’s third eyelid has two fundamental functions:

Scratch safety

Your cat’s exterior eyelids shut over the attention vertically, however the third eyelid closes diagonally. It sweeps throughout your cat’s eyes from one other angle—somewhat like a windshield wiper—to take away grime and particles from the attention. This supplies an additional layer of safety on your cat’s eyes in order that they don’t get scratched as simply.

Added moisture

The third eyelid additionally incorporates a tear gland that helps moisten your cat’s eyes. In actual fact, specialists estimate this eyelid produces round 30% of all of the tears in your cat’s eyes. Cats who don’t have a 3rd eyelid could expertise points like power dry eye.

JacquesKloppers via iStock

How Do A Cat’s Eyes Work?

Structurally talking, a cat’s eyes operate similarly to a human’s eyes. Right here’s how:

• When mild hits the eye, it first passes via the cornea. That is the clear, dome-shaped outer layer that helps the attention focus.
• A few of the mild enters the attention via the pupil. That is the black opening. The iris, which is the coloured a part of your cat’s eye, could make the pupil larger or smaller to regulate the quantity of sunshine that comes via.
• After that, mild passes via the lens. That is the clear layer within the internal eye that helps the cornea focus.
• Lastly, mild hits the retina. This layer behind the attention turns mild into electrical indicators. These indicators journey via the optic nerve and into the mind, the place your cat interprets them.

What makes a cat’s eye particular?

In fact, a cat’s eyes have a couple of further bonuses that people don’t. Cats are crepuscular, which implies they’re primarily lively in the course of the morning and night hours. So, they’ve a couple of distinctive traits that give them an optical edge in low-light environments.

A few of these distinctive traits embody:

• Low mild imaginative and prescient: Each cats and people have rod cells of their retinas. These cells assist with peripheral imaginative and prescient and low-light imaginative and prescient. However cats have 6-8 times the quantity of rod cells that individuals do. This doesn’t fairly give them evening imaginative and prescient, however it could actually assist them see extra clearly in dim circumstances.
• Reflective eyes: Cat eyes have one other distinctive construction absent in human eyes: the tapetum lucidum. This mirror-like construction behind a cat’s retina displays mild backward into the eyes, serving to cats see extra clearly at evening. You could acknowledge it because the glowing yellow-green “ghost eyes” watching you in a darkish room.
• Vertical pupils: Your cat’s slit-shaped pupils do greater than make them look smug. They management the quantity of sunshine your cat’s eyes absorb and will help your cat see more details when searching. In actual fact, cat pupils can dilate 10-15 instances greater than human pupils.

These traits, mixed with the third eyelid that gives further safety and moisture, assist cats thrive as low-light hunters. It’s no surprise they’ll discover and assault your toes at 3 a.m.!

Can cats blink their third eyelid?

Nope! Cats don’t have any impartial management over their third eyelid, Dr. Carter explains, and blinking it’s a completely passive response. This eyelid may shut mechanically if one thing presses on a cat’s higher eyelid.

This occurs as a result of muscle tissues that contract the third eyelid are linked to the sympathetic nervous system, which triggers the “struggle or flight” response.

You possibly can consider this as the same response to your pupils dilating. You don’t have any management over this—your physique does it for you!

Why Can I See My Cat’s Third Eyelid?

Dr. Carter says you sometimes shouldn’t be capable to discover a cat’s third eyelid except you’re in search of it. That stated, some medical circumstances could cause the third eyelid to noticeably protrude, together with:

Horner’s syndrome

Horner’s syndrome is a set of signs attributable to harm to the sympathetic nervous system.

Along with an elevated third eyelid, your cat may additionally have:

• • Constricted pupils
  • Drooping fundamental eyelids
  • Retracting eyeballs

Whereas these indicators could seem somewhat scary, Horner syndrome doesn’t trigger your cat ache or have an effect on their imaginative and prescient. In actual fact, you don’t even have to deal with it, although in some instances a vet or veterinary ophthalmologist could prescribe phenylephrine eye drops to assist relieve these signs.

Cherry eye

The tear gland inside your cat’s third eyelid is held in place by tissue fibers. However typically, particular person fibers weaken, inflicting the gland to drop down. The tear gland has a reddish shade, so when it protrudes, it’s referred to as “cherry eye.”

In case your cat has cherry eye, a vet or vet ophthalmologist could counsel surgical procedure to tuck the protruding gland again the place it belongs.

Haws syndrome

Haws syndrome is one other widespread situation that may trigger your cat’s third eyelid to protrude. It doesn’t often have an effect on your cat’s imaginative and prescient, however it could actually trigger different signs, like diarrhea.

Whereas the reason for Haws syndrome technically stays unknown, specialists usually affiliate it with underlying gastrointestinal illness, in keeping with Dr. Carter.

Haw’s syndrome usually goes away by itself and sometimes doesn’t want remedy.

Different causes

Moreover, the third eyelid may protrude because of non eye-related well being points, like tetanus or parasitic infections. Dr. Carter additionally says cats experiencing ache, both of their eyes or one other a part of their physique, may additionally have protruding eyelids.

So, for those who discover something amiss in your cat’s eyes, a vet appointment might assist you uncover any underlying causes and assist your cat really feel like their previous self once more.

Nonetheless, for those who discover a protruding third eyelid in your cat or every other uncommon signs, it’s a good suggestion to make a vet appointment as quickly as attainable. Your vet will help pinpoint the reason for modifications in your cat’s eyes and assist rule out any severe well being circumstances.