These facts about a dog's eyelids will help you learn more about these fascinating body parts and the many important roles they play.
Just as it happens in humans, a dogs' eyelids aren't there just for decoration. The eyes are very important for survival purposes, and therefore, it makes sense for them to need as much protection as possible.
Let's therefore discover more about eyelids in dogs and the different functions they carry out. So, what are eyelids exactly? And why are they so important?
A Lesson in Anatomy
Dogs have an upper and lower eyelid and both eyelids are hairy.
The upper and lower eyelids meet at the medial canthus nearby the nose, and the lateral canthus nearby the ear as seen in the picture below.
Protection of the Dog's Eyeball
Eyelids are composed by muscle and skin and one of their primary roles is to shield the eyeball from damage.
After all, imagine how vulnerable a dogs' eyes would be if they weren't protected by eyelids! A dog walking around among grass and bushes would risk getting their eyes scratched every day.
In particular, the dog's lower eyelid helps protects the eye from dirt, seeds and foreign matter. A loose lower lid is undesirable in dogs because it allows the accumulation of dirt and debris which risks getting into direct contact with the dog's eyeball.
For this reason, loose lower eyelids are frowned upon in many breeds, with exclusion of a few breeds with loose skin such as the bloodhound.
We can therefore say that a dog's eyelids protect the anterior surface of the dog's eyeball from mechanical injury as it can happen through exposure to many potentially harmful things such as dust, debris and foreign bodies.
Did you know? Courtesy of your dog's blink reflex, should any object get close enough to the eye, the eyelids will reflexively close to prevent any trauma.
This happens courtesy of the concentration of trigeminal nerve endings in the area which create an extreme sensitivity to any stimulation.
On top of this sensitivity, to further help readily detect any danger to a dog's vulnerable eyes, dogs have special whiskers known as "supraorbital whiskers."
These sort of act like extensions of the dog's eyelashes, protecting the eyes and triggering the reflexive protective blinking reaction mentioned above.
Protection From Light
The eyelid is made up of several layers of delicate tissue that protects the eye from light.
When puppies are born, their eyelids are shut. This is a protective mechanism considering that puppies are born helpless and underdeveloped. Intrigued? Discover more about dogs being altricial animals.
Since their development is still in an immature state, their eyes are not ready to deal with light. Only once the eyes are better developed, puppies will open their eyes which happens at around 12 to 14 days after birth.
Once the eyes are opened, pups' eyes will initially have a cloudy look. This should clear in about 24 hours.
Although puppies are starting to see, their eyes are still quite vulnerable during this stage. Exposure to bright lights should be avoided.
Once pups grow, they are able to better tolerate light. However, the eyelids will keep on assuming a protective role towards exposure to the sun's harmful rays or bright lights by eliciting squinting.
If you notice your dog squinting without being in the sun or exposed to a bright light, please report to your vet as there may an underlying eye condition at play.
Did you know? Dog eyelashes are located along the edge of the upper eyelid and consist of 2 to 4 rows of cilia. The dog's lower eyelid does not have cilia, explains board-certified veterinary ophthalmologist Dr. Ellen B. Belknap, in an article for Today's Veterinary Practice.
Protection from "Noise"
Interestingly, as it happens in humans, when dogs are exposed to loud noises, they'll also close their eyes. This eyelid closure isn't voluntary, but occurs as an automatic response, an innate protective mechanism.
However, this eyelid closure, is likely not protecting us and our dogs from noises directly. Rather, possibly, our bodies, and the bodies of our dogs, have come to associate loud sounds with the danger of projectile fragments. It's technically called the "acoustic startle reflex."
Did you know? A dog's eyelids may also close as a response to eye pain. This is called "blepharospasm" and is seen in many painful eye disorders. Please have your dog see the vet as soon as possible if he is squinting.
Production of Tears
Dogs produce two main types of "tears": the watery version which acts like a natural saline solution and an oily version which helps lubricate the eyes and prevent the watery tears from evaporating.
Lacrimal glands (tear glands) are located at the side of the eyelid and produce the watery tears. The watery tears are responsible for keeping a dog's eyes moist.
Meibomian glands, (also known as tarsal glands), instead are located on the eyelid margin (approximately 20 to 40 ducts found on the eyelid's edge attached to the hair follicle of every eyelash) and secrete a fatty substance known as meibum.
Meibum floats over the eye and is responsible for lubricating the surface of the dog's eye and preventing moisture from evaporating.
There are several eye disorders that can cause excessive tearing of the eyes in dogs.
The medical term for excessive tearing in dogs is "epiphora" and it can originate from an obstruction of the dog's tear duct or an overproduction of tears with a drainage system that cannot keep up.
Did you know? A recent study has revealed that dogs can cry tears of emotions. Discover about it here: can dogs cry tears as humans?
Dogs Have a Third "Eyelid"
We all know that dogs have two eyelids just like us; the upper eyelid and the lower eyelid. Many though may not known that dogs have also a third eyelid!
The dog's third eyelid, also known as nictating membrane or "haw," originates from the medial canthus, and therefore, tends to hide in the corner of the dog's eye, but you occasionally notice it when your dog is sleeping with his eyelids semi-open.
The third eyelid covers your dog's eyes when your dog is sound asleep so to prevent his eyes from drying up too much. Your dog doesn't blink in his sleep so this membrane takes over the role of keeping those eyes nice and moist.
So yes, it's a good thing if you notice your dog's "red eyes" when he's sleeping. What you are seeing is the third eyelid doing its job.
You need to worry instead when the third eyelid doesn't normally retract once your dog's eyes are open and your dog is wide awake, as this can be a sign of something wrong. More about this is covered here: why is my dog's third eyelid showing?
On top of preventing the eyes from drying up, the third eyelid has a special gland (called the nictitans gland) that's responsible for about 40 to 50 percent of your dog's tear production.
The "Spread" of Tears
When dogs blink, the eyelids help distribute protective tears over the eyeball. Such even spread, over the surface of the eye, ensures the eyes retain their needed moisture, preventing them from drying up.
A dog's third eyelid also acts as a "windshield-wiper" for the cornea, effectively removing debris or mucus from it when the dog is asleep.
Did you know? On top of spreading tears, the eyelids help trap any foreign material and debris pushing it towards the medial canthus.
Excess tears that collect by the medial canthus then drain into the nasal cavity through the nasolacrimal duct.
This is why you get a runny nose when you cry and why when you put eye drops or shampoo ends up in your eyes, you may taste it. Your nasal cavity connects to the mouth at the base of the throat!
Problems With Dog Eyelids
Your dog's eyelids can also be subjected to a varied host of disorders. If you notice anything unusual, please report to your vet.
Inflammation of the Dog's Eyelids
Blepharitis is a fancy medical term for eyelid inflammation. Blepharitis manifests with visual changes of the eyelids, including intense swelling, redness, and scabbing.
Blepharitis can stem from bacterial infections, parasites (external or insect bites), or systemic conditions such as hypothyroidism (underactive thyroid gland).
Although there are many potential culprits, the most common cause of blepharitis in dogs is demodectic mange, a skin condition caused by the Demodex parasite, which causes skin lesions in different parts of the body but almost always affects the area around the eyes, explains veterinarian Dr. Ivana.
Defects of the Dog's Eyelids
Entropion is a congenital disorder in which the eyelid is abnormally rolled inward. It can affect one (unilateral) or both (bilateral) eyes and the lower eyelid, upper eyelid, or both.
As a result, the eyelashes rub the eye causing constant irritation (redness and weeping). A dog with entropion will scratch its eyes, squint a lot, or even keep its eye closed.
Eye Lid Growths in Dogs
Eyelid growths often develop in older dogs, although they can affect a dog of any age.
While most eyelid mass growths are benign, meaning that they won't spread to other parts of the body (metastasize), when they grow in size, they may be capable of causing problems to the surface of the dog's eyes (cornea).
Sometimes, a small number of eyelid growths in dogs may be malignant and require systemic therapy.
If your dog has developed a suspicious lump or bump on his eyelids, contact your veterinarian for an examination.
Prolapsed Third Eyelid
As discussed, sometimes a dog's third eyelid may not retract. This happens when a small ligament responsible for holding the gland in place, may stretch or break for unknown reasons, explains veterinary ophthalmologist Rhea V. Morgan.
When this happens, you'll notice a visible reddish mass in the corner of the dog's eye that is commonly referred to as "cherry eye."
This mostly happens in certain dog breeds that are prone to it such as cocker spaniels, basset hounds, lhasa apso, shih-tzu, poodle, beagles, Newfoundlands, Chinese shar-pei, Saint Bernards and bulldogs.
To correct this, dog must undergo cherry eye surgery so to get that gland replaced so that it can continue to produce tears as it was meant to and to prevent the emergence of complications such as a chronic dry eye.
Veterinary Ophthalmology. (2021). United Kingdom: Wiley.